Photosynthetic light acclimation in fully developed leaves of the juvenile and adult life phases of Hedera helix

The aim of this study was to investigate the extent to which fully developed leaves of Hedera helix L. are capable of acclimating to new light conditions and how this ability is determined by the life phase of the plant. To this end juvenile and adult plants were transferred from a low (L) to a moderately high (H) light regime and vice versa and changes of photosynthetic gas exchange, RuBP carboxylase (EC 4.1.1.39) activity and specific anatomy were monitored in leaves that were fully developed prior to the transfer.
Immediately after transfer from L to H there was a decrease in the rate of net photosynthesis (F_n). This photoinhibition was particularly pronounced in leaves of the adult life phase. F_n recovered after 10 to 20 days at H, and 40 to 65 days after transfer the rate exceeded that of control plants by about 20% in leaves of the adult life phase and by about 50% in leaves of the juvenile life phase. If H plants were transferred to L, F_n had declined only slightly after 30 to 40 days and regained its initial level within a few days, when the plants were returned to the original high light regime.
The increased rates of F_n per unit leaf area in leaves transferred from L to H were associated with higher light levels necessary to saturate F_n, higher carboxylation efficiencies, higher contents of soluble protein and higher activities of RuBP carboxylase, whereas the quantum yield did not change. Although fully differentiated before transfer, the leaves had formed a further cell layer in the palisade parenchyma. Related to leaf volume there was no increase in F_n.
Our results indicate that in the adult life phase of ivy phenotypie light acclimation occurs mainly during leaf development, whereas in juvenile plants fully expanded leaves still possess a rather wide modulativc acclimation plasticity.