Photosynthetic Light-use Efficiency in Rice (Oryza sativa L.) Leaf under Light with Fluctuating Intensities at Two Different Ambient Humidities

Abstract

Photosynthetic and transpiration rates of rice leaf were measured under light with an intensity fluctuating in a wide range of frequencies (2.8 × 10–4∼4 Hz). The time course of photosynthetic rate differed with the frequency. Under the light with an intensity fluctuating at a frequency of lower than 0.05 Hz, the photosynthetic rate oscillated regularly, and the photosynthetic light-use efficiency (LUE : mean photosynthetic rate under fluctuating light) was constant, independent of the frequency. On the other hand, under the light with an intensity fluctuating at a high frequency ( > 0.05 Hz), the time course of photosynthetic rate was constant without oscillation, and LUE increased with increasing frequency. The effect of the fluctuating light intensity was affected by ambient humidity. LUE was higher at the high ambient humidity, although the difference between the photosynthetic rates at high and low ambient humidities was reduced under the light with an intensity fluctuating at a high frequency (> 2 Hz). The present findings suggests that brief leaf movements by natural factors such as wind (leaf flutter) may provide effective intermittent light penetration to lower leaves, and increase the total photosynthetic rate in the canopy. Rapidly fluctuating light intensity may also alleviate the photosynthetic depression caused by stomatal closure frequently observed in the afternoon (known as midday depression).