光強度と光合成速度、蒸散速度ならびに水利用効率との関係からみたC₃，C₄作物の特性の違い

To make clear the response differences of photosynthesis，transpiration and water use efficiency to light intensity in C₃ and C₄ crops belonging to Gramineae，the CO₂ exchange and transpiration rates were measured with the attached leaves. Also，water use efficiency， leaf conductance and intercellular CO₂ concentration were calculated based on these measured data. The response of photosynhetic and transpiration rates to light intensity were simultaneously measured using the open system assimilation chamber method provided with a by-pass to dehydrate the water transpired by the leaves. In each crop，light intensity was changed in 9 steps from 110 klx to total darkness. During the measuring，temperature and relative humidity in the chamber were maintained at 30±O.5℃ and 64.3±1.24%，respectively. The results obtained were as follows : 1) Light saturation point，maximum photosynthetic rate and initial slope in the photosynthetic light response curve of C₄ crops were higher compared with those of C₃ crops. 2) The response of the transpiration and water use efficiency to light intensity in both crops saturated at 30 Klx and 10 klx，respectively. The transpiration of C₄ crops was three fourths that of C₃ crops. On the other hand，the water use efficiency of C₄ crops was about twice that of C₃ crops. 3) Leaf conductance and intercellular CO₂ concentration of C₃ crops were larger than those of C₄ crops. 4) There were，in the case of the present experiment，high positive correlations between transpiration and photosynthetic rates in each of the crops. 5) The relations of water use efficiency and intercellular CO₂ concentration with leaf conductance in all crops under high light intensity showed a high negative correlation for water use efficiency and a high positive correlation for intercellular CO₂ concentrations. 6) The higher water use efficiency of C₄ crops was mainly caused by restricting the loss of water vapour from leaves with the smaller Ieaf conductance than those of C₃ crops. 7) Under such small Ieaf conductance，it is considered that the C₄ crops maintain their high photosynthetic rate by means of the high carboxylation efficiency and of the large CO₂ gradient (from the atmosphere to the inside leaves).