Photosynthetic Control Factors in a Single Leaf of Sweet Potato, Ipomoea batatas Lam : 3. Estimation of in vivo rubisco activity from the CO₂ exchange rate of a peeled leaf

Abstract

Rubisco (ribulose 1, 5-bisphosphate carboxylase/oxygenase) activity is the prime determinant for CO₂ exchang rate (CER) at the beginning of the photosynthetic reaction, and hence the rubisco activity in a leaf can be deduced from the initial rise of CER. The activity of rubisco is also regulated by two main internal factors, the activation state of rubisco and the availability of RuBP (ribulose 1, 5-bisphosphate). A peeled leaf (Ipomoea batatas, cv. Koganesengan) without stomatal resistance was preilluminated in PAR of 900 μ mol m^-2 s^-1 at CO₂ concentration ([CO₂]) of 20, 60 or 350) μmol mol^lt;-1>. Directly after the dark interruption following preillumination, the response of CER of a peeled leaf (CER_P1) to reillumination (PAR of 900 μmol m^-2 s^-1) was measured at [CO₂] of 350 μmol mol^-1. The dark time affected the response of CER_P1. The light-activated state of rubisco, which was deduced from the response of the initial rise of CER_P1 (IR-CER_P1), could remain constant for at least 5 min in the dark. The leaves preillluminated at different [CO₂] levels had no or little difference in IR-CER_P1 which was determined after the dark interruption of 2.5 min. This is an evidence that rubisco in the intact mesophyll tissues was substantially light-activated even at such a low [CO₂] level as 20 or 60 μmolmol^-1] and the rubisco activity was primarily determined by the activation state of rubisco but was independent of the RuBP pool level. IR-CER_P1 can be proposed as an indicator for the rubisco activation state in vivo or the rubisco activity unrestricted by RuBP availability.