Photosynthetic Responses of Pisum sativum to an Increase in Irradiance During Growth. I. Photosynthetic Activities

Abstract

The extent and time course of changes in photosynthetic activities of leaves and isolated chloroplasts was followed in pea plants which were adapted to low light (60 µmol photons m^-2 s^-1, 400-700 nm, 16 h light/ 8 h dark cycles), and subsequently transferred to higher light (390 µmol photons m^-2 s^-1). The photosynthetic rates of leaves in CO₂-saturating conditions, measured at light saturation or subsaturation, increased with no noticeable lag, doubling within 1 week after transfer to high light. In contrast, the increase of in vitro ribulose-1,5-bisphosphate carboxylase activity (~ 130%) and photosystem II electron transport capacity (~ 60%) occurred with an apparent lag of - 1 day after transfer to high light. The capacity for uncoupled whole-chain electron transport also increased slowly (~ 70%). Whilst the total chlorophyll (Chl) per unit leaf area remained steady, the Chl a/Chl b ratio increased with no apparent lag phase from 2.7 in low irradiance to 3.2 in high irradiance within 1 week. The results demonstrate that, following an increase of growth irradiance, pea leaves readily increase the capacity for utilising high light effectively, even when the total chlorophyll per unit leaf area remained constant. However, a better understanding of the time course of response requires measurements of other chloroplast parameters.