Varietal differences in photosynthetic rate, transpiration rate and leaf conductance for leaves of rice plants.

Abstract

In order to examinc the varietal differences in photosynthetic rate, transpiration rate and leaf conductance for leaves of rice plants, rates of photosysthesis and transpiration of fifty cultivars as shown in Table 1 were simultaneously measured under the controlled conditions : 1800 μmol·m-2·s-1 light intensity, 30.4°C leaf temperature, 14.5 mbar vapour pressure difference and 340 μl·l-1 CO2 concentration. The measurements were performed three times during the period from maximum tiller number stage to panicle heading stage. We designate these as periods I, II, and III with period I being the maximum tiller number stage, period II being midway between periods I and III, and period III being the heading stage. From the data obtained in this experiment, water use elficiency, leaf conductance, mesophyll conductance and intercellular CO2concentration (Ci) were calculated. In addition, chlorophyll content, nitrogen content and specific leaf area (SLA) were measured. Using these parameters, the causes of differences in photosynthesis for the different rice cultivars w'ere analyzed in relation to the diffusion pathway of carbon dioxide from the atmosphere to the chloroplasts and to its biochemical activity. The results obtained were as follows : 1. The maximum rate of photosynthesis was 51 mgCO2·dm-2·hr-1 in 'Century Patna 231' and the minimum rate was 22 mgCO2·dm-2·hr-1 in 'Senbon asahi' rice cultivars. The mean value for fifty rice cultivars was 39.8, 36.2 and 33.1 mgCO2·dm-2·hr-1 for periods I, II and III, respectively. The coefficients of variation for each of these periods was 12.1, 11.1 and 13.4%, respectively (Table 1). 2. The transpiration rates lbr the cultivars ranged from 2.21 to 4.88 with an average of 3.70, 3.59 and 3.40 gH2O·dm-2·hr-1 for periods I, II and III, respectively. The cocfficients of variation for each period were 12.6, 15.5 and 16.4%, respectively (Table 1). 3. From the linear regression analysis, there were close correlations between photosynthetic rate and leaf or mesophyll conductance. There were loose correlations, however, between photosynthetic rate and chlorophyll content, nitrogen content, or SLA. Intercellular CO2 concen-tration, which is considered to be onc of the parameters strongly regulating the photosynthetic rate, was not associated with photosynthetic rate in the rice cultivars (Table 2). 4. Using the least squares method, we tried to obtain some information on the relationships between photosynthetic rate and leaf or mesophyll conductance in the rice cultivars. Both of the relationships were approximated by a fburth order polynomial. It appeared from the former relationship that the photosynthetic rate has an optimum leaf conductance ; that is, for values up to 1 cm·s-1 of leLlf conductance, photosynthetic rate increased with increasing in leaf conductance. Beyond tlle value, photosynthetic rate decreased (Fig. 1-A). On the other hand, photosynthetic rate was linealy correlated with mesophyll conductance (Fig. 1-B). Consequently, it is clear that differences in photosynthetic rate among the rice cultivars examined was due to the differences in mesophyll conductance.