Photosynthetic carbon assimilation and associated metabolism in relation to water deficits in higher plants

Abstract

Summary: Experimental studies on CO 2 assimilation of mesophytic C3 plants in relation to relative water content (RWC) are discussed. Decreasing RWC slows the actual rate of photosynthetic CO 2 assimilation (A) and decreases the potential rate (A pot ). Generally, as RWC falls from c . 100 to c. 75%, the stomatal conductance (g s ) decreases, and with it A. However, there are two general types of relation of A pot to RWC, which are called Type 1 and Type 2. Type 1 has two main phases. As RWC decreases from 100 to c. 75%, A pot is unaffected, but decreasing stomatal conductance (g s ) results in smaller A, and lower CO 2 concentration inside the leaf (C i ) and in the chloroplast (C c ), the latter falling possibly to the compensation point. Down‐regulation of electron transport occurs by energy quenching mechanisms, and changes in carbohydrate and nitrogen metabolism are considered acclimatory, caused by low C i and reversible by elevated CO 2 . Below 75% RWC, there is metabolic inhibition of A pot , inhibition of A then being partly (but progressively less) reversible by elevated CO 2 ; g s regulates A progressively less, and C i and CO 2 compensation point, Γ rise. It is suggested that this is the true stress phase, where the decrease in A pot is caused by decreased ATP synthesis and a consequent decreased synthesis of RuBP. In the Type 2 response, A pot decreases progressively at RWC 100 to 75%, with A being progressively less restored to the unstressed value by elevated CO 2 . Decreased g s leads to a lower C i and C c but they probably do not reach compensation point: g s becomes progressively less important and metabolic limitations more important as RWC falls. The primary effect of low RWC on A pot is most probably caused by limited RuBP synthesis, as a result of decreased ATP synthesis, either through inhibition of Coupling Factor activity or amount due to increased ion concentration. Carbohydrate synthesis and accumulation decrease. Type 2 response is considered equivalent to Type 1 at RWC below c. 75%, with A pot inhibited by limited ATP and RuBP synthesis, respiratory metabolism dominates and C i and Γ rise. The importance of inhibited ATP synthesis as a primary cause of decreasing A pot is discussed. Factors determining the Type 1 and Type 2 responses are unknown. Electron transport is maintained (but down‐regulated) in Types 1 and 2 over a wide range of RWC, and a large reduced/oxidized adenylate ratio results. Metabolic imbalance results in amino acid accumulation and decreased and altered protein synthesis. These conditions profoundly affect cell functions and ultimately cause cell death. Type 1 and 2 responses may reflect differences in g s and in sensitivity of metabolism to decreasing RWC.