Abstract
In the previous paper, it was pointed out that the extent of depression in photosynthesis by soil water deficit treatment was different from part to part of the wheat plant. The objective of the present study was to find out the physiological basis for this phenomenon from the viewpoint of water potential and CO2 diffusion resistances, i.e. leaf resistance (r1) and mesophyll resistance (rm) using the same materials as those for the previous paper. The results are summarized as follows : 1. Water potential of each plant part was decreased by water deficit treatment. The extent of decrease in water potential was the same in every plant part. It was concluded form this finding that the difference in photosynthetic sensitivity among plant parts is not be attributable to the difference in the extent of decrease in water potential of plant parts (Figs. 1, 2). 2. Both r1 and rm were increased by the water deficit treatment. The extent of increase in r1 and rm was largest just after the start of the water deficit treatment. However, when the water deficit condition was prolonged, r1 and rm tended to decrease and come close to those of unstressed plants (Figs. 6-9). 3. Both r1 and rm increased to a larger extent in lower leaves than in upper ones by the water deficit treatment (Figs. 3-4). The extent of increase in r1 and rm resulted from water deficit was always larger in lower leaves than in upper ones (Figs. 5-9). Since there was no difference in the extent of decreasse in water potential by the water deficit treatment between lower and upper leaves, both stomata and the photosynthetic mechanism of lower leaves seemed to have greater sensitivity to decrease of water potential than those of upper leaves.
