Responses of foliar photosynthetic electron transport, pigment stoichiometry, and stomatal conductance to interacting environmental factors in a mixed species forest canopy

We studied limitations caused by variations in leaf temperature and soil wa ter availability on photosynthetic electron transport rates calculated from foliar chlorophyll fluorescence analysis (theta) in a natural deciduous fo rest canopy composed of shade-intolerant Populus tremula L. and shade-toler ant Tilia cordata Mill. In both species, there was a positive linear relati onship between light-saturated theta (theta(max)) per unit leaf area and me an seasonal integrated daily quantum flux density (S-s, mol m(-2) day(-1)). Acclimation of leaf dry mass per area and nitrogen per area to growth irra diance largely accounted for this positive scaling. However, the slopes of the theta(max) versus S-s relationships were greater on days when leaf temp erature was high than on days when leaf temperature was low. Overall, theta (max) varied 2.5-fold across a temperature range of 20-30 degrees C. Maximu m stomatal conductance (G(max)) also scaled positively with S-s. Although G (max) observed during daily time courses, and stomatal conductances during theta(max) measurements declined in response to seasonally decreasing soil water contents, theta(max) was insensitive to prolonged water stress, and w as not strongly correlated with stomatal conductances during its estimation . These results suggest that photorespiration was an important electron sin k when intercellular CO2 concentration was low because of closed stomata. G iven that xanthophyll cycle pool size (VAZ, sum of violaxanthin, antheraxan thin, and zeaxanthin) may play an important role in dissipation of excess e xcitation energy, the response of VAZ to fluctuating light and temperature provided another possible explanation for the stable theta(max). Xanthophyl l cycle carotenoids per total leaf chlorophyll (VAZ/Chl) scaled positively with integrated light and negatively with daily minimum air temperature, wh ereas the correlation between VAZ/Chl and irradiance was best with integrat ed light averaged over 3 days preceding foliar sampling. We conclude that t he potential capacity for electron transport is determined by long-term acc limation of theta to certain canopy light conditions, and that the rapid ad justment of the capacity for excitation energy dissipation plays a signific ant part in the stabilization of this potential capacity. Sustained high ca pacity of photosynthetic electron transport during stress periods provides an explanation for the instantaneous response of theta to short-term weathe r fluctuations, but also indicates that theta restricts potential carbon ga in under conditions of water limitation less than does stomatal conductance .