[Assimilation of CO2, NADP and PGA reduction and ATP synthesis in intact leaf cells in relation to water content]

1. Photosynthesis, NADP and PGA reduction and ATP synthesis in strongly dehydrated intact leaves or parts of leaves of fodder beet were investigated. 2. Water stress leads to inhibition of photosynthesis. However, even loss of a major proportion of the water content of cells (up to 80%) does not completely suppress photosynthesis. Under these conditions the rate of photosynthesis is still comparable to that of respiration. Loss of water exceeding about 80% of the total water of leaf cells depresses photosynthesis to a level lower than that of respiration, but respiration is also reduced. Photosynthesis of leaf cells which survived strong dehydration is not restored parallel to rehydration and exhibits a pronounced lag phase in restoration. 3. In vivo in the light, NADP and PGA reduction and ATP synthesis are observed even if up to nearly 85% of the total water content of the leaf cells is removed by wilting. This finding is consistent with the results obtained in vitro with isolated chloroplasts which were dehydrated by exposure to solutions of different tonicity, and it agrees with measurements of photosynthesis in intact leaves. 4. The significance of the results in relation to drought resistance of plants is discussed.