No evidence of saturation of the xanthophyll cycle pool size vs. integrated quantum flux density relationship in tree canopies

Abstract

The content of xanthophyll cycle carotenoids per unit of total chlorophyll (VAZ/Chl) or per unit of total carotenoids (VAZ/C) generally scales positively with long term quantum flux density (Qint) in plant canopies. However, in natural conditions, the light constantly fluctuates, but the limits and the speed of acclimation responses have not been studied extensively. To determine the extent of foliar adjustment of xanthophyll cycle carotenoid pool size and the kinetic constants of acclimation, we provided extra light (500 µmol m-2 s-1 for 12 h photoperiod) to 24 individual leaves of Tilia cordata Mill. and Populus tremula L. along the light gradient in mature tree canopies (4-25 m). We expected the VAZ/Chl and VAZ/C vs. Qint relationships to saturate in time as the leaves reach a new equilibrium with the changed incident irradiance conditions. The leaves were monitored for 11 days, during which both the epoxidation state of the leaves as well as VAZ/Chl steadily increased. In 11 days, the epoxidation state of the uppermost canopy leaves reached from 50% to 99%, but that of the lower canopy leaves to 75%, indicating a limited potential for acclimation. There were also up to three-fold increases in VAZ/Chl in the upper canopy, but non-significant changes in the lower canopy. When all leaves were examined simultaneously, the kinetic constants of acclimation were directly proportional to the previous leaf light environment. However, there was no evidence of saturation of VAZ/Chl vs. Qint relationships in time, suggesting that the xanthophyll cycle pool did not reach the poise with the changed leaf light environment during 11 days. We conclude that foliar xanthophyll cycle pool size is generally not in steady-state under natural conditions, and that the biochemical acclimation capacity is directly proportional to long-term leaf light environment.