A Comparison of the Effects of Chilling on Leaf Gas Exchange in Pea ( Pisum sativum L.) and Cucumber ( Cucumis sativus L.)

Abstract

The effects of chilling on the photosynthesis of a chilling-resistant species, pea (Pisum sativum L. cv Alaska) and a chilling-sensitive species, cucumber (Cucumis sativus L. cv Ashley) were compared in order to determine the differences in the photosynthetic chilling sensitivity of these two species. For these experiments, plants were chilled (5°C) for different lengths of time in the dark or light. Following a 1 hour recovery period at 25°C, photosynthetic activity was measured by gas exchange (C02 uptake and H20 release), quantum yield, and induced chlorophyll fluorescence. The results show that pea photosynthesis was largely unaffected by two consecutive nights of chilling in the dark, or by chilling during a complete light and dark cycle (15 hours/9 hours). Cucumber gas exchange was reduced by one night of chilling, but its quantum yield and variable fluorescence were unaffected by dark chilling. However, chilling cucumber in the light led to reduced C02 fixation, increased internal leaf C02 concentration, decreased quantum yield, and loss of variable fluo-rescence. These results indicate that chilling temperatures in conjunction with light damaged the light reactions of photosynthesis, while chilling in the dark did not. Chilling-induced photosynthetic inhibition appears to be due to direct inhibition of the photosynthetic process in chilling-sensitive plants, and not simply to the limitation of CO2 supply by chilling-decreased stomatal conductance (1, 12). Chilling stress in combination with light is more inhibitory to photosyn-thesis than chilling in the absence of light (8, 11, 15, 16, 19, 20). Further, oxygen is a contributory factor to chilling-induced pho-tosynthetic inhibition in the light, which distinguishes it from the oxygen-independent photoinhibition observed at higher temperatures (14). In this paper, the effects of chilling on net CO2 uptake in pea and cucumber are reported. Although there is a substantial literature concerning the effects ofchilling on net photosynthesis (6), very few reports make direct comparisons of the effects of whole-plant chilling on a chilling-resistant and a chilling-sensitive species. The goal ofthese experiments was to compare photosyn-thetic responses of pea (chilling-resistant) and cucumber (chill-ing-sensitive) to chilling stress in the light and dark. In addition to using net CO2 uptake as a measure of photosynthesis, values for stomatal conductance to H20, photosynthetic quantum yield, and induced Chl fluorescence were obtained for the two species 'Supported by National Science Foundation grants PCM-84049 1 1 to AWN and BSR 83-14925 to the Duke University Phytotron. TX 78713. 3Abbreviation: F,, variable fluorescence. following chilling treatments, thus providing an in-depth analysis of the effects of chilling on various aspects of photosynthesis. The results reported here show that cucumber photosynthesis was inhibited by the imposed chilling stress, while pea photosyn-thesis was unaffected, directly demonstrating the differential sensitivities of these two species to this stress. In addition, our results confirm reports in the literature suggesting that the combination of chilling and light reduce photosynthesis more than dark chilling alone. The results also provide the groundwork at the whole-plant level for further research into the nature of photosynthetic chilling injury (13). MATERIALS AND METHODS Pea (Pisum sativum L. cv Alaska) and cucumber (Cucumis sativus L. cv Ashley) plants were grown from seed in the air-conditioned greenhouses of the Duke University Phytotron (4). Seedlings 10 to 14 d old were placed under control conditions (26°C day/l 8C night, 15 h/9 h light/dark regime) in a growth chamber for several days prior to a chilling treatment. Irradiation was provided by a combination of fluorescent and incandescent lamps and was 350 ,umol m 2s-' at the sample leaf level in the chamber. Plants were watered twice daily throughout the experiment , in the morning with deionized water and in the afternoon with half-strength Hoagland's solution. The dark chilling treatment involved dropping the chamber temperature to 5C during the 9 h dark period. Light and chilling conditions were imposed by maintaining the temperature at 5°C day and night. The chilling treatment was initiated between 1 and 2 h into the light period. Leaftemperature was approximately 10°C in the light when the chamber air temperature was 5oC. Chilled plants were allowed to recover for 1 h at 25°C in the light before gas exchange, quantum yield or induced fluorescence measurements were made. Therefore, the results reported here are concerned with the inhibition of photosynthesis following a chilling treatment. Control measurements were made during the 26°C day/l 8°C night cycle prior to the onset of the chilling treatments. Gas exchange and stomatal conductance measurements were made with a Photosynthetic Analysis and Control System 9900 (Data Design Group, PO Box 3318, La Jolla, CA 92038). All measurements were made at 25°C, 350 gl/L CO2 and 45% relative humidity. Relative humidity was low because the leaf being measured provides all of the humidity in the cuvette in this system. Chilling-stressed cucumber leaves, with reduced stomatal conductance, provided only 45% relative humidity. Light for the measurements was 1000 ,umol m-2s-', and found to be saturating. The most recently fully expanded leaf was used for gas exchange measurements. The leaf area sampled was approximately 20 cm2 for cucumber and 10 cm2 for pea. Leaf internal CO2 concentrations (Ci) during chilling in both the light and dark were calculated from the gas exchange data (18). The results presented for net photosynthesis and gas exchange are 143 https://plantphysiol.org Downloaded on November 29, 2020.-Published by