Plant Physiology and Biochemistry 73 (2013) 359e367
Contents lists available at ScienceDirect
Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy
Research article
Down-regulation of photosynthesis following girdling, but contrasting effects on fruit set and retention, in two sweet cherry cultivars A.G. Quentin a, b, *, D.C. Close a, L.M.H.P. Hennen a, c, E.A. Pinkard b a
Perennial Horticulture Centre, Tasmanian Institute of Agriculture, University of Tasmania, Private Bag 54, Hobart, Tasmania 7001 Australia CSIRO Ecosystem Sciences, Private Bag 12, Hobart 7001, Australia c University of Applied Science, HAS Den Bosch, 5223 DE’s-Hertogenbosch, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2013 Accepted 10 October 2013 Available online 24 October 2013
Sweet cherry (Prunus avium) trees were manipulated to analyse the contribution of soluble sugars to sink feedback down-regulation of leaf net CO2 assimilation rate (Anet) and fruit set and quality attributes. Total soluble sugar concentration and Anet were measured in the morning on fully expanded leaves of girdled branches in two sweet cherry cultivars, ‘Kordia’ and ‘Sylvia’ characterised typically by low and high crop load, respectively. Leaves on girdled trees had higher soluble sugar concentrations and reduced Anet than leaves on non-girdled trees. Moreover, RuBP carboxylation capacity of Rubisco (Vcmax) and triosephosphate utilisation (TPU) were repressed in the girdled treatments, despite Jmax remaining unchanged; suggesting an impairment of photosynthetic capacity in response to the girdling treatment. Leaf Anet was negatively correlated to soluble sugars, suggesting a sink feedback regulatory control of photosynthesis. Although there were significantly less fruit set and retained in ‘Kordia’ than ‘Sylvia’; girdling had contrasting effects in each cultivar. Girdling significantly increased fruit set and fruitlet retention in ‘Sylvia’ cultivar, but had no effect in ‘Kordia’ cultivar. We propose that low inherent sink demand for photoassimilates of ‘Kordia’ fruit could have contributed to the low fruit retention rate, since both non-girdled and girdled trees exhibited similar retention rate and that increases in foliar carbohydrates was observed above the girdle. In ‘Sylvia’ cultivar, the carbohydrate status may be a limiting factor for ‘Sylvia’ fruit, since girdling improved both fruit set and retention, and leaf soluble solids accumulation. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: Prunus avium Photosynthetic capacity Soluble sugars Sink feedback Fruit set and retention Sink demand
1. Introduction In order to intensify sweet cherry production, a good knowledge of the physiological aspects related to fruit set and development is required. In Australia, blooming, fruit set, fruit growth, and ripening of cherry fruit occur from September to JanuaryeFebruary on shoots that grew the previous year. The supply of adequate carbohydrate to fruit is crucial for fruit set, fruit growth, ripening, and final fruit quality (colour, size, firmness). Initial bloom and early fruit growth is dependent on re-mobilised carbohydrate reserves [1], accumulated by the tree during winter [2] when the sink demand exceeds the capacity of the tree to synthesise carbohydrates [3]. Subsequently developing fruit, compete with shoot growth for newly assimilated and accumulated carbohydrates. In sweet cherry, most assimilates are supplied by leaves on the same shoot where
* Corresponding author. CSIRO Ecosystem Sciences, Private Bag 12, Hobart 7001, Australia. Tel.: þ61 3 6237 5689; fax: þ61 3 62375601. E-mail address: [email protected] (A.G. Quentin). 0981-9428/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.plaphy.2013.10.014
the fruit is attached [4]. Sourceesink modification can enable a better understanding of the mechanisms controlling photosynthesis and dry-matter production and partitioning. In fruit trees such as apple [5], peach [6] and coffee [7,8], fruit load has a large effect on dry matter production and partitioning on vegetative and reproductive organ [9]. For instance, increasing fruit load in apple trees led to increases in dry matter production per unit leaf area and partitioning to fruit, but reduced fruit size, percentage fruit dry matter, dry matter partitioning to new shoot growth, thickening of existing woody tissue and root growth [5]. The partitioning of dry matter among various groups of organs depends on the number of organs per group and on their sink strength, i.e. their competitive ability to attract assimilates [9]. At high fruit loads, there is competition for carbohydrates among fruits that strongly affects fruit size and quality [8]. Consistency of yield is a major problem in fruit production in Prunus, including sweet cherry (Prunus avium). Abscission of fruitlets is a factor that can significantly reduce yield. The causes are unclear, but environmental influences, on photoassimilate production, and limited availability of stored carbohydrates are likely
360
A.G. Quentin et al. / Plant Physiology and Biochemistry 73 (2013) 359e367
causal factors [10]. Sourceesink manipulations are commonly used to optimise yield and quality of perennial horticultural crops [11e 13]. Branch girdling is traditionally used to investigate the effect of assimilate partitioning between the tree and individual branches and evaluate how carbohydrate transfer can be altered and affect tree performance [14]. Girdling often promotes flower-bud initiation, fruit set and growth, and increases yield [15e19], but reduces vegetative growth [8,14,20] and main stem girdling is often employed in commercial practice because of these benefits. Branch girdling has been reported to alter leaf characteristics by increasing leaf mass ratio and decreasing leaf net photosynthesis (Anet) of numerous fruit trees [5,20e30] and accumulation of carbohydrates [7,15,18,23,28,31]. Numerous studies have reported inhibitory feedback of leaf photosynthesis in response to the accumulation of leaf carbohydrate reserves [7,23,28,31]; however, the mechanisms whereby Anet is decreased following carbohydrate accumulation are not well understood. Accumulation of starch in the chloroplasts has been shown to lead to sharp decrease in Anet due to disruption of the thylakoid membranes [32]. However, the sink limitation effect on Anet due to an accumulation of end products is not universally observed. It has also been demonstrated that some photosynthetic genes are sugar-repressible [33,34] and that the limiting availability of inorganic phosphate (Pi) can inhibit photosynthetic rates [35,36]. Cheng et al. [37] demonstrated that decreased Anet in sink-limited coffee trees was rather directly related to a lower CO2 availability as a result of lower leaf stomatal conductance (gs) and stomatal closure. In view of the strong sink strength of fruits, especially during maturation, information is needed to evaluate the effect of foliar carbohydrate accumulation on sweet cherry leaf photosynthesis and fruit size. Sweet cherry is unique among temperate deciduous tree fruits in the short time elapsing between bloom and harvest (60e70 days in Southern Australia). Tasmania produces almost 35% of the Australia cherry crop, with exports accounting for over 50% of the national export. Although the cultivars most frequently grown in the region are considered to be well adapted to the local environment, there have been few field studies on carbohydrate requirements, which can impact on horticultural performance. The effects of sink-limitation were evaluated in high- and low-cropload bearing commercial trees of ‘Sylvia’ and ‘Kordia’. The main objective of this research was to determine the influence of decreased sink demand on photosynthesis and foliar carbohydrate production. Girdling of branches offered a convenient system for the study of decreased sink demand as it interrupts the movement of photosynthates produced by leaves through the phloem.
a
b
c
d
2. Results 2.1. Photosynthetic response We evaluated photosynthesis and biochemical parameters to determine if the cultivar and/or girdling treatment had affected overall photosynthetic capacity of fully expanded leaves of sweet cherry trees. Anet, gs, and Ci, showed seasonal variation (P < 0.001; Fig. 1a, b and d), with substantial increase throughout spring (Nov to early Dec), and a decrease in autumn (Mar) as photoperiod and temperature decreased. In contrast, A/gs decreased throughout the pre-harvest period and increased post-harvest (Fig. 1c). Decrease in gas exchange in autumn, or six weeks following the harvest, was larger in the ‘Kordia’ cultivar than in ‘Sylvia’ (P < 0.001; Fig. 1). Branch girdling reduced significantly Anet (10.4%), and gs (21.3%), compared to non-girdled trees irrespective of the cultivar (P < 0.05; Fig. 1a and b). As a result of larger decrease in gs, A/gs was significantly higher in girdled trees (63.1 mmol mol1) than in non-
Fig. 1. Mean (SE) on seasonal variation of net photosynthetic rate (Anet), stomatal conductance (gs), A/gs ratio, and sub-stomatal CO2 concentration (Ci) in leaves of non-girdled and girdled fruiting branches of two sweet cherry cultivars varying in crop load: ‘Kordia’ (low cropping) and ‘Sylvia’ (high cropping) during the season 2011/2012. For each measuring time, points represent the mean of fifteen trees. The arrow indicates the time of the harvest. Gas exchange reduced significantly in response to girdling (P < 0.05); but the effect differed between cultivar with no effect in ‘Sylvia’.
A.G. Quentin et al. / Plant Physiology and Biochemistry 73 (2013) 359e367
girdled trees (55.8 mmol mol1) (P ¼ 0.0013). Although there was no significant interaction between cultivar and treatment (P > 0.05), the effects of girdling on Anet, gs, and A/gs were stronger in ‘Kordia’ (11.5, 27.7, and 15.8%) than in ‘Sylvia’ (9.2, 14.8, and 10.6%), during the pre-harvest period. In non-girdled branches, no differences between cultivars were observed in Anet, gs, A/gs, and Ci (P > 0.05), except on the last measurement postharvest (P < 0.001; Fig. 1). Although there were no significant interactions between time and cultivar and/or treatment pre-harvest (P > 0.05), substantial decrease in Anet, gs, and Ci was observed due to girdling in ‘Kordia’ two weeks after the treatment application, while there was no differences in ‘Sylvia’ cultivar. Potential rates of biochemical reactions of photosynthesis were significantly different between cultivars (Table 1; Fig. 2). Vcmax, Jmax, TPU and G* were significantly lower in the ‘Sylvia’ cultivar than in the ‘Kordia’ cultivar, but J/Vcmax were higher in ‘Sylvia’ cultivar than in the ‘Kordia’ cultivar (Table 1). There were no significant differences in Rd, gm, and Ls between both cultivars (Table 1). The maximum carboxylation rate of RuBisCo decreased marginally in leaves of girdled branches compared with non-girdled branches (43.1 and 33.1 mmol m2 s1, respectively) (P ¼ 0.095), but no changes were observed in the Jmax (99.1 and 98.9 mmol m2 s1, respectively). 2.2. Effects on foliar carbohydrate concentrations Concentration of NSC, calculated on dried weight (DW) basis, of fully expanded young leaves was determined from week two after the girdling treatment application (Fig. 3), and varied significantly throughout the measuring period (P < 0.001; Table 2). The data indicated that the foliar NSC concentration levels were higher early spring and then decreased significantly throughout fruit growth (P < 0.001; Fig. 3). Foliar starch concentration, as a measure of glucose released, remained low throughout the experiment (data not shown) and represented less than 5% of TNC, while the other sugars ranged between 5 and 55% of TNC. Sorbitol was the major sugar (data not shown). Girdling on fruiting branches caused significant accumulation of NSC in the foliage of both cultivars (P < 0.05), except for sucrose and starch (Table 2). Accumulation was mostly observed early after the treatment application (two weeks) (P < 0.001; Fig. 3). However, accumulation in fructose and glucose was very transient as the effect of girdling disappeared by week four onwards (Fig. 3a and b),
Table 1 Characteristics of photosynthetic capacity of leaves from non-girdled and girdled fruiting branches of two sweet cherry cultivars varying in crop load: “Kordia” (low cropping) and “Sylvia” (high cropping) during the vegetative season 2011/2012: the maximal rate of carboxylation (Vcmax, mmolCO2 m2 s1), the light-saturated rate of electron transport (Jmax, mmol m2 s1), triose phosphate utilisation (TPU), the rate of CO2 evolution in the light resulting from processes other than photorespiration (Rd, mmol m2 s1), mesophyll conductance (gm, mmol m2 s1), photocompensation point in the absence of non-photorespiratory CO2 evolution (G*), relative stomatal limitation (Ls, %), and J/Vcmax. Measured parameters were derived from ACi curves performed on leaves of 10-year-old trees at saturating light (PPFD ¼ 1800 mmol m2 s1) and leaf temperature ¼ 20 C. Means are presented SE. Kordia Vcmax Jmax TPU Rd gm G* Ls J/Vcmax
48.4 109.1 9.7 0.05 0.041 38.0 16.7 2.35
Sylvia
4.4 5.3 0.9 a 0.36 0.003 1.4 1. 9 0.10
35.1 88.9 8.0 0.13 0.042 35. 8 13.5 2.57
P-value
2.7 5.5 0.7b 0.24 0.009 0.5 1.3 0.05
Contents lists available at ScienceDirect
Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy
Research article
Down-regulation of photosynthesis following girdling, but contrasting effects on fruit set and retention, in two sweet cherry cultivars A.G. Quentin a, b, *, D.C. Close a, L.M.H.P. Hennen a, c, E.A. Pinkard b a
Perennial Horticulture Centre, Tasmanian Institute of Agriculture, University of Tasmania, Private Bag 54, Hobart, Tasmania 7001 Australia CSIRO Ecosystem Sciences, Private Bag 12, Hobart 7001, Australia c University of Applied Science, HAS Den Bosch, 5223 DE’s-Hertogenbosch, The Netherlands b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 July 2013 Accepted 10 October 2013 Available online 24 October 2013
Sweet cherry (Prunus avium) trees were manipulated to analyse the contribution of soluble sugars to sink feedback down-regulation of leaf net CO2 assimilation rate (Anet) and fruit set and quality attributes. Total soluble sugar concentration and Anet were measured in the morning on fully expanded leaves of girdled branches in two sweet cherry cultivars, ‘Kordia’ and ‘Sylvia’ characterised typically by low and high crop load, respectively. Leaves on girdled trees had higher soluble sugar concentrations and reduced Anet than leaves on non-girdled trees. Moreover, RuBP carboxylation capacity of Rubisco (Vcmax) and triosephosphate utilisation (TPU) were repressed in the girdled treatments, despite Jmax remaining unchanged; suggesting an impairment of photosynthetic capacity in response to the girdling treatment. Leaf Anet was negatively correlated to soluble sugars, suggesting a sink feedback regulatory control of photosynthesis. Although there were significantly less fruit set and retained in ‘Kordia’ than ‘Sylvia’; girdling had contrasting effects in each cultivar. Girdling significantly increased fruit set and fruitlet retention in ‘Sylvia’ cultivar, but had no effect in ‘Kordia’ cultivar. We propose that low inherent sink demand for photoassimilates of ‘Kordia’ fruit could have contributed to the low fruit retention rate, since both non-girdled and girdled trees exhibited similar retention rate and that increases in foliar carbohydrates was observed above the girdle. In ‘Sylvia’ cultivar, the carbohydrate status may be a limiting factor for ‘Sylvia’ fruit, since girdling improved both fruit set and retention, and leaf soluble solids accumulation. Ó 2013 Elsevier Masson SAS. All rights reserved.
Keywords: Prunus avium Photosynthetic capacity Soluble sugars Sink feedback Fruit set and retention Sink demand
1. Introduction In order to intensify sweet cherry production, a good knowledge of the physiological aspects related to fruit set and development is required. In Australia, blooming, fruit set, fruit growth, and ripening of cherry fruit occur from September to JanuaryeFebruary on shoots that grew the previous year. The supply of adequate carbohydrate to fruit is crucial for fruit set, fruit growth, ripening, and final fruit quality (colour, size, firmness). Initial bloom and early fruit growth is dependent on re-mobilised carbohydrate reserves [1], accumulated by the tree during winter [2] when the sink demand exceeds the capacity of the tree to synthesise carbohydrates [3]. Subsequently developing fruit, compete with shoot growth for newly assimilated and accumulated carbohydrates. In sweet cherry, most assimilates are supplied by leaves on the same shoot where
* Corresponding author. CSIRO Ecosystem Sciences, Private Bag 12, Hobart 7001, Australia. Tel.: þ61 3 6237 5689; fax: þ61 3 62375601. E-mail address: [email protected] (A.G. Quentin). 0981-9428/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.plaphy.2013.10.014
the fruit is attached [4]. Sourceesink modification can enable a better understanding of the mechanisms controlling photosynthesis and dry-matter production and partitioning. In fruit trees such as apple [5], peach [6] and coffee [7,8], fruit load has a large effect on dry matter production and partitioning on vegetative and reproductive organ [9]. For instance, increasing fruit load in apple trees led to increases in dry matter production per unit leaf area and partitioning to fruit, but reduced fruit size, percentage fruit dry matter, dry matter partitioning to new shoot growth, thickening of existing woody tissue and root growth [5]. The partitioning of dry matter among various groups of organs depends on the number of organs per group and on their sink strength, i.e. their competitive ability to attract assimilates [9]. At high fruit loads, there is competition for carbohydrates among fruits that strongly affects fruit size and quality [8]. Consistency of yield is a major problem in fruit production in Prunus, including sweet cherry (Prunus avium). Abscission of fruitlets is a factor that can significantly reduce yield. The causes are unclear, but environmental influences, on photoassimilate production, and limited availability of stored carbohydrates are likely
360
A.G. Quentin et al. / Plant Physiology and Biochemistry 73 (2013) 359e367
causal factors [10]. Sourceesink manipulations are commonly used to optimise yield and quality of perennial horticultural crops [11e 13]. Branch girdling is traditionally used to investigate the effect of assimilate partitioning between the tree and individual branches and evaluate how carbohydrate transfer can be altered and affect tree performance [14]. Girdling often promotes flower-bud initiation, fruit set and growth, and increases yield [15e19], but reduces vegetative growth [8,14,20] and main stem girdling is often employed in commercial practice because of these benefits. Branch girdling has been reported to alter leaf characteristics by increasing leaf mass ratio and decreasing leaf net photosynthesis (Anet) of numerous fruit trees [5,20e30] and accumulation of carbohydrates [7,15,18,23,28,31]. Numerous studies have reported inhibitory feedback of leaf photosynthesis in response to the accumulation of leaf carbohydrate reserves [7,23,28,31]; however, the mechanisms whereby Anet is decreased following carbohydrate accumulation are not well understood. Accumulation of starch in the chloroplasts has been shown to lead to sharp decrease in Anet due to disruption of the thylakoid membranes [32]. However, the sink limitation effect on Anet due to an accumulation of end products is not universally observed. It has also been demonstrated that some photosynthetic genes are sugar-repressible [33,34] and that the limiting availability of inorganic phosphate (Pi) can inhibit photosynthetic rates [35,36]. Cheng et al. [37] demonstrated that decreased Anet in sink-limited coffee trees was rather directly related to a lower CO2 availability as a result of lower leaf stomatal conductance (gs) and stomatal closure. In view of the strong sink strength of fruits, especially during maturation, information is needed to evaluate the effect of foliar carbohydrate accumulation on sweet cherry leaf photosynthesis and fruit size. Sweet cherry is unique among temperate deciduous tree fruits in the short time elapsing between bloom and harvest (60e70 days in Southern Australia). Tasmania produces almost 35% of the Australia cherry crop, with exports accounting for over 50% of the national export. Although the cultivars most frequently grown in the region are considered to be well adapted to the local environment, there have been few field studies on carbohydrate requirements, which can impact on horticultural performance. The effects of sink-limitation were evaluated in high- and low-cropload bearing commercial trees of ‘Sylvia’ and ‘Kordia’. The main objective of this research was to determine the influence of decreased sink demand on photosynthesis and foliar carbohydrate production. Girdling of branches offered a convenient system for the study of decreased sink demand as it interrupts the movement of photosynthates produced by leaves through the phloem.
a
b
c
d
2. Results 2.1. Photosynthetic response We evaluated photosynthesis and biochemical parameters to determine if the cultivar and/or girdling treatment had affected overall photosynthetic capacity of fully expanded leaves of sweet cherry trees. Anet, gs, and Ci, showed seasonal variation (P < 0.001; Fig. 1a, b and d), with substantial increase throughout spring (Nov to early Dec), and a decrease in autumn (Mar) as photoperiod and temperature decreased. In contrast, A/gs decreased throughout the pre-harvest period and increased post-harvest (Fig. 1c). Decrease in gas exchange in autumn, or six weeks following the harvest, was larger in the ‘Kordia’ cultivar than in ‘Sylvia’ (P < 0.001; Fig. 1). Branch girdling reduced significantly Anet (10.4%), and gs (21.3%), compared to non-girdled trees irrespective of the cultivar (P < 0.05; Fig. 1a and b). As a result of larger decrease in gs, A/gs was significantly higher in girdled trees (63.1 mmol mol1) than in non-
Fig. 1. Mean (SE) on seasonal variation of net photosynthetic rate (Anet), stomatal conductance (gs), A/gs ratio, and sub-stomatal CO2 concentration (Ci) in leaves of non-girdled and girdled fruiting branches of two sweet cherry cultivars varying in crop load: ‘Kordia’ (low cropping) and ‘Sylvia’ (high cropping) during the season 2011/2012. For each measuring time, points represent the mean of fifteen trees. The arrow indicates the time of the harvest. Gas exchange reduced significantly in response to girdling (P < 0.05); but the effect differed between cultivar with no effect in ‘Sylvia’.
A.G. Quentin et al. / Plant Physiology and Biochemistry 73 (2013) 359e367
girdled trees (55.8 mmol mol1) (P ¼ 0.0013). Although there was no significant interaction between cultivar and treatment (P > 0.05), the effects of girdling on Anet, gs, and A/gs were stronger in ‘Kordia’ (11.5, 27.7, and 15.8%) than in ‘Sylvia’ (9.2, 14.8, and 10.6%), during the pre-harvest period. In non-girdled branches, no differences between cultivars were observed in Anet, gs, A/gs, and Ci (P > 0.05), except on the last measurement postharvest (P < 0.001; Fig. 1). Although there were no significant interactions between time and cultivar and/or treatment pre-harvest (P > 0.05), substantial decrease in Anet, gs, and Ci was observed due to girdling in ‘Kordia’ two weeks after the treatment application, while there was no differences in ‘Sylvia’ cultivar. Potential rates of biochemical reactions of photosynthesis were significantly different between cultivars (Table 1; Fig. 2). Vcmax, Jmax, TPU and G* were significantly lower in the ‘Sylvia’ cultivar than in the ‘Kordia’ cultivar, but J/Vcmax were higher in ‘Sylvia’ cultivar than in the ‘Kordia’ cultivar (Table 1). There were no significant differences in Rd, gm, and Ls between both cultivars (Table 1). The maximum carboxylation rate of RuBisCo decreased marginally in leaves of girdled branches compared with non-girdled branches (43.1 and 33.1 mmol m2 s1, respectively) (P ¼ 0.095), but no changes were observed in the Jmax (99.1 and 98.9 mmol m2 s1, respectively). 2.2. Effects on foliar carbohydrate concentrations Concentration of NSC, calculated on dried weight (DW) basis, of fully expanded young leaves was determined from week two after the girdling treatment application (Fig. 3), and varied significantly throughout the measuring period (P < 0.001; Table 2). The data indicated that the foliar NSC concentration levels were higher early spring and then decreased significantly throughout fruit growth (P < 0.001; Fig. 3). Foliar starch concentration, as a measure of glucose released, remained low throughout the experiment (data not shown) and represented less than 5% of TNC, while the other sugars ranged between 5 and 55% of TNC. Sorbitol was the major sugar (data not shown). Girdling on fruiting branches caused significant accumulation of NSC in the foliage of both cultivars (P < 0.05), except for sucrose and starch (Table 2). Accumulation was mostly observed early after the treatment application (two weeks) (P < 0.001; Fig. 3). However, accumulation in fructose and glucose was very transient as the effect of girdling disappeared by week four onwards (Fig. 3a and b),
Table 1 Characteristics of photosynthetic capacity of leaves from non-girdled and girdled fruiting branches of two sweet cherry cultivars varying in crop load: “Kordia” (low cropping) and “Sylvia” (high cropping) during the vegetative season 2011/2012: the maximal rate of carboxylation (Vcmax, mmolCO2 m2 s1), the light-saturated rate of electron transport (Jmax, mmol m2 s1), triose phosphate utilisation (TPU), the rate of CO2 evolution in the light resulting from processes other than photorespiration (Rd, mmol m2 s1), mesophyll conductance (gm, mmol m2 s1), photocompensation point in the absence of non-photorespiratory CO2 evolution (G*), relative stomatal limitation (Ls, %), and J/Vcmax. Measured parameters were derived from ACi curves performed on leaves of 10-year-old trees at saturating light (PPFD ¼ 1800 mmol m2 s1) and leaf temperature ¼ 20 C. Means are presented SE. Kordia Vcmax Jmax TPU Rd gm G* Ls J/Vcmax
48.4 109.1 9.7 0.05 0.041 38.0 16.7 2.35
Sylvia
4.4 5.3 0.9 a 0.36 0.003 1.4 1. 9 0.10
35.1 88.9 8.0 0.13 0.042 35. 8 13.5 2.57
P-value
2.7 5.5 0.7b 0.24 0.009 0.5 1.3 0.05
