Relationship between Leaf Water Content and Temporary Increase in the Net Photosynthetic Rate Caused by Leaf Excision from Leaf Sheath in the Rice Plant

Abstract

The photosynthetic rate and the water content in the rice leaf of potted plant, cv. Yamabiko, Ochikara and Araki, were measured simultaneously by the assimilation chamber method and by β-ray transmittance of ⁹⁹Tc-source through the leaf blade. During measurement, leaf was excised from its leaf sheath and the measurement was continued until the photosynthetic rate reached a minimum. Within one minute after leaf excision, the photosynthetic rate increased quickly with a concomitant decrease in leaf water. This phenomenon was clearly recognized in the leaf with a lower rate of photosynthesis and higher water content except for leaf showing a higher rate of photosynthesis and yellowy old leaf. It could generally be expressed by following formula. ΔPn=4.950exp (-0.06Pn^*)·ΔI/I₀ r=0.965^** Where, ΔPn is the difference between the rate before excision (Pn^*) and the maximum rate of photosynthesis induced by leaf excision. ΔI/I₀ is the difference in β-ray transmittance between before excision and when the maximum photosynthetic rate was induced by leaf excision. It is considered that photosynthesis of the leaf in which the stomata was insufficiently opened had been depressed by excess leaf water due to intercepting CO₂ exchange between the intercellular space and mesophyll tissue. This postulation was supported by the experimental finding that the temporary increase in photosynthesis did not change with the stomatal aperture, and the air amount in the leaf blade had a negative correlation with leaf water content. Depression of photosynthesis by the excess water content in leaf was recognized with outdooor measurement under conditions at early morning and in the daytime on cloudy days.