Carbon Balance of Sorghum Plants during Osmotic Adjustment to Water Stress

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Abstract
The daily (24-h) C balances of whole sorghum plants were continuously measured throughout 15 days of water stress, followed by rewatering and 4 more days of measurements. The plants were grown under controlled environment conditions typical of warm, humid, sunny days. During the first 12 days, osmotic potentials decreased in parallel with decreased water potentials to maintain pressure potentials near 0.5 kJ/kg (5 bars). Immediately before rewatering on day 15, the water potential was -3.0 kJ/kg. Osmotic adjustment at this point was 1.0 kJ/kg, as measured by the decrease in the water potential at zero turgor from its initial value of -1.4 kJ/kg. Gross input of C was less but the fraction retained was greater because a smaller fraction was lost through respiration in stressed plants than in unstressed plants. This was attributed to a lower rate of biomass synthesis, and conversely a higher rate of storage of photosynthate, due to inhibition of leaf expansion. The reduction in the cost associated with biomass synthesis more than balanced any metabolic cost of osmotic adjustment. The net daily gain of C was always positive in the stressed plants. There was a large burst of respiration of rewatering, due to renewed synthesis of biomass from stored photosynthate. Over the next 3 days, osmotic adjustment was lost and the daily C balance returned to that typical of nonstressed plants. Osmotic adjustment allowed the stressed plants to accumulate biomass C throughout the cycle, with little additional metabolic cost. C stored during stress was immediately available for biomass synthesis on rewatering.