PhotosynthesisResearch28: 99-108, 1991. © 1991KluwerAcademicPublishers.Printedin the Netherlands. Regular paper

CO 2 and water vapour exchange in four alpine herbs at two alti and under varying light and temperature conditions

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A.S. Rawat & A.N. Purohit

High Altitude Plant Physiology Research Centre, H.N.B. Garhwal University, Srinagar Garhwal-246174, U.P. India Accepted in revised form 9 April 1991

Key words: alpine, CO 2 exchange, transpiration, stomatal conductance, water use efficiency, quantum use efficiency, altitude Abstract

CO 2 and water vapour exchange rates of four alpine herbs namely; Rheum emodi, R. moorcroftianum, Megacarpaea polyandra and Rumex nepalensis were studied under field conditions at 3600 m (natural habitat) and 550 m altitudes. The effect of light and temperature on CO 2 and water vapour exchange was studied in the plants grown at lower altitude. In R. moorcroftianum and R. nepalensis, the average photosynthesis rates were found to be about three times higher at 550 m as compared to that under their natural habitat. However, in M. polyandra, the CO 2 exchange rates were two times higher at 3600 m than at 550 m but in R. emodi, there were virtually no differences at the two altitudes. These results indicate the variations in the CO 2 exchange rates are species specific. The change in growth altitude does not affect this process uniformly. The transpiration rates in R. emodi and M. polyandra were found to be very high at 3600 m compared to 550 m and are attributed to overall higher stomatal conductance in plants of these species, grown at higher altitude. The mid-day closure of stomata and therefore, restriction of transpirational losses of water were observed in all the species at 550 m altitude. In addition to the effect of temperature and relative humidity, the data also indicate some endogenous rhythmic control of stomatal conductance. The temperature optima for photosynthesis was close to 30°C in M. polyandra and around 20°C in the rest of the three species. High temperature and high light intensity, as well as low temperature and high light intensity, adversely affect the net rate of photosynthesis in these species. Both light compensation point and dark respiration rate increased with increasing temperature. The effect of light was more prominent on photosynthesis than the effect of temperature, however, on transpiration the effect of temperature was more prominent than the effect of light intensity. No definite trends were found in stomatal conductance with respect to light and temperature. Generally, the stomatal conductance was highest at 20°C. The study reveals that all these species can easily be cultivated at relatively lower altitudes. However, proper agronomical methodology will need to be developed for better yields.

Introduction

Various herbs of medicinal and other economic value growing in the higher reaches of Central

Himalaya are under immense pressure due to uncontrolled extraction and increasing grazing pressure. Some of them have been declared threatened, rare or endangered. Being a chief

100 source of the well-known drug 'Rhubarb' and therefore extracted indiscriminately from the wilds, Rheum emodi and R. moorcroftianum are now facing similar threats. It is in this connection that strategies are being developed for the conservation of Himalayan herbs for which the possibilities are being explored for their cultivation at relatively lower altitudes. Since the ability of a species to acclimate and adapt the contrasting environments is associated with the ability to acclimate at the level of photosynthesis (Pearcy 1977) this study was undertaken to find out the changes in CO 2 exchange as well as water vapour exchange in four alpine herbs in their natural habitat (3600 m) and at lower altitude (550 m). The effect of light and temperature on CO 2 and water vapour exchange was also studied in the plants grown at lower altitude. It is expected that the present study will provide valuable basic information on the mechanism as well as the potential of physiological adjustment to low altitude environments in alpine herbs.

Materials and methods

Gas and water vapour exchange studies in four alpine and sub-alpine species namely; Rheum emodi, R. moorcrofiianum, Megacarpaea polyandra and R. nepalensis were made under field conditions at 3600 m (natural habitat) and 550 m altitudes. The effect of light and temperature on CO 2 and water vapour exchange was studied in the plants grown at lower altitude. For studies under field conditions at Tungnath (3600m), young plants growing in the Alpine Garden were used. Measurements were made in the month of August 1987 when the sky was clear for most of the day and plants were showing active growth. Plants of all four species were also raised from the seeds in December-January 1985-86 at Srinagar (550 m). When these plants were about four months old they were used for gas and water vapour exchange measurements under field conditions at 550 m altitude. Three plants, one mature leaf from each plant, were used throughout the measurements. To study the effect of temperature and light on gas and water vapour exchange, plants were raised from the seeds and were transplanted in

earthen pots. The plants were reared at Srinagar for about 8-9 months. Four plants were used for gas and water vapour exchange measurements between 10 to 40°C with 10°C increments under 0, 160, 320, 480 and 640/xmol m - 2 s -1 irradiance at plant level in a Growth Chamber, Model E-15, (CEL, Canada). Since the steady state photosynthesis reaches within 20 min, the plants were kept for about half an hour under each set of conditions before the observations were recorded. Gas and water vapour exchange rates under all irradiances were first recorded at lowest temperatures (10°C) and subsequently at increasing temperatures. At all the temperatures and under each irradiance, four observations on each species were recorded. Care was taken to avoid excessive build-ups of CO 2 inside the Growth Chamber during measurements. A closed flow Portable Photosynthesis System Model Li-6000, (LI-COR, USA) was used for the measurements. Before recording CO 2 exchange through leaves, the chamber CO z concentration was brought to 310 ppm. The relative humidity was kept nearly constant all through the measurement by regulating the flow rate. The clamped leaf in the chamber receives about 85% of the total light intensity falling on the top of the leaf chamber. To calculate the quantum use efficiency, percentage light absorption by leaves (between 400700 nm) was measured with the help of Li-1800 spectroradiometer (LI-COR, USA). Results

The light and temperature data for the two altitudes during gas and water vapour exchange measurements are shown in Fig. 1. Since most active growth period for these species at Srinagar (550m) was found to be in April and at Tungnath (3600 m) during the month of August, the gas and water vapour exchange rates at these two altitudes were recorded in April and August, respectively. The measurements were made at two hourly intervals from 8 a.m. to 2 p.m.

Gas and water vapour exchange in plants grown at two altitudes The maximum and minimum values of net

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(0) altitudes. The atmospheric pressure at 3600m (Tungnath) and 550 m (Srinagar) was 644 mb and 955 mb, respectively. The ambient CO2 concentration was 240 ppm and 330 ppm at Tungnath and Srinagar, respectively. photosynthesis (Pn), stomatal conductance (g) and transpiration (E) recorded at 550m and 3600m altitudes during the total observation period (8 a.m. to 2 p.m.) are shown in Table 1. Daily means of Pn, g and E are also shown in the table. While in M. polyandra, the mean daily net photosynthetic rate was two times higher at 3600 m as compared to that recorded at 550 m, in case of R. moorcroftianum and R. nepalensis it was about three times higher when the plants were grown at 550 m compared to those grown at 3600 m. In R. emodi, mean daily net photosynthetic rate was almost similar at the two altitudes. It was found that maximum rate of net photosynthesis at both the altitudes occurred in morning hours. In all of them, while the rate was highest around 8 a.m. at 3600 m, at 550 m most of the species showed maximum rate of net photosynthesis occurring late in the morning, around 10a.m. After reaching its maxima in b e f o r e n o o n hours, in all of them the rate of net photosynthesis decreased towards the middle of the day at both the altitudes. Similar to net photosynthetic rate, stomatal conductance in all the species at 550 m was also high in b e f o r e n o o n hours and low during middle

of the day. At 3600 m altitude, however, stomatal conductance reached its maxima around 2 p . m . and was lowest in beforenoon hours in most of the species. While mean daily stomatal conductance in R. emodi and M. polyandra at 3600 m was considerably higher compared to that at 550 m, in R. nepalensis the trend was opposite. In R. moorcroftianum the stomatal conductance in plants grown at the two altitudes was almost identical. In the former two species, mean daily stomatal conductance was considerably higher as compared to R. moorcroftianum and R. nepalensis. It is interesting to note that at 550 m the rate of transpiration was invariably significantly higher during beforenoon hours but low around noon. Contrary to this, in most of the species, highest transpiration rates were recorded during the middle of the day at 3600 m altitude. The trend of the differences in mean daily transpiration rate with respect to altitude are the same as described for stomatal conductance. Leaf and air temperatures recorded simultaneously with gas and water vapour exchange measurements revealed that the leaves of all the species were warmer than the air at both the altitudes. Among them, average leaf-to-air tern-

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103 net photosynthesis was maximum at 30°C, the average net photosynthesis in the rest of the species was maximum at 20°C. In all of them the overall rate of net photosynthesis was minimum at 40°C. Based on the data given in Fig. 2, the light compensation value seems to be very high at 40°C as compared to that at other temperatures in all the four species. In M. polyandra, R. moorcroftianum and R. nepalensis, the light compensation value increased almost linearly with increasing temperature. In two species of

perature differences ranged between 0.45 to 1.37°C at 550m and 1.3 to 3.15°C at 3600m altitude.

Light and temperature interactions Net photosynthesis The effect of light and temperature on net photosynthesis is shown in Fig. 2. In general, the net photosynthetic rate increased with increasing photosynthetically active radiation (PhAR). Except in M. polyandra where the overall rate of 0.8

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the basis of multivariate general linear hypothesis (data not shown), for photosynthesis as a dependent variable on light and temperature revealed that the combined effect of light and temperature on photosynthesis was less than the individual effect of these variables. High temperature and high light intensity, as well as low temperature and high light intensity invariably, adversely affect the CO 2 exchange rate in all the four species.

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nepalensis, on the other hand, photosynthetic light saturation was not seen even up to 640/xmol m -2 s -1 when exposed to 40°C. The dark respiration rate, generally, increased with increasing temperatures in all the species. Multiple regression analysis, worked out on

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Transpiration The rate of transpiration under different light and temperatures (Fig. 3) clearly indicate that transpiration is more dependent on temperature rather than on light intensity. In all the species, the rate of transpiration increased with increasing temperature irrespective of the light intensity.

peratures and light intensities, the water use efficiency (WUE) was worked out for all the four species. The results are shown in Fig. 4. It was found that the water use efficiency decreases with increasing temperature and increases with light intensity in all the four species. The minimum water use efficiency was recorded in M. polyandra, whereas R. emodi showed maximum water use efficiency irrespective of the temperature and light. At higher temperatures the effect of light on water use efficiency was less prominent in all the four species as compared to that observed at lower temperatures. Invariably, at 10°C, the water use efficiency decreased beyond 480 p~mol m -2 s ; light intensity.

Water use efficiency Based on the data of transpiration and net photosynthetic rates recorded at different tem-

Quantum use efficiency Micromoles of C O 2 fixed per quantum of light absorbed (QUE), for the four species under

Stomatal conductance No definite trends in stomatal conductance with respect to light and temperature effects were observed in any of the species. In general, at 10°C, stomatal conductance was found to be the lowest and the highest was observed at 20°C.

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different light and temperature conditions are shown in Fig. 5. Invariably, maximum quantum use efficiency was recorded at 20°C and minimum at 40°C. While at 10, 20 and 30°C, in all the four species the quantum use efficiency was maximum at lowest light intensity, at 40°C, excepting in R. moorcroftianum and M. polyandra, in other two species the quantum use efficiency was lowest at lowest light intensity.

Discussion Generally, the variation in the photosynthetic rates reflect adjustments to the respective growth environment and also the resistance to climatic rigors of any kind. The maximum values of net photosynthesis recorded at 3600 and 550 m, see Table 1, clearly bring out the fact that, excepting M. polyandra, the remaining three species show higher maxima in photosynthesis when grown at

550m as compared to the plants grown at 3600 m. Since all the species do not show identical trends at two altitudes, it can be said that the variation in net photosynthetic rates are species specific as well as influenced by the habitat under which a species is grown. Since the rates of photosynthesis in three species were higher at lower elevation, the possibility of their cultivation at lower elevation, from a physiological view point, are evident, However, as rhizome and roots are the only exploitable plant parts in these species, it would be necessary to evaluate the rate of translocation of photosynthates from shoot to the underground parts before the final recommendation can be made in this regard. The maximum net photosynthetic rates at 3600 m altitude occurred around 8 a.m., whereas at 550 m, around 10 a.m. The differences in time for maximum photosynthesis at two altitudes may be attributed to the differences in photosynthetic temperature optima, probably in-

107 fluenced by the prevailing environmental conditions during the growth of the plants at these two altitudes. Moreover, the temperature optima of net photosynthesis for the plants grown at lower altitude (550 m) seems to be higher as compared to the plants growing in their natural habitat at 3600 m. There are reports indicating shifting of the temperature optima for net photosynthesis towards higher temperature when plants of the same species were grown at lower altitude (Mooney and West 1964) and also with respect to prevailing growth temperatures under controlled conditions (Billings and Mooney 1968, Chabot and Chabot 1977). Since there were no significant differences in air temperatures at two altitudes (Fig. lb) during the gas exchange measurements in this study the differences due to environmental factors could be mainly due to differences in PhAR or the ambient CO 2 concentrations at the two altitudes. However, keeping in view the behaviour of M. polyandra it can be concluded that all these species do not react similarly to ambient CO 2 concentration and/or light intensity. The transpiration rates were higher in R. emodi and M. polyandra plants grown at 3600 m which could be attributed to overall higher stomatal conductance at this altitude in these two species. There were no specific changes in transpiration rates of R. moorcroftianum plants grown at two altitudes similar to the stomatal conductance. In young leaves of adult trees of Eucalyptus pauciflora, Korner and Cochrane (1985) found higher leaf diffusive conductance for water vapour and therefore, higher transpiration in leaves of plants from high elevation as compared to those from lower elevation. However, it was shown that sharply decreasing humidity deficit with increasing altitude lessened the expected large differences in transpiration rates per unit leaf area along the altitudinal transact. A depression in stomatal conductance during mid-day (i.e., 12 noon to 2 p.m.) in all species at 550 m could be attributed to the combined effect of high temperature and low relative humidity which might lead to very high evaporative demands during this period. Similar mid-day depressions in leaf conductance have also been reported by Turner et al. (1978) and Bjorkman et al. (1972). However, our data indicate that in

R. emodi and M. polyandra the minimum value of conductance is observed at 12 noon whereas the air temperature increased gradually right from 8 a.m. to 2 p.m. and the relative humidity came to its lowest level around 2p.m. These results suggest that a depression in stomatal conductance at 12 noon might be a phenomenon of endogenous rhythm. Similar indications of endogenous rhythmic control of stomatal conductance were also found in the diurnal gas and water vapour exchange study in these plants (unpublished). Interestingly, despite the relatively higher evaporative demand during mid-day (12 noon to 2p.m.) in all of the species transpiration during these hours was considerably low compared to that in the beforenoon hours (8 a.m. to 10 a.m.). The fact that stomatal conductance in all of them was also lowest during mid-day indicates that stomatal closure at these hours is restricting the transpirational losses of water irrespective of the available temperature and humidity. The temperature optimal for photosynthesis m R. emodi, R. moorcroftianum and R. nepalensis was found to be around 20°C whereas in M. polyandra the optima was close to 30°C. Although relatively high temperature optima for photosynthesis in alpine plants from Europe and North America have been reported (Mooney and Billings 1961, Moser 1970, Korner 1982) these results are in contrast to our observation in Himalayan alpine plants. The differences could be due to the original habitats of the plants or different genetic makeup of the species in these areas. It is worthwhile mentioning that even in a recent publication by Korner and Diemer (1987) there are striking examples of variability in optimum temperatures for photosynthesis in alpine plants from Austrian Alps. The light compensation seems to be influenced by the temperatures to which the plants are exposed, the higher the temperature, the higher becomes the compensation value in all the species. Similar results have also been reported by Scott and Billings (1964) for alpine plants from North America which might reflect either the increase in rate of photorespiration with increasing temperature or decrease in efficiency of carboxylase enzymes at higher temperatures. Invariably, light saturation reaches at relatively low light intensities at low

108 temperatures as well as at high temperatures. However, the behaviour of R. nepalensis at 40°C with respect to light saturation seems to be different than the remaining three species which might reflect for its very high potential of growth even under relatively higher temperatures. Our results on interaction of light and temperature demonstrate that high temperature and high light intensities together adversely affect the CO 2 exchange rate and also low temperature and high light intensities in combination are inhibitory. Similar to the conclusions derived by Korner (1982), based on the coefficients of correlation, we find PhAR is more important than temperature in regulating the photosynthetic rate in these plants. In contrast with the alpine plants from other areas (Mooney et al. 1964), we find maximum percentage of the rate of photosynthesis at optimum temperature, occurring 10°C above the optimum temperature whereas Mooney et al. (1964) have shown maximum percentage 10°C below the optimum temperature. Again this difference could be attributed to the differences in the basic genetic makeup of the plants from different alpine areas. Similar to Scott and Billings (1964), we also find almost linear increase in dark respiration rates with increasing temperature in the Himalayan alpine plants. The data on interaction of light and temperature demonstrate that neither light nor temperature has any significant effect on stomatal conductance. In fact, in all the four species, highest conductance value was found at 20°C. Irrespective of the light intensity, the rate of transpiration increases with increasing temperature and the analysis of the data brings out a highly significant positive correlation between temperature and transpiration ( p = 0.001).

Acknowledgements This work was conducted with the financial assistance from the Department of Environment and Forestry, Govt. of India. The assistance of

Dr M.C. Nautiyal, Mr A.S. Pharswan and Mr J.K. Rawat in conducting these studies is duly acknowledged.

References Billings WD and Mooney (1968) The ecology of arctic and alpine plants. Biol Rev 43:481-529 Bjorkman O, Ludlow MM and Morrow PA (1972) Photosynthetic performance of two rainforest species in their native habitat and analysis of their gas exchange. Carnegie Inst Wash Yb 71/72:94-102 Chabot BF and Chabot JF (1977) Effect of light and temperature on leaf anatomy and photosynthesis in Fragaria vesca. Oecologia (Berlin) 26:363-377 Korner Ch (1982) CO~ exchange in alpine sedge Carex curvula as influenced by canopy structure, light and temperature. Oecologia (Berlin) 53:98-104 Korner Ch and Cochrane PM (1985) Stomatal responses and water relations of Eucalyptus pauciflora in summer along an elevational gradient. Oecologia (Berlin) 66:443-455 Korner Ch and Diemer M (1987) In situ photosynthetic responses to light, temperature and carbon dioxide in herbaceous plants from low and high altitude. Functional Ecology 1:179-194 Mooney HA and Billings WD (1961) Comparative physiological ecology of arctic and alpine populations of Oxyria digyna. Ecol Monog 31:1-29 Mooney HA and West M (1964) Photosynthetic acclimation of plants of diverse origin. Am J Bot 51:825-827 Mooney HA, Wright RD and Strain BE (1964) The gas exchange capacity of plants in relation to vegetation zonation in the White Mountains of California. Am Midl Nat 72:281-297 Moser W (1970) Okophysiologische untersuchungen an Nivalpflanzen. Mitteilungen der Ostalpin-Dinarischen. Geseiischaft fur Vegetationskunde 11:121-134 Pearcy RW (1977) Acclimation of photosynthetic and respiratory carbon dioxide exchange to growth temperature in Atriplex lentiformis (Torr.) Wars. Plant Physiology 59: 795-799 Scott D and Billings WD (1964) Effect of environmental factors on standing crop and productivity of an alpine trundra. Ecol Monog 34:243-270 Turner NC, Begg JE, Rawson HM, English SD and Heard AB (1978) Agronomic and physiological responses of soybean and sorghum crops to water deficits III. Components of leaf water potential, leaf conductance, CO 2 photosynthesis and adaptation to water deficits. Aust J Plant Physiol 5:179-194