Accepted Article
Received Date : 07-May-2014 Revised Date : 24-Aug-2014 Accepted Date : 29-Aug-2014 Article type
: Primary Research Articles
Response of wheat restricted-tillering and vigorous growth traits to variables of climate change
Eduardo A. Dias de Oliveira1,2,3, Kadambot H.M. Siddique2, Helen Bramley2,4, Katia Stefanova2 and Jairo A. Palta1,3
1
*
CSIRO Plant Industry, Private Bag No 5, Wembley, WA 6913 Australia.
2
The UWA Institute of Agriculture (M082), The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia 3 School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia 4 Present address: Plant Breeding Institute, Faculty of Agriculture and Environment, The University of Sydney, 12656 Newell Highway, Narrabri NSW 2390, Australia
*
Corresponding author:
Dr Jairo A. Palta CSIRO Plant Industry Private Bag No. 5, Wembley WA 6913, Australia This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/gcb.12769 This article is protected by copyright. All rights reserved.
Accepted Article
Tel: +61 (0) 8 9333 6611 Fax: +61 (0) 8 9387 8991 Email: [email protected]
Abstract The response of wheat to the variables of climate change includes elevated CO2, high temperature and drought which vary according to the levels of each variable and genotype. Independently, elevated CO2, high temperature and terminal drought affect wheat biomass and grain yield, but the interactive effects of these three variables are not well known. The aim of this study was to determine the effects of elevated CO2 when combined with high temperature and terminal drought on the high-yielding traits of restricted-tillering and vigorous growth. It was hypothezised that elevated CO2 alone, rather than combined with high temperature, ameliorates the effects of terminal drought on wheat biomass and grain yield. It was also hypothesized that wheat genotypes with more sink capacity (e.g. hightillering capacity and leaf area) have more grain yield under combined elevated CO2, high temperature and terminal drought. Two pairs of sister lines with contrasting tillering and vigorous growth were grown in poly-tunnels in a four-factor completely randomized splitplot design with elevated CO2 (700 µL L–1), high day time temperature (3°C above ambient) and drought (induced from anthesis) in all combinations to test whether elevated CO2 ameliorates the effects of high temperature and terminal drought on biomass accumulation and grain yield. For biomass and grain yield, only main effects for climate change variables were significant. Elevated CO2 significantly increased grain yield by 24–35% in all four lines and terminal drought significantly reduced grain yield by 16–17% in all four lines, while high temperature (3°C above the ambient) had no significant effect. A trade-off between yield components limited grain yield in lines with greater sink capacity (free-tillering lines). This
This article is protected by copyright. All rights reserved.
Accepted Article
response suggests that any positive response to predicted changes in climate will not overcome the limitations imposed by the trade-off in yield components.
keywords: high- yielding traits, climate change components, source–sink relationships, isogenic lines, wheat, tunnel houses.
Introduction In Mediterranean-type environments, climate change has a direct impact on wheat production because growth and grain yield will be affected by the predicted increase in temperature and predicted decrease and variability in precipitation during the growing season (Ragab and Prudhomme 2002; Asseng et al., 2004). Total biomass and grain yield in wheat may increase in a climate scenario that combines elevated CO2 and temperatures moderately above the current ambient temperatures, regardless of whether the crop is irrigated or under terminal drought. (Dias de Oliveira et al. 2013). However, large increases in temperature may restrict any positive effects of elevated CO2 on above or belowground biomass and grain yield (Schütz and Fangmeier 2001; Benlloch-Gonzalez et al. 2014). The amplitude of these responses may vary according to genotype growth habit and source–sink capacity (Dias de Oliveira et al. 2013) , since these traits affect carbon assimilation and grain yield components . For instance, if carbon assimilation exceeds sink capacity, respiration may increase (Aranjuelo et al. 2011).
Dias de Oliveira et al. (2013) showed wheat responses to combined elevated CO2 and temperatures +2°C, +4°C and +6°C above ambient, but did not identify the main driver for biomass and grain yield response or explain the interaction between the drivers of climate
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change. Elevated CO2 and high temperature are climate change drivers that, when combined are likely to have an interactive effect on biomass and grain yield (Mitchell et al. 1993; Kimball et al. 1995; Moot et al. 1996; Manderscheid and Weigel 1997; Fangmeier et al. 1999; Manderscheid et al. 2003). The outcome of this interaction may include a reduced positive effect of elevated CO2, amelioration of the effect of high temperature and a synergistic effect where high temperature increases the positive effect of elevated CO2 (Mitchell et al. 1993; Rawson 1995; Batts et al. 1997; Batts et al. 1998; van Oijen et al. 1999; Martinez-Carrasco et al. 2005). The interactive effect may be influenced by genotypic differences in the ability of wheat to adapt to changes in CO2 concentration, temperature and drought as well as the levels of these variables. Reductions in biomass and grain yield caused by terminal drought were partially ameliorated in two wheat genotypes contrasting in vigorous growth when grown under elevated CO2 +2°C above ambient temperature. The amelioration in grain yield resulted from an increase in the number of ears m-2, but it was not possible to determine whether the driver of this response was elevated CO2 or high temperature (Dias de Oliveira et al. 2013).
In genotypes with vigorous growth, phenology can be a major constraint for improving grain yield under elevated CO2 and high temperature. This is because flowering and physiological maturity, are inherently earlier in vigorous than non-vigorous genotypes (Rebetzke and Richards 1999; Botwright et al. 2002) and elevated CO2 and high temperature are likely to accelerate phenology (Rawson 1986; Sadras and Monzon 2006; Asseng et al.2004 ). Acceleration in phenology not only restricts tiller production and hence ear number, but also duration of grain filling (Dias de Oliveira et al. 2013). Tiller production in wheat depends on the amount of radiation that the crop receives over a period of time (Evans 1978; Rawson
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1986) and restricted tiller production reduces sink capacity, photosynthetic enhancement and grain yield (Fischer 2011; Aranjuelo et al. 2011). Therefore one could expect that genotypes with increased tillering and hence more ears per m – 2 would benefit more when grown under elevated CO2.
The aims of this study were (i) to identify the main driver in the response of biomass and grain yield in wheat crops grown under elevated CO2 and high temperature and (ii) determine whether the effects of CO2, temperature and terminal drought had significant interaction when applied at the same time. Two pairs of sister lines contrasting in tillering and vigorous growth were chosen to represent differences in sink strength and source size (Palta et al. 2007; Palta et al. 2011; Benlloch-Gonzalez et al. 2013). The four lines were grown in polytunnels under elevated CO2, high temperature and terminal drought. It was hypothesized that (i) elevated CO2 alone, rather than combined with high temperature, ameliorates the effects of terminal drought on wheat biomass and grain yield and (ii) wheat genotypes with more sink capacity (e.g. high tillering capacity and leaf area) will have more grain yield under combined elevated CO2, high temperature and terminal drought.
Materials and Methods Location A field experiment was conducted between May and November 2011 at The University of Western Australia’s (UWA) Research Station at Shenton Park, Western Australia (31º 57’S, 115º 45’E). The site is located in an area with a long-term average rainfall (80 years) of 710 mm for the May–November growing season. The soil is a free-draining infertile Spearwood
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clayey sand (McArthur and Bettenay 1960) consisting of brown, fine sandy clay with less than 1% organic matter. The pH, measured in a 1:5 suspension of soil in 0.01 M CaCl2 was 5.1–6.8 in the surface 0–10 cm and 4.1–4.5 in lower layers of the soil profile.
Plant material Two pairs of sister lines of wheat (Triticum aestivum L.) representing contrasting genotypes for tillering and vigorous growth were used in this study. The lines for tillering 7750N (free) and 7750PF (reduced) and early vigor CV97 (high) and CV207 (low) were selected by Dr G Rebetzke from CSIRO Plant Industry. The 7750N and 7750PF lines are F5-derived, sister lines for restricted-tillering in a Lang genetic background (Wyalkatchem//3*Silverstar/971/3/Lang). The CV97 and CV207 lines are F2-derived F5:7 lines selected from a cross between the reduced-vigor, Rht8 donor Chuan-mai 18 and a tall vigorous donor Vigour 18. Sister lines were selected with similar Rht8-based dwarf stature but contrasting in leaf areas of leaves 1 and 2 (Benlloch-Gonzalez et al., 2013). The two pairs of sister lines were grown in the field under poly-tunnels in a four-factor completely randomized design with elevated CO2, high temperature and terminal drought in all combinations in a split-plot design within each poly-tunnel. The day before seeding, plots in each poly-tunnel (24 plots) were cultivated by hand to a depth of 6 cm using a wide rake. Plants were sown in six plots (0.80 × 0.80 m) per sister line by hand to a density of 150 plants m-2 on 18 May 2011. The equivalent of 60 kg N ha–1 as urea, 75 kg P ha–1 as amended superphosphate (with Cu, Zn, Mo, S) and 55 kg K ha–1 as KCl was buried with the seed. A top-dressing application of 33 kg N, 38 kg P and 28 kg K ha–1 was made when plants were at the 3–4 leaf stage (Z13–Z14; Zadoks scale of growth) (Zadoks et al. 1974). An additional
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application of N as urea was made by the end of stem elongation (Z39) at a rate of 50 kg N ha–1.
Poly-tunnels Details of the poly-tunnels have been described in Dias de Oliveira et al. (2013). Briefly, each poly-tunnel, 10m long × 2.5m wide × 2.75m tall, consisted of a steel frame covered with a double sheet of F-clean greenhouse film (200 mm) (AGC Chemicals Americas, Inc. Exton, PA, USA). This film allows 96% light transmission and is impermeable to CO2, and inflation between the double walls insulates against external temperature. Air flow through each tunnel was provided by a CPD 0454 FHP multi-speed fan (Fantech, Melbourne, Vic., Australia) capable of moving 1333 L s–1 of air. Each fan moves the outside air through a cardboard radiator window mounted in the opposite wall of the tunnel. The fan speed varied continuously with an ACS150 Drive (ABB Inc., New Berlin, WI, US) to maintain a set temperature inside the tunnel, which was monitored with a Techni-temp Resistant Thermometer Detector (Technitemp, Kewdale, WA, Australia; model TWA 27708) at the end of the tunnel. Certified CO2 gas was pulsed from a gas vessel (BOC Special Gases, Chatswood, NSW, Australia) through a 1.5m × 15mm diameter plastic hose connected onto a solenoid valve into the inlet stream of each tunnel. The last meter of hose was finely perforated to distribute the pulse of CO2 evenly into the inlet stream. The rate of the pulse of the solenoid valve was determined by measuring the CO2 level in the outlet stream of the tunnel, using a GAS-CO2-002-K infrared gas analyser (Gas Alarm System, Sydney, NSW, Australia). The CO2 concentration inside the tunnels was maintained at 700 mL L–1 with no discernible gradients of CO2 or temperature along the tunnels at any time. The system was automatically regulated so that when CO2 uptake by plants increased at high irradiance, the
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(α=0.05); n=3 replicates; ns (non-significant). ACO2, ambient CO2; ECO2, elevated CO2; AT, ambient temperature; HT, +3oC above ambient temperature
Source
of Ears m–2
Grains ear–1 Grain dry weight (mg)
ACO2
392.7
36.1
44.0
ECO2
403.4
44.5
43.3
AT
382.5
41.5
43.8
HT
413.6
39.1
43.5
High-vigour
366.1
43.3
43.0
Low-vigour
430.0
37.3
44.2
WW
421.1
39.7
45.8
TD
375.0
40.9
41.5
l.s.d.
35.8
2.6
1.2
variation
WW TD
Interaction
ns
ns
ACO2+AT
45.8
41.4
ACO2+HT
47.7
41.0
ECO2+AT
46.2
41.6
ECO2+HT
43.3
42.0
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watered). Well-watered and droughted plots were separated by a vertical plastic barrier buried to 1.2m in the soil profile to prevent lateral movement of water. The well-watered plots received approximately 450 mm m–2 of water during the whole season.
Sampling and measurements The dates of the developmental stages (phenostages) for anthesis and physiological maturity (flag leaves had turned yellow; Hanft and Wych 1982) were recorded for each plot in each poly-tunnel. Daily observations were made and the phenostage noted when 50% of the plants in each plot had achieved the particular stage. Phenostages were defined using the Zadoks’ scale of cereal development (Zadoks et al. 1974). Comparisons between lines in each pair were made in days after sowing (DAS).
Aboveground biomass was measured by harvesting five plants per plot (30 plants per treatment) at 50% anthesis (Z65). Twenty plants per plot were sampled at final harvest, equating to three replicates per treatment due to the two watering regimes imposed from anthesis. Plants from edge rows were not sampled to avoid edge effects. On each occasion, plants and sampling sites were assigned randomly and plant material was separated into leaves and stems, before being dried in a fan-forced oven at 70°C for at least 48 h, and then weighed. Where appropriate, ears were counted and threshed by hand, and grain was redried and weighed. Grain number and weight was determined for each sample. Green leaf area was measured at 50% anthesis using a Li-Cor LI-3100 leaf area meter (Li-Cor, US). Tiller number was recorded at 50% anthesis and final harvest.
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Details of the measurements of leaf water potential (Ψleaf), leaf net photosynthesis and transpiration rates are described in Dias de Oliveira et al. (2013). Briefly, Ψleaf was measured near midday (1200–1400 hours) on clear sunny days every 5–6 days after anthesis. Ψleaf was measured in three flag leaves per plot (treated as one replicate) that were well illuminated on randomly-selected plants from each plot under each combination of CO2, temperature and watering regime. Leaves were enclosed in a polyethylene bag immediately before cutting (Turner 1988) and Ψleaf was measured using a Scholander pressure chamber (Series 3000, Soil Moisture Equipment Corp,Santa Bárbara, CA, US). Replicate analyses agreed within 0.05 MPa. The net photosynthesis and transpiration rates of four flag leaves per plot (treated as one replicate) were measured using a portable CIRAS-II open gas exchange system (PP Systems, Amesbury, MA, USA). Measurements were made from 50% anthesis at 5–6 day intervals between 1030 and 1330 hours using natural light on cloud-free days, when PPFD was above 900 mmol m–2 s–1, the level at which photosynthesis is saturated in wheat (Austin 1990; Henson et al. 1990). The temperature and CO2 concentration used in the gas exchange measurements were those at which the wheat crops in each poly-tunnel were grown, with the CO2 concentration maintained by the CO2 cartridge. The air flow was 200mL min–1 and vapour pressure deficit remained between 0.7 kPa and 1.2 kPa regardless of the poly-tunnel or day of measurement.
Experimental design and statistical analysis The general design was a randomized complete block design accommodating a four factorial treatment structure. The experimental settings allowed two levels of CO2 and two levels of temperature to be allocated to the four poly-tunnels. Within each tunnel the experimental layout consisted of two columns reflecting each watering regime, terminal drought and well-
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watered. Each column consisted of 12 plots, randomly allocating two sister lines (freetillering and restricted-tillering; high-vigor and low-vigor) to six replicates at anthesis and three at final harvest. There were five repeated measures over time of the response variables of interest.
The statistical models used for the analyses involved ANOVA and regression techniques which accounted for the blocking structure and the above-described treatment structure. The repeated measures data were based on destructive sampling, so there was no need to model the possible correlation of e measurements over time. Moreover, the main interest was to analyze the changes with time in leaf net photosynthesis, Ψleaf , and leaf transpiration rate. The changes were analyzed using an additive-type second-order polynomial model, representing a quadratic curve which showed a very good fit of the data. Thermal time (accumulation of daily temperature averages) was preferred and used as an independent variable because the measurements were made on different calendar days and anthesis was used as 0°C degree day. CO2, temperature, watering regime and sister lines were used as grouping factors. Data were analyzed using GenStat 15th edition (VSN International Ltd, UK). An overall analysis was conducted in an attempt to explain the networks of mechanisms for the responses of grain yield constraints. Prior to that, an exploratory data analysis using MANOVA plotted the response variables of yield components and grain yield to elucidate their correlation.
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Results For some of the measured variable responses, two, three or even four-factor interactions were significant. Main effects of treatments are described only when there was no significant interaction between factors. Each measured response variable is individually discussed below. The exploratory data analysis of plotting the response variables of yield components and grain yield showed a strong multicollinearity for total grain yield and total number of grains for the sister lines contrasting in vigor (Fig. 1supplementary; 1s ). Plotting revealed strong multicollinearity for total grain yield and total number of grains and for ear number and total number of grain, despite the moderate correlation (0.7) for total grain yield and ear number for both lines contrasting in tillering (Fig. 2s ). These correlations were expected from the ANOVA analysis. Results from MANOVA confirmed the significance results from the single analyses (ANOVA). The correlation pattern found above and the presence of only four variables (three yield components plus total grain yield per m2) suggested no need for further PCA (Principal Component Analysis) or Factor Analysis (FA). As a result Fig. 1s and 2s are presented as supplementary figures.
Tunnel houses performance
During the season (June – November 2011) the tunnel houses accurately maintained the temperature and CO2 concentration set for each treatment, simulating some future climate predictions for wheat crops. The CO2 concentration of approximately 700 µL L–1 predicted for the second half of this century (IPCC, 2007) was simulated with a high level of accuracy (Figure 1a). The daytime temperature differences intended between tunnels were attained and maintained throughout the growing season (Figure 1b) based on 30-min average data. During
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the season, each tunnel received 10–12.5 h of daylight, with an average maximum PPFD inside the tunnels of 948 ± 19 mmol m–2 s–1, measured at 1300h. Night temperature in each tunnel was passive, so the average minimum temperature in each tunnel was similar, 6.2 ± 0.4°C from sowing to anthesis and 7.0 ± 0.5°C from anthesis to physiological maturity. The average maximum temperature from sowing to anthesis and from anthesis to physiological maturity, respectively were 21.6 ± 0.2°C and 22.3 ± 0.09°C for ACO2 + AT; 21.8 ± 0.2°C and 22.5 ± 0.1°C for ECO2 + AT; 24.6 ± 0.3°C and 25.4 ± 0.4°C for ACO2 + HT; and 24.5 ± 0.3°C and 25.2 ± 0.3°C for ECO2 + HT.
Phenology Time to anthesis and physiological maturity were affected by different interactions. At anthesis there was a significant interaction of CO2 × temperature in the sister lines contrasting in tillering (P = 0.013) and contrasting in vigorous growth (P = 0.026) (Table 1). In all four lines, elevated CO2 reduced time to anthesis by an average of 2 days under ambient temperature only. High temperature tended to reduce time to anthesis regardless of the CO2 concentration. Time to physiological maturity in lines contrasting in tillering was decreased by 9 days by main effects of terminal drought, 6 days by high temperature and 6 days by elevated CO2 (P
Received Date : 07-May-2014 Revised Date : 24-Aug-2014 Accepted Date : 29-Aug-2014 Article type
: Primary Research Articles
Response of wheat restricted-tillering and vigorous growth traits to variables of climate change
Eduardo A. Dias de Oliveira1,2,3, Kadambot H.M. Siddique2, Helen Bramley2,4, Katia Stefanova2 and Jairo A. Palta1,3
1
*
CSIRO Plant Industry, Private Bag No 5, Wembley, WA 6913 Australia.
2
The UWA Institute of Agriculture (M082), The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia 3 School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia 4 Present address: Plant Breeding Institute, Faculty of Agriculture and Environment, The University of Sydney, 12656 Newell Highway, Narrabri NSW 2390, Australia
*
Corresponding author:
Dr Jairo A. Palta CSIRO Plant Industry Private Bag No. 5, Wembley WA 6913, Australia This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/gcb.12769 This article is protected by copyright. All rights reserved.
Accepted Article
Tel: +61 (0) 8 9333 6611 Fax: +61 (0) 8 9387 8991 Email: [email protected]
Abstract The response of wheat to the variables of climate change includes elevated CO2, high temperature and drought which vary according to the levels of each variable and genotype. Independently, elevated CO2, high temperature and terminal drought affect wheat biomass and grain yield, but the interactive effects of these three variables are not well known. The aim of this study was to determine the effects of elevated CO2 when combined with high temperature and terminal drought on the high-yielding traits of restricted-tillering and vigorous growth. It was hypothezised that elevated CO2 alone, rather than combined with high temperature, ameliorates the effects of terminal drought on wheat biomass and grain yield. It was also hypothesized that wheat genotypes with more sink capacity (e.g. hightillering capacity and leaf area) have more grain yield under combined elevated CO2, high temperature and terminal drought. Two pairs of sister lines with contrasting tillering and vigorous growth were grown in poly-tunnels in a four-factor completely randomized splitplot design with elevated CO2 (700 µL L–1), high day time temperature (3°C above ambient) and drought (induced from anthesis) in all combinations to test whether elevated CO2 ameliorates the effects of high temperature and terminal drought on biomass accumulation and grain yield. For biomass and grain yield, only main effects for climate change variables were significant. Elevated CO2 significantly increased grain yield by 24–35% in all four lines and terminal drought significantly reduced grain yield by 16–17% in all four lines, while high temperature (3°C above the ambient) had no significant effect. A trade-off between yield components limited grain yield in lines with greater sink capacity (free-tillering lines). This
This article is protected by copyright. All rights reserved.
Accepted Article
response suggests that any positive response to predicted changes in climate will not overcome the limitations imposed by the trade-off in yield components.
keywords: high- yielding traits, climate change components, source–sink relationships, isogenic lines, wheat, tunnel houses.
Introduction In Mediterranean-type environments, climate change has a direct impact on wheat production because growth and grain yield will be affected by the predicted increase in temperature and predicted decrease and variability in precipitation during the growing season (Ragab and Prudhomme 2002; Asseng et al., 2004). Total biomass and grain yield in wheat may increase in a climate scenario that combines elevated CO2 and temperatures moderately above the current ambient temperatures, regardless of whether the crop is irrigated or under terminal drought. (Dias de Oliveira et al. 2013). However, large increases in temperature may restrict any positive effects of elevated CO2 on above or belowground biomass and grain yield (Schütz and Fangmeier 2001; Benlloch-Gonzalez et al. 2014). The amplitude of these responses may vary according to genotype growth habit and source–sink capacity (Dias de Oliveira et al. 2013) , since these traits affect carbon assimilation and grain yield components . For instance, if carbon assimilation exceeds sink capacity, respiration may increase (Aranjuelo et al. 2011).
Dias de Oliveira et al. (2013) showed wheat responses to combined elevated CO2 and temperatures +2°C, +4°C and +6°C above ambient, but did not identify the main driver for biomass and grain yield response or explain the interaction between the drivers of climate
This article is protected by copyright. All rights reserved.
Accepted Article
change. Elevated CO2 and high temperature are climate change drivers that, when combined are likely to have an interactive effect on biomass and grain yield (Mitchell et al. 1993; Kimball et al. 1995; Moot et al. 1996; Manderscheid and Weigel 1997; Fangmeier et al. 1999; Manderscheid et al. 2003). The outcome of this interaction may include a reduced positive effect of elevated CO2, amelioration of the effect of high temperature and a synergistic effect where high temperature increases the positive effect of elevated CO2 (Mitchell et al. 1993; Rawson 1995; Batts et al. 1997; Batts et al. 1998; van Oijen et al. 1999; Martinez-Carrasco et al. 2005). The interactive effect may be influenced by genotypic differences in the ability of wheat to adapt to changes in CO2 concentration, temperature and drought as well as the levels of these variables. Reductions in biomass and grain yield caused by terminal drought were partially ameliorated in two wheat genotypes contrasting in vigorous growth when grown under elevated CO2 +2°C above ambient temperature. The amelioration in grain yield resulted from an increase in the number of ears m-2, but it was not possible to determine whether the driver of this response was elevated CO2 or high temperature (Dias de Oliveira et al. 2013).
In genotypes with vigorous growth, phenology can be a major constraint for improving grain yield under elevated CO2 and high temperature. This is because flowering and physiological maturity, are inherently earlier in vigorous than non-vigorous genotypes (Rebetzke and Richards 1999; Botwright et al. 2002) and elevated CO2 and high temperature are likely to accelerate phenology (Rawson 1986; Sadras and Monzon 2006; Asseng et al.2004 ). Acceleration in phenology not only restricts tiller production and hence ear number, but also duration of grain filling (Dias de Oliveira et al. 2013). Tiller production in wheat depends on the amount of radiation that the crop receives over a period of time (Evans 1978; Rawson
This article is protected by copyright. All rights reserved.
Accepted Article
1986) and restricted tiller production reduces sink capacity, photosynthetic enhancement and grain yield (Fischer 2011; Aranjuelo et al. 2011). Therefore one could expect that genotypes with increased tillering and hence more ears per m – 2 would benefit more when grown under elevated CO2.
The aims of this study were (i) to identify the main driver in the response of biomass and grain yield in wheat crops grown under elevated CO2 and high temperature and (ii) determine whether the effects of CO2, temperature and terminal drought had significant interaction when applied at the same time. Two pairs of sister lines contrasting in tillering and vigorous growth were chosen to represent differences in sink strength and source size (Palta et al. 2007; Palta et al. 2011; Benlloch-Gonzalez et al. 2013). The four lines were grown in polytunnels under elevated CO2, high temperature and terminal drought. It was hypothesized that (i) elevated CO2 alone, rather than combined with high temperature, ameliorates the effects of terminal drought on wheat biomass and grain yield and (ii) wheat genotypes with more sink capacity (e.g. high tillering capacity and leaf area) will have more grain yield under combined elevated CO2, high temperature and terminal drought.
Materials and Methods Location A field experiment was conducted between May and November 2011 at The University of Western Australia’s (UWA) Research Station at Shenton Park, Western Australia (31º 57’S, 115º 45’E). The site is located in an area with a long-term average rainfall (80 years) of 710 mm for the May–November growing season. The soil is a free-draining infertile Spearwood
This article is protected by copyright. All rights reserved.
Accepted Article
clayey sand (McArthur and Bettenay 1960) consisting of brown, fine sandy clay with less than 1% organic matter. The pH, measured in a 1:5 suspension of soil in 0.01 M CaCl2 was 5.1–6.8 in the surface 0–10 cm and 4.1–4.5 in lower layers of the soil profile.
Plant material Two pairs of sister lines of wheat (Triticum aestivum L.) representing contrasting genotypes for tillering and vigorous growth were used in this study. The lines for tillering 7750N (free) and 7750PF (reduced) and early vigor CV97 (high) and CV207 (low) were selected by Dr G Rebetzke from CSIRO Plant Industry. The 7750N and 7750PF lines are F5-derived, sister lines for restricted-tillering in a Lang genetic background (Wyalkatchem//3*Silverstar/971/3/Lang). The CV97 and CV207 lines are F2-derived F5:7 lines selected from a cross between the reduced-vigor, Rht8 donor Chuan-mai 18 and a tall vigorous donor Vigour 18. Sister lines were selected with similar Rht8-based dwarf stature but contrasting in leaf areas of leaves 1 and 2 (Benlloch-Gonzalez et al., 2013). The two pairs of sister lines were grown in the field under poly-tunnels in a four-factor completely randomized design with elevated CO2, high temperature and terminal drought in all combinations in a split-plot design within each poly-tunnel. The day before seeding, plots in each poly-tunnel (24 plots) were cultivated by hand to a depth of 6 cm using a wide rake. Plants were sown in six plots (0.80 × 0.80 m) per sister line by hand to a density of 150 plants m-2 on 18 May 2011. The equivalent of 60 kg N ha–1 as urea, 75 kg P ha–1 as amended superphosphate (with Cu, Zn, Mo, S) and 55 kg K ha–1 as KCl was buried with the seed. A top-dressing application of 33 kg N, 38 kg P and 28 kg K ha–1 was made when plants were at the 3–4 leaf stage (Z13–Z14; Zadoks scale of growth) (Zadoks et al. 1974). An additional
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application of N as urea was made by the end of stem elongation (Z39) at a rate of 50 kg N ha–1.
Poly-tunnels Details of the poly-tunnels have been described in Dias de Oliveira et al. (2013). Briefly, each poly-tunnel, 10m long × 2.5m wide × 2.75m tall, consisted of a steel frame covered with a double sheet of F-clean greenhouse film (200 mm) (AGC Chemicals Americas, Inc. Exton, PA, USA). This film allows 96% light transmission and is impermeable to CO2, and inflation between the double walls insulates against external temperature. Air flow through each tunnel was provided by a CPD 0454 FHP multi-speed fan (Fantech, Melbourne, Vic., Australia) capable of moving 1333 L s–1 of air. Each fan moves the outside air through a cardboard radiator window mounted in the opposite wall of the tunnel. The fan speed varied continuously with an ACS150 Drive (ABB Inc., New Berlin, WI, US) to maintain a set temperature inside the tunnel, which was monitored with a Techni-temp Resistant Thermometer Detector (Technitemp, Kewdale, WA, Australia; model TWA 27708) at the end of the tunnel. Certified CO2 gas was pulsed from a gas vessel (BOC Special Gases, Chatswood, NSW, Australia) through a 1.5m × 15mm diameter plastic hose connected onto a solenoid valve into the inlet stream of each tunnel. The last meter of hose was finely perforated to distribute the pulse of CO2 evenly into the inlet stream. The rate of the pulse of the solenoid valve was determined by measuring the CO2 level in the outlet stream of the tunnel, using a GAS-CO2-002-K infrared gas analyser (Gas Alarm System, Sydney, NSW, Australia). The CO2 concentration inside the tunnels was maintained at 700 mL L–1 with no discernible gradients of CO2 or temperature along the tunnels at any time. The system was automatically regulated so that when CO2 uptake by plants increased at high irradiance, the
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(α=0.05); n=3 replicates; ns (non-significant). ACO2, ambient CO2; ECO2, elevated CO2; AT, ambient temperature; HT, +3oC above ambient temperature
Source
of Ears m–2
Grains ear–1 Grain dry weight (mg)
ACO2
392.7
36.1
44.0
ECO2
403.4
44.5
43.3
AT
382.5
41.5
43.8
HT
413.6
39.1
43.5
High-vigour
366.1
43.3
43.0
Low-vigour
430.0
37.3
44.2
WW
421.1
39.7
45.8
TD
375.0
40.9
41.5
l.s.d.
35.8
2.6
1.2
variation
WW TD
Interaction
ns
ns
ACO2+AT
45.8
41.4
ACO2+HT
47.7
41.0
ECO2+AT
46.2
41.6
ECO2+HT
43.3
42.0
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watered). Well-watered and droughted plots were separated by a vertical plastic barrier buried to 1.2m in the soil profile to prevent lateral movement of water. The well-watered plots received approximately 450 mm m–2 of water during the whole season.
Sampling and measurements The dates of the developmental stages (phenostages) for anthesis and physiological maturity (flag leaves had turned yellow; Hanft and Wych 1982) were recorded for each plot in each poly-tunnel. Daily observations were made and the phenostage noted when 50% of the plants in each plot had achieved the particular stage. Phenostages were defined using the Zadoks’ scale of cereal development (Zadoks et al. 1974). Comparisons between lines in each pair were made in days after sowing (DAS).
Aboveground biomass was measured by harvesting five plants per plot (30 plants per treatment) at 50% anthesis (Z65). Twenty plants per plot were sampled at final harvest, equating to three replicates per treatment due to the two watering regimes imposed from anthesis. Plants from edge rows were not sampled to avoid edge effects. On each occasion, plants and sampling sites were assigned randomly and plant material was separated into leaves and stems, before being dried in a fan-forced oven at 70°C for at least 48 h, and then weighed. Where appropriate, ears were counted and threshed by hand, and grain was redried and weighed. Grain number and weight was determined for each sample. Green leaf area was measured at 50% anthesis using a Li-Cor LI-3100 leaf area meter (Li-Cor, US). Tiller number was recorded at 50% anthesis and final harvest.
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Details of the measurements of leaf water potential (Ψleaf), leaf net photosynthesis and transpiration rates are described in Dias de Oliveira et al. (2013). Briefly, Ψleaf was measured near midday (1200–1400 hours) on clear sunny days every 5–6 days after anthesis. Ψleaf was measured in three flag leaves per plot (treated as one replicate) that were well illuminated on randomly-selected plants from each plot under each combination of CO2, temperature and watering regime. Leaves were enclosed in a polyethylene bag immediately before cutting (Turner 1988) and Ψleaf was measured using a Scholander pressure chamber (Series 3000, Soil Moisture Equipment Corp,Santa Bárbara, CA, US). Replicate analyses agreed within 0.05 MPa. The net photosynthesis and transpiration rates of four flag leaves per plot (treated as one replicate) were measured using a portable CIRAS-II open gas exchange system (PP Systems, Amesbury, MA, USA). Measurements were made from 50% anthesis at 5–6 day intervals between 1030 and 1330 hours using natural light on cloud-free days, when PPFD was above 900 mmol m–2 s–1, the level at which photosynthesis is saturated in wheat (Austin 1990; Henson et al. 1990). The temperature and CO2 concentration used in the gas exchange measurements were those at which the wheat crops in each poly-tunnel were grown, with the CO2 concentration maintained by the CO2 cartridge. The air flow was 200mL min–1 and vapour pressure deficit remained between 0.7 kPa and 1.2 kPa regardless of the poly-tunnel or day of measurement.
Experimental design and statistical analysis The general design was a randomized complete block design accommodating a four factorial treatment structure. The experimental settings allowed two levels of CO2 and two levels of temperature to be allocated to the four poly-tunnels. Within each tunnel the experimental layout consisted of two columns reflecting each watering regime, terminal drought and well-
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watered. Each column consisted of 12 plots, randomly allocating two sister lines (freetillering and restricted-tillering; high-vigor and low-vigor) to six replicates at anthesis and three at final harvest. There were five repeated measures over time of the response variables of interest.
The statistical models used for the analyses involved ANOVA and regression techniques which accounted for the blocking structure and the above-described treatment structure. The repeated measures data were based on destructive sampling, so there was no need to model the possible correlation of e measurements over time. Moreover, the main interest was to analyze the changes with time in leaf net photosynthesis, Ψleaf , and leaf transpiration rate. The changes were analyzed using an additive-type second-order polynomial model, representing a quadratic curve which showed a very good fit of the data. Thermal time (accumulation of daily temperature averages) was preferred and used as an independent variable because the measurements were made on different calendar days and anthesis was used as 0°C degree day. CO2, temperature, watering regime and sister lines were used as grouping factors. Data were analyzed using GenStat 15th edition (VSN International Ltd, UK). An overall analysis was conducted in an attempt to explain the networks of mechanisms for the responses of grain yield constraints. Prior to that, an exploratory data analysis using MANOVA plotted the response variables of yield components and grain yield to elucidate their correlation.
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Results For some of the measured variable responses, two, three or even four-factor interactions were significant. Main effects of treatments are described only when there was no significant interaction between factors. Each measured response variable is individually discussed below. The exploratory data analysis of plotting the response variables of yield components and grain yield showed a strong multicollinearity for total grain yield and total number of grains for the sister lines contrasting in vigor (Fig. 1supplementary; 1s ). Plotting revealed strong multicollinearity for total grain yield and total number of grains and for ear number and total number of grain, despite the moderate correlation (0.7) for total grain yield and ear number for both lines contrasting in tillering (Fig. 2s ). These correlations were expected from the ANOVA analysis. Results from MANOVA confirmed the significance results from the single analyses (ANOVA). The correlation pattern found above and the presence of only four variables (three yield components plus total grain yield per m2) suggested no need for further PCA (Principal Component Analysis) or Factor Analysis (FA). As a result Fig. 1s and 2s are presented as supplementary figures.
Tunnel houses performance
During the season (June – November 2011) the tunnel houses accurately maintained the temperature and CO2 concentration set for each treatment, simulating some future climate predictions for wheat crops. The CO2 concentration of approximately 700 µL L–1 predicted for the second half of this century (IPCC, 2007) was simulated with a high level of accuracy (Figure 1a). The daytime temperature differences intended between tunnels were attained and maintained throughout the growing season (Figure 1b) based on 30-min average data. During
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the season, each tunnel received 10–12.5 h of daylight, with an average maximum PPFD inside the tunnels of 948 ± 19 mmol m–2 s–1, measured at 1300h. Night temperature in each tunnel was passive, so the average minimum temperature in each tunnel was similar, 6.2 ± 0.4°C from sowing to anthesis and 7.0 ± 0.5°C from anthesis to physiological maturity. The average maximum temperature from sowing to anthesis and from anthesis to physiological maturity, respectively were 21.6 ± 0.2°C and 22.3 ± 0.09°C for ACO2 + AT; 21.8 ± 0.2°C and 22.5 ± 0.1°C for ECO2 + AT; 24.6 ± 0.3°C and 25.4 ± 0.4°C for ACO2 + HT; and 24.5 ± 0.3°C and 25.2 ± 0.3°C for ECO2 + HT.
Phenology Time to anthesis and physiological maturity were affected by different interactions. At anthesis there was a significant interaction of CO2 × temperature in the sister lines contrasting in tillering (P = 0.013) and contrasting in vigorous growth (P = 0.026) (Table 1). In all four lines, elevated CO2 reduced time to anthesis by an average of 2 days under ambient temperature only. High temperature tended to reduce time to anthesis regardless of the CO2 concentration. Time to physiological maturity in lines contrasting in tillering was decreased by 9 days by main effects of terminal drought, 6 days by high temperature and 6 days by elevated CO2 (P
