Estimation of Photosynthetic Activity from the Electron Transport Rate of Photosystem 2 in a Film-Sealed Leaf of Sweet Potato,Ipomoea batatas Lam.

In order to evaluate the photosynthetic activity of a C₃ leaf from the electron transport rate (ETR) of photosystem 2 (PS2), a new method was devised and examined using leaves of sweet potato. In this method, both surfaces of a leaf were sealed with transparent films to stop the gas exchange between the leaf and the atmosphere; hence the functions of both photosynthetic assimilation (CO₂ uptake) and photorespiration (CO₂ release) are restricted to the inside of the leaf. After both functional rates became equally balanced, ETR of the sealed leaf (ETR_seal) was determined from the chlorophyll fluorescence. The measurements were conducted at different irradiances and leaf temperatures and by using leaves of different age. Under each measurement condition, ETR_seal showed a close positive relationship with the photosynthetic potential, or the gross photosynthetic rate measured in the air of 2 % O₂ (P _G2%) before sealing. ETR_seal may become an indicator to estimate or evaluate the photosynthetic activity of C₃ leaves.