Carbon Balance of Water‐Deficient Grain Sorghum Plants1

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Experiments were performed to determine how water deficits affect the rates of production and utilization of photosynthetic substrates in whole plants. Substrate utilization was characterized by two parameters which were estimated from an analysis of the C fluxes into and out of plants during a 24‐hour period. These were the efficiency of conversion of substrate used in growth processes to new plant dry‐matter (Yg, growth efficiency), and the daily substrate requirement per unit dry‐matter for maintenance of existing plant material (m, maintenance coefficient).Measurements were made on grain sorghum [Sorghum bicolor (L.) Moench cv. RS610] plants at the eight‐leaf growth stage, in which daytime leaf water potentials ranged from −1.4 to −24.6 bars, and nighttime potentials from −1.0 to −10.9 bars. Carbon dioxide exchange rate (CER) measurements were made in an assimilation chamber at air temperature 30 C, dewpoint 22 C, and photosynthetic photon flux density 1,800 µE.sec‐1.m ‐2. Leaf water potentials were measured regularly. Leaf areas and plant dry weights were measured at the end of each test.Water deficits did not affect Yg, its mean value being 0.71 ± 0.02, comparable to previously reported values for unstressed plants. As water deficits became more severe, m decreased from 60 to 30 mg.g‐1.24hours‐1. The cause of the decrease could not be determined from these experiments.The dry‐matter accumulation rate of the plants was greatly reduced by water deficit. This reduction occurred for two reasons: (i) the leaf area index was reduced, (ii) the substrate production rate per unit of leaf area by photosynthesis decreased.The total C efflux rate from the plants in the dark was decreased by water deficit. This decrease was mainly attributable to the reduced quantity of available substrate, as shown by separating the C efflux into growth and maintenance components. Of the 58% reduction of the C efflux rate, 22% was attributable to the reduction of the maintenance component and 36% was caused by the reduction of the growth component.