Photosynthesis and photorespiratory CO2 evolution of water-stressed sunflower leaves

Abstract

Rates of true photosynthesis (TPS), apparent photosynthesis (APS) and photorespiration (PR) of sunflower (Helianthus annuus L., Var. Mennonite) leaves were measured in air (21% O₂, 300 vpm CO₂) at 25° C and 400 μEinsteins m^-2 s^-1 radiant flux density. The plants were water stressed by application of osmoticum (polyethylene glycol 4000) to the root system. TPS and APS decreased linearly from maxima at-4 bar leaf-water potential (ψ) to become very small and zero respectively at about-18 bar ψ; at smaller potential CO₂ was evolved from the leaf. Statistical analysis shows that TPS and APS were more closely correlated with ψ than stomatal conductance (r _s ^-1), because r _s ^-1 changed only in the range-4 to-13 bar but ψ exerted an effect at smaller potential. Photorespiration decreased linearly with stress and at-18 bar was 30% of the control plant rate; ψ and TPS accounted for only part of the variance in PR, both independently and in combination, and r _s ^-1 accounted for little of the variance. Tricarboxylic acid cycle respiration of leaves placed for 20 min in darkness, remained almost constant with changing ψ and r _s ^-1. It was one-third of photorespiration in control plants but increased as a proportion in severely stressed plants. The relative specific activity (RSA) of the CO₂ released by PR of wellwatered plants was 90% after 20 min photosynthesis in ¹⁴CO₂ but decreased to 18% at-18 bar ψ. Therefore, under stress mpre CO₂ was derived by respiration from reserve materials and less from immediate photosynthate. Elimination of CO₂ production by the glycollate pathway with small oxygen concentration (1.5%), showed that the contribution of TCA cycle respiration to photorespiration was small in unstressed plants but increased at small ψ to almost the same rate as photorespiration. It is concluded that desiccation decreased photosynthesis by decreasing the stomatal conductance to CO₂ diffusion and by changing the balance between CO₂ assimilation and production of the leaf. As a consequence carbon flux through the glycollate pathway decreased as did the rate of CO₂ produced by it. However, TCA cycle respiration in the light increased with stress, so that total photorespiration remained large. The importance of maintaining carbon flux through the glycollate pathway and TCA cycle is discussed.