Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosynthetic proteins were measured in fourth leaves of Lolium temulentum grown until full expansion at 360 μmol quanta m−2 s−1 and subsequently at the same irradiance or shaded to 90 μmol m−2 s−1. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), light-harvesting chlorophyll a/b protein of photosystem II (LHCII), 65 kDa protein of photosystem I (PSI), cytochrome f (Cytf) and coupling factor 1 (CF1) declined steadily in amount throughout senescence in unshaded leaves. In shaded leaves, however, the decrease in LHCII and the 65 kDa protein was delayed until later in senescence whereas the amount of Cyt f protein decreased rapidly following transfer to shade and was lower than that of unshaded leaves at the early and middle stages of senescence. Decreases in the Rubisco and CF1 of shaded leaves occurred at slightly reduced rates compared with unshaded leaves. These results indicate that chloroplast proteins in fully-expanded leaves are controlled individually, in a direction appropriate to acclimate photosynthesis to a given irradiance during senescence