Effect of Shading on Gas Exchange of Cotton Leaves Under Conditions of Different Soil Water Contents

Pedosphere 10(1): 77~80, 200017ISSN 1002-0160/CN 32- L315/P回2000 SCIENCE PRESS, BEIJING有Efect of Shading on Gas Exchange of Cotton LeavesUnder Conditions of Diferent Soil Water Contents*1LIU XIANZHAO, KANG SHAOZHONG and SHAO MINGANInstitute of Soil and Water Conservation, the Chinese Academy of Sciences and the Ministry of WaterResounces, Yangling 712100 (China)(Received October 25, 1999; revised December 15, 1999)ABSTRACTEffect of diferent shading levels (no shading, 80% shading, and 40% shading) on pbotosynthetic andstomatal responses in cotton leaves were investigated under conditions of different soil water contents insummer midday. All cotton leavea exhibited similar basic responses to shading, including decreased netphotoaynthetic rates, a tendency to decrease in transpiration rates, and increased stomatal conductance andintercellular COz concentration. The leaf conductance of 80% shaded and 40% shaded plants increased by28% and 16.7% compared with po-shaded plants at high water, reepectively, but the net photoeyntheticrates of 80% shaded and 40% shaded plants declined by 50% and 14.73%, respectively. Results showed thatcombined efect of soil moisture and shading on photosynthetic and stomatal responses in coton leavas wasvery remarkable.Key Words: gas exchange, leaf conductance, shading, transpirationPlant growth depends mainly on photosynthetic carbon that is dependent on resourceavailability. Sunlight and water are the most variable and essential resources for plants. Cloudcover and within-canopy shading are two major factors making sunlight intensity variableto individual leaves (Knapp and Fay, 1997). Reduction in solar radiation (e.g., man-madeshading) usually reduces leaf photosynthesis and causes rapid stomatal closure that decreasesleaf transpiration. However, little is known about the effect of reduced solar radiation byshading in summer midday on leaf conductance. The objectives of this work were to study theeffect of shade on cotton leaf photosynthetic rate, stomatal conductance and transpirationrate, etc, under conditions of different soil water contents in summer midday.MATERIALS AND METHODSFive or six cotton seeds were sown in each pot (18 cm in diamneter and 21 cm in depth)after the seeds were surface sterilized in a 980 g kg~ 1 sulfuric acid solution. The pots wereplaced in a greenhouse with temperature of 25/15 °C (day/night), photoperiod of 14/10 h(day/night), relative humidity of 70% and light intensity of 250 pumol m-2 s-1. Each potwas flled with exact 6 kg of a mixture of dry sand (0.45 kg), gravel (0.5 kg), compost (0.05+1 Project (No. 49725102) eupported by the National Outstanding Youth Science Foundation of China.中国煤化工MYHCNMHG78x. Z. LIU et al.kg) and light loam soil (5 kg) with a bulk density of 1.15 g cm-3 and field capacity of 24.3%(by mass). By the time the first pair of permanent leaves had expanded, only one plant perpot was left for uniformity.The experiment was laid out in 3 randomized blocks with the no-shade control (CK) andthe two different shading nets providing a 40% (sparse net, SN) and 80% (dense net, DN)reduction in photosynthetic active radiation (PAR), respectively. Each block included threelevels of water supply, namely 85%~100% of field capacity (high water, HW), 65%~85%of fled capacity (medium water, MW) and 45%~65% of field capacity (low water, LW),respectively. During the experimental period, two different rectangular shade nets (4 m x3m) were implemented by placing horizontally above plants in the open ground. At the sametime, a time-domain reflectometer was used to monitor the soil water content every 3 days.When the water content was going to go out the designed values, watering was carried outto maintain the water supply levels. Three replications were implemented in the experiment.When the seedlings had three true leaves, the initiation of shade treatments began. Thenet photosynthetic rate, transpiration rate, stomatal conductance, leaf temperature, inter-cellular CO2 concentration, air temperature and relative humidity of the uppermost fullyexpanded main-stem leaves were measured between 11:00 and 14:30 of sunny midday witha portable photosynthesis analysis system (CI 301PS, CID, USA). From the young stage tothe bud stage, measurements were carried out once every two days. Data in all figures andtables were means of three replicates and the vertical bars are standard error.RESULTS AND DISCUSSIONEffect of shading on stomatal conductance and photosynthetic rateMidday sunlit stomatal conductance of the uppermost fully expanded main-stem leavesof control plants ranged from 110.7+19.8 to 122.4+16.3 mmol m~ 2 s-1. However, leaf con-ductance under the shade nets was significantly higher in both treatments than in the controlplants. The leaf conductance of densely shaded and sparsely shaded plants increased by 28%and 16.7% compared with unshaded control plants at HW, respectively. Shading had similar200口CK DN■ISN1601204(HWMWWLWTreatmentToatmentFig. 1 Leaf stomatal conductance (Ca) and net photosynthetic rates (Pn) of unshaded plants and shadedplants under diferent soil water content conditions.中国煤化工MYHCNMHGEFFECT OF SHADING ON COTTON LEAF GAS EXCHANGE79effect on leaf conductance under otber soil water treatments (Fig. 1A). However, the netphotosynthetic rate of shaded plants was not increased by the large increases in leaf conduc-tance in the shaded cotton. In contrast, at HW, MW and LW after initiation of shading, thenet photosynthetic rates of densely shaded plants declined by 50%, 30% and 40% comparedwith unshaded plants (Fig. 1B).Effect of shading on tmanspiration rate and interellular CO2 concentrationThe data in Table I showed clearly that the leaves of shaded plants exhibited a higherintercellular CO2 concentrations than the unshaded control plants at all treatments. Thetranspiration rates under three diferent water levels showed a tendency to decrease in shadedplants compared with controlled plants (Table I). Densely shaded leaves and sparsely shadedleaves at HW decreased transpiration rates by 0.3~1.3 mmol m^ 2 s-1 and 0.1~1.3 mmol m^ -2s-', respectively, but transpiration rates under MW and LW were not significantly diferentfrom the control.TABLE 1Efect of shading on Jeaf transpiration rate and intercellular CO2 concentrationWaterTranspiration raleIntercellular CO2 concentrationtreatmentControlDense shadeSparse shademmol m-2 g-1μ CO2 L-1 airHW4.240.7 3.4士0.23.9+1.3 .283.5+18.9 336.1+10.2306.4+29.5MW3.9+1.43.8士1.03.8土0.7273.6士9.4333.0土8.6294.9土29.7LW3.7+1.93.9士1.73.5士1.5282.9士12.4339.9士16.0310.5士18.6Our results indicated that leaves of shaded plants had a higher stomatal conductance,higher iterellular CO2 concentrations and lower photosynthetic rates than those of thecontrol plants, the infuence of shading on leaf transpiration rate was very small. Thisdemonstrated that the stomatal conductance and intercellular CO2 concentration were notthe major factors causing the decrease in leaf photosynthetic rate of shaded plants. Similarobservations have been reported by Pasternak and Wilson (1973) who indicated that at lowlight Aux, the decrease in net pbotosynthetic rate appeared to depend mainly on factorsother than stomatal closure. The reason for this was the low PAR decreased photosyntheticelectron transport as well as the activity of Calvin cycle enzymes (Crookston et al, 1975; Sageand Reid, 1994), resulting in low leaf carbon assimilation. A recent work that large changesin leaf conductance had been shown to have no efect on photosynthetic rate is anotberpiece of evidence (Jones, 1998), which shows that the dynamic changes in photosyntheticrate response to changes in irradiance. Leaf conductance of cotton in summer is generallytypifed by low midday values due to stomatal closure. There are contradictory. findingsthat under very short-term (9 min) shade conditions, leaf conductance of field-grown cottonwas reduced by 35% to 42% (Bauer et al, 1997). On the other hand, average middayleaf conductance in the field conditions is significantly negatively correlated with leaf-to-airvapor pressure difference (VPD) (Cohen and Cohen, 1983). The different results may be中国煤化工MYHCNMHGX. Z. LIU et al.In this study, several possible environmental infuences on leaf conductance can account for itsincrease in leaves of shaded cotton. Reduced leaf temperatures and leaf-to air vapor pressurediferences (Table II) are two examples. High VPD often results in stomatal closure (Marocoet al, 1997). The negative infuence of VPD on stomatal opening has been found in allcitrus cultivars (Syvertsen and Lloyd, 1994). Photoinhibition in leaves of unshaded cottonat high irradiance would increase internal CO2 concentrations known to cause stomatalclosure, but internal CO2 concentrations of control plants were lower than those of shadedplants in this experiment. It is therefore unlikely that internal CO2 concentrations increaseenough to account for the changes in leaf conductance. Previous studies have demonstratedthat leaf conductance response to VPD results from the adjustment of stomatal aperture inorder to maintain constant leaf transpiration at diferent values of VPD. This relationshipis considered to be responsible for the insensitivity of cotton leaf transpiration to differentenvironments.TABLE IEffect of shading on leaf temperatures and leaf-to-air vapor pressure diferenceWaterLeaf temperatureLeaf-to air vapor pressure differencetreatmentControlDense shadeSparse shade_。. MPaHW34.3+2.7 31.5士2.231.0+3.23.5+0.52.6+0.52.6+0.5 .MW34.7士3.1 31.3+2.031.3+3.33.7土0.72.6士0.92.7土0.5LW36.1+2.4 30.9+2.431.1+2.94.0土0.32.8士0.62.7士0.4REFERENCESBauer, P. J, Sadler, E. J. and Prederick, J. R. 1997. Interittent shade on gas exchange of cotton leaves inthe humid Southeastern USA. Agron. J. 89: 163~166.Cohen, s. and Cohen, Y. 1983. Field studies of leaf conductance response to environmental variables incitrus. Joumnal of Applied Ecology. 20: 561~570.Crookston, R. K., Trebarne, K. J. and Ozbun, J. L. 1975. Response of beans to shading. Crop Sci 15:412~416.Jones, G, 1998. Stomatal control of photosynthesis and transpiration. J. Exrp. Bot. 49: 387~398.Knapp, A. K. and Fay, F. A.1997. Plant strategies for coping with variable light regimes. In Basra, A. s.and Basra, R. K. (eds.) Mechanisms of Environmental Stress Resistance in Plants. Harwood Academnic,Amsterdam. pp. 191~212.Maroco, J. P, Perrier, J. s. and Chaves, M. M. 1997. Stomatal reeponses to leaf-to air vapor pressure defcitin Sahelian species. Australian Jourmal of Plant Physiology. 24: 381~387.Pasteroak, D. and Wilson, G. L. 1973. Iluminance, stomatal opening and photoeyntheis in sorghum andcotton. Aust. J. Agric. Res. 24: 527~532.jage, R. F. and Reid, C. D. 1994. Photosynthetic response to mechanisms to environment change in C3plants. In Wilkinson, R. E. (ed.) Plant-Environment Interaction. Dekker, New York. pp. 413~419.Syvertsen, J. P. and Lloyd, J. J. 1994. Citrus. In Schaffer, B. and Andersen, P. C. (eds.) Handbook ofEnvironmental Physiology of Fruit Crope. CRC Pres, Boca Raton. pp. 65~99.中国煤化工MYHCNMHG