Plant and environmental effects on the stomatal regulation of photosynthesis and transpiration in sorghum

Abstract

The stomatal behaviour of sorghum, and consequent effects on net photosynthesis and transpiration was studied mainly in leaf chambers, using psyohrometers and infra red gas analysis. Particular attention was paid to effects of water supply and light, and some observations were made on effects of night temperatures and wind speed. In some experiments the behaviour of sorghum was compared to that of maize and cotton.The reduction of photosynthesis in consequence of water deficits could be largely attributed to stomatal closure. This was true for leaves which underwent a rapid development of water deficit (2 hours), and from plants which were subjected to 3-7 days of water shortage.The importance of stomatal behaviour in the adaptability of sorghum to moisture deficits is considered in relation to water use efficiency, stomatal reaction to water status, and the cuticular control of water loss. The reaction of whole plants to water deficits during grain filling was also investigated. The high water use efficiency of sorghum is attributed to very low mesophyll resistance, efficient fixation of carbon dioxide and in addition an effective closure of stomata at reduced light intensity which minimizes water loss from leaves with low photosynthesis. The stomata started to close at a very high relative water content (97 percent), corresponding to a leaf water potential of -0.1 bars. The high cuticular resistance (~70 seo cm-1.) was responsible for a reduction of transpiration to 1-2 percent of maximum. Cuticular resistance was greater (~300 seo.cm-1) for leaves of plants which were exposed to cycles of wetting and drying. Both the high sensitivity of stomata to water deficits and the effective cuticular control of water loss are discussed in terms of moisture conservation during periods of limited water supply. Another possible adaptation to conditions of water shortage lies in the capacity of the head to photosynthesise at high rates during periods of moisture shortage, when photosynthesis by the leaves has virtually ceased due to stomatal closure.The stomata of sorghum are very sensitive to low light intensity, closure starting below 9,000 f.c. and becoming more pronounced at less than 3,000 f.c. Increases of photosynthesis with light above 3,000 f.c. are attributed solely to decreases in leaf resistance. At lower light intensities (1,300 f.o.) other factors were involved, but photosynthesisis nevertheless still largely limited by stomatal resistances. The behaviour of sorghum is compared to that of cotton whose photosynthesis was limited by the photochemical processes at intermediate and low light intensities. Variations in net photosynthesis among eight varieties of sorghum at saturating light (11,000 f.c.) and among four varieties at intermediate (5,000 f.c.) and low (1,300 f.c.) light intensities could be largely attributed to variations in leaf resistances. There was a lag in stomatal opening and recovery of photosynthesis on the return of leaves to saturating light after shading at intensities below 3,000 f.c. This lag increased with increasing shade. Shading in the afternoon reduced stomatal apertures and photosynthesis more than in the morning, and time for recovery increased. These results are discussed in term of utilisation of radiation for photosynthesis during periods of intermittent clouds.The effects of wind were investigated in the glasshouse. Winds above 8k.p.h. induced stomatal closure and reduced dry matter production and grain yield. Grain yield of plants exposed to a 10 k.p.h. wind was seventy percent of that of plants at 8k.p.h., and further reduced at 32k.p.h. Continual exposure to wind resulted in marked decreases incuticular resistances.Exposure of whole plants to cold (5oC) or hot (30oC) nights, or exposure of shoots only to a range of low night temperatures (5-15oC) reduced both the rate and extent of stomatal opening during the succeeding day. Photosynthesis was correspondingly reduced. The reduction in the rate of stomatal opening could be attributed to a development of water deficits in the leaf. Stomatal closure after cold nights (7oC) resulted also in a significant reduction in transpiration rates.