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

Abstract

SUMMARYExperimental 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 Γ Γ 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 Γ Γ 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.

Tolerance to water deficiency between two soybean cultivars: transgenic versus conventional

“…This reduction in ATP synthesis is due to decreased electron transport and photophosphorylation caused by the loss of membrane-associated reactions. The membranes suffer structural damage because of the reduction in free water (LAWLOR & CORNIC, 2002). The results from the electrolyte leakage analysis (Table 1) indicated that the cellular membranes of leaves from conventional cultivars had significant more damage caused by water deficiency than those from the transgenic cultivars grown under the same water conditions.…”

Leaf Development Monitoring and Early Detection of Water Deficiency by Low Field Nuclear Magnetic Resonance Relaxation in Nicotiana tabacum Plants

“…At the first stage (42 DAS), plants were stressed for only two weeks and the water stress was mild, and thus no differences in photosynthetic related parameters were detected. However, the stress did have a negative impact on these parameters at 57 DAS (Figure 2) as expected [48,49].…”

Reduced plant water status under sub-ambientpCO2limits plant productivity in the wild progenitors of C3and C4cereals

“…When mild water deficits develop, stomatal closure is one of the first leaf responses, followed by a decline in P i and a downregulation of photosynthetic capacity to match the available carbon substrate (Brodribb, 1996;Chaves et al, 2002;Lawlor and Cornic, 2002). In both C 4 species grown at glacial pCO 2 , A declined alongside changes in g s , suggesting that stomatal closure had brought P i down to the steep initial slope of the A/P i curve, where C 4 photosynthesis becomes highly sensitive to changes in pCO 2 Gifford, 1995, 1996).…”