Recovery After Water Stress of Leaf Gas Exchange in Panicum maximum var. trichoglume

Abstract

Net photosynthesis of the last fully expanded leaf of P. maximum var. trichoglume was able to recover from leaf water potentials as low as -92 bar. The degree of stress experienced during the single drying cycle did not influence the maximum net photosynthetic rate attained during recovery, but the time taken to reach the maximum increased with the degree of stress experienced. During the first 24 h, the rate of recovery of net photosynthesis was mainly determined by the rate at which the water status improved. Leaves which experienced water potentials less than c. -40 bar had a slower rate of recovery of water potential than less stressed leaves. This was partially offset by higher rates of net photosynthesis. Furthermore, the relationship between leaf water potential and net photosynthesis recorded during the drying cycle was different from those measured during recovery. Thus different relationships must be used in models simulating behaviour during water stress and subsequent recovery.

Stomatal resistance exerted greater control than intracellular resistance over net photosynthesis in the recovery phase, irrespective of the water potential before rewatering or whether plants were preconditioned to stress. Although abscisic acid concentration was positively related to leaf water potential and stomatal resistance during the drying cycle, the relationship between abscisic acid concentration and stomatal resistance during recovery was poor or absent. Sucrose and amino-acid nitrogen accumulated during stress and decreased during recovery. However, the level of non- structural carbohydrates or nitrogen compounds in the recovery phase did not appear to influence net photosynthetic rate or its components. In fact, the reverse appeared to occur: the rate of photosynthesis and growth seemed to determine the levels of these compounds.