Diurnal courses of photosynthesis, transpiration, and diffusive conductance in the single-leaf of the rice plants grown in the paddy field under submerged condition.

Abstract

It had been reported by the authors that leaf stomatal aperture in the rice plant under submerged condition decreased in fine midday with high evaporative demand and that close relation was found between diffusive conductance and photosynthetic rate in rice leaves. From these results it had been predicted that photosynthesis would reach the maximum early in the morning and then decrease gradually toward the afternoon with increase in light intensity and vapour pressure deficit. This study was conducted to ascertain this prediction by measuring the diurnal courses of photosynthetic rate (P), transpiration rate (T), P/T ratio (water use efficiency) and diffusive conductance in the single-leaf of the rice plant in paddy field under submerged condition on fine and cloudy days and to clarify the factors determining the diurnal courses of photosynthesis in a fine day using the simultaneous measurement system of photosynthesis and transpiration developed by KOCH, SCHULTZ and LANGE (Siemens Co. Ltd.). The photosynthetic rate on a fine day increased with increase in light intensity, reached the maximum early in the morning and then gradually decreased down to 75% of the maximum rate toward afternoon even under sufficient light intensity and this was accompanied by a decrease of diffusive conductance (Figs. 2A and 4). The photosynthetic rate in the morning was higher than that in the afternoon under the same light intensity more than 600 μE/m2/sec (Fig. 3A). Transpiration rate increased toward the midday with the increase of light intensity and vapor pressure deficit, and reached the maximum at about 13:00 PM. Water use efficiency was higher early in the morning and late in the evening and lower in the midday (Fig. 2A). The photosynthetic rate, transpiration rate and diffusive conductance on a cloudy day changed according to the change in light intensity (Figs. 2B and 3B). Therefore, light intensity was the dominant factor determining diurnal courses of photosynthetic rate and transpiration rate on a cloudy day. It was found that the time of the daily maximum of diffusive conductance was different from the time of that of photosynthetic rate in their diurnal changes, that is, diffusive conductance had already started to decrease even before the photosynthetic rate reached the maximum early in the morning. Under sufficient light intensity with artificially reflected sunlight by the mirror when diffusive conductance reached the maximum early in the morning, the time when both photosynthetic rate and diffusive conductance reached the maximum coincided. The maximum rater of the photosynthesis was higher than that of the day without the reflected light (Fig. 5). This result suggested that leaf photosynthetic capacity was not always fully realized in diurnal courses of photosynthesis on a fine day. To examine the factor reducing the photosynthetic rate on a fine day, diurnal courses of photosynthetic rate was measured under lower vapor pressure deficit by humidifying air in the chamber. The photosynthetic rate was much higher under lower vapor pressure deficit at least up to noon compared with that under the same vapor pressure deficit as outside (Fig. 6). This fact showed that water stress related to high vapor pressure deficit was the main factor for the decrease of photosynthesis in the midday even though the effects of photosynthate accumulation in the leaf blade and photoinhibition could not be neglected. From these results it was clarified that leaf photosynthetic rate in the rice plants under submerged condition decreased to some extent in the midday on fine days and they could not utilize high solar energy fully for dry matter production. [the rest omitted]