Interspecific differences in photosynthesis and stomatal conductance to water stress in C3 and C4 plants of the Gramineae family

Plant photosynthesis is significantly affected by water stress. The rate of photosynthesis is determined by the rate of CO2 diffusion into the leaf through the stomata and the activity of the photosynthetic system in the mesophyll cells. In other words, the amount of photosynthesis exhibited by the leaves of higher plants is a value that is regulated by the stomatal conductance, which is determined by the water conditions inside and outside the plant body at that time, and the activity of the photochemical system and CO2 fixation system in the mesophyll. Therefore, when considering the drought tolerance of plants, it is necessary to consider both the mesophyll photosynthetic activity and the stomatal conductance. In order to analyse the photosynthetic rate of leaves under water stress into the above two aspects, in this study, we used C3 and C4 types of Gramineae plants as materials. First, (1) environmental and plant factors affecting the diurnal variation of stomatal response under the natural growth conditions were investigated. As the solar radiation increases from early morning to mid-morning, the stomatal conductance and transpiration rate increase in C3-type plants, but the conductance decreases rather sharply. At that time, the mesophyll water potential (YL) dropped, and the vessel water potential (YX) dropped even more (??Perhaps, they could not measure mesophyll water potential properly. Terashima). In the case of C4-type plants, stomatal conductance was increased in the morning, but the values were much smaller than those in C3-type plants. Moreover, the difference between YL and YX during the daytime hardly increased. In a nutshell, C4 type plants seem to keep the stomatal opening small to reduce water consumption. In other words, the control mechanism of plant water content by stomatal opening seems to work extremely well in C4 plants. Next, we examined (2) the activity of the photosynthetic system in mesophyll cells. In this study, we employed the liquid phase O2 electrode method to measure the photosynthetic activity of mesophyll. Thus, the activity was not constrained by stomatal conductance. Leaf tissues were dissected and their O2 evolution rate in the liquid was measured. The relationship between the water potential and the photosynthetic activity of the mesophyll tissue was investigated. As YL decreased, the mesophyll photosynthetic rate was decreased. The declining trends differed depending on the subfamily. The drought resistance of the mesophyll tissue as measured by the photosynthetic activity of was strong in the so-called typical C3 plants such as those in the bamboo subfamily (Bambusoideae), the gramineae subfamily (Oryzoideae), the Arundo subfamily (Arundinoideae), and the fescue subfamily (Pooideae), while weak in C4 species such as those in the millet subfamily (Panicoideae), the lovegrass subfamily (Eragrostideae), etc. The results showed that the C4-type plants were more susceptible to low water potential. The present results seem to contradict the hypothesis that C4 plants evolved from C3 plants as a result of adaptation to dry conditions. My discussion will be focused on this point