Photosynthetic Rate in Willow Leaves during Water Stress and Changes in the Chloroplast Ultrastructure, with Special Reference to Crystal Inclusions

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Abstract
The response of CO2 uptake and transpiration of individual leaves to intercellular CO2 concentration was studied in well-watered plants of Salix ‘aquatica gigantea’ and in similar plants in three different water stress treatments. Chloroplast structure was examined by electron microscopy. With decreasing water potentials, the net CO2 uptake decreased at all measured concentrations of intercellular CO2, which was due mainly to increased resistance of the mesophyll to CO2 diffusion. Two days after rewatering, in plants suffering from moderate and severe stress, the CO2 uptake had increased at all concentrations of intercellular CO2, but not to the rate of the well-watered plants. Stomatal resistance had returned to almost the same value as for well-watered controls, but factors causing increased mesophyll resistance reverted more slowly. With electron microscopy, the leaves exposed to water stress showed small changes in the lamellar structure of the chloroplasts. In well-watered plants, the plastoglobuli were small and strongly osmiophilic, but with increasing water stress they became large and less osmiophilic. Large crystal inclusions were observed in the chloroplast stroma in well-watered controls. These crystals were similar to crystals of ribulose 1,5-bisphosphate carboxylase that have been found in chloroplasts. With increasing water stress the number of crystals decreased; in severely stessed plants, the crystals were absent.