Abstract
In order to clarify the dominant factor responsible for the reduction in photosynthetic rate caused by 1) an increase in leaf-air vapor pressure difference and 2) a decrease in leaf water potential in rice plants, changes of intercellular CO2 concentration were investigated as the leaf-air vapor pressure difference increased and leaf water potential decreased. As the leaf-air vapor pressure difference increascd, intercellular CO2 concentration decrcased (Fig. 1 and 2). As leaf water potential decreased, intercellular CO2 concentration decreased in the leaves with water potential higher than -5-6 bars, and conversely, increased when the water potential declined to lower than -5-6 bars (Fig. 2, 3 and 5). It was concluded that when leaf water potential was higher than -5-6 bars, the dominant factor responsible for the reduction in photosynthetic rate caused either by increasing the leaf air vapor pressure difference or by decreasing leaf water potential was the decrease of CO2 supply through stomata to the mesophyll; whereas at leaf water potential lower than -5-6 bars, the dominant factor was the decrease in mesophyll photosynthetic activity. There was a change in the dominant factor responsible for the reduction of photosynthetic rate as leaf water potential altered. The critical values of leaf water potential changing the dominant factor were the same irrespective of the leaf position on the stem, leaf age, or nitrogen contcnt in the leaf (Table 1). However, a comparison of plants grown in soil and those cultured in solution shows that the critical values of leaf water potential were lower in the soil-grown plants (Fig. 5). The leaf water potential of the soil-grown plants declined slowly by withholding water from the soil, whereas the potential quickly dropped in mannitol-added culture solution. The fact suggested that the decrease in the mesophyll photosynthetic activity caused by a decrease in leaf water potential might be reduced owing to osmotic adjustment.
