Abstract
Transpiration and net photosynthesis of lettuce cultivars ‘Amanda Plus’ and ‘Sucrine’ were measured at decreasing plant water potentials. Whole plants were used in an open system and 6 levels of irradiance were applied. Mesophyll resistance ( rm) and stomatal resistance ( rs) to diffusion of CO 2 were calculated from the gas exchange data. Rates of photosynthesis and transpiration were higher for ‘Sucrine’ than for ‘Amanda Plus’. For all the levels of irradiance, gas exchange rates decreased at plant water potentials below −6.17 and −9.09 bars for ‘Amanda Plus’ and ‘Sucrine’, respectively, the corresponding values for controls being −4.02 and −2.55 bars. Reduction of photosynthesis is thought to have been mainly due to stomatal closure. Water stress had also direct effects on photosynthesis through increases in rm and/or internal CO 2 concentration of leaves. Droughted plants were rewatered and the above measurements were carried out 24 hours after rewatering. Photosynthesis did not recover completely due to aftereffects of stress on the stomates.
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