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

The response of CO² uptake and transpiration of individual leavesto intercellular CO² concentration was studied in well-wateredplants of Salix ‘aquatica gigantea’ and in similarplants in three different water stress treatments. Chloroplaststructure was examined by electron microscopy. With decreasingwater potentials, the net CO² uptake decreased at all measuredconcentrations of intercellular CO², which was due mainly toincreased resistance of the mesophyll to CO² diffusion. Twodays after rewatering, in plants suffering from moderate andsevere stress, the CO² uptake had increased at all concentrationsof intercellular CO², but not to the rate of the well-wateredplants. Stomatal resistance had returned to almost the samevalue as for well-watered controls, but factors causing increasedmesophyll resistance reverted more slowly. With electron microscopy,the leaves exposed to water stress showed small changes in thelamellar structure of the chloroplasts. In well-watered plants,the plastoglobuli were small and strongly osmiophilic, but withincreasing water stress they became large and less osmiophilic.Large crystal inclusions were observed in the chloroplast stromain well-watered controls. These crystals were similar to crystalsof ribulose 1,5-bisphosphate carboxylase that have been foundin chloroplasts. With increasing water stress the number ofcrystals decreased; in severely stessed plants, the crystalswere absent. Key words: Photosynthetic rate, Chloroplast ultrastructure, Crystal inclusions.