Niinemets, Ü; Bilger, W; Kull, K; Tenhunen, JD: Responses of foliar photosynthetic electron transport, pigment stoichiometry, and stomatal conductance to interacting environmental factors in a mixed species forest canopy, Tree Physiology, 19, 839-852 (1999)
Abstract: We studied limitations caused by variations in leaf temperature and soil water availability on photosynthetic electron transport rates calculated from foliar chlorophyll fluorescence analysis (theta) in a natural deciduous forest canopy composed of shade-intolerant Populus tremula L. and shade-tolerant Tilia cordata Mill. In both species, there was a positive linear relationship between light-saturated theta (thetamax) per unit leaf area and mean seasonal integrated daily quantum flux density (Ss, mol m-2day-1). Acclimation of leaf dry mass per area and nitrogen per area to growth irradiance largely accounted for this positive scaling. However, the slopes of thetamax versus Ss relationships were greater on days when leaf temperature was high than on days when leaf temperature was low. Overall, thetamax varied 2.5-fold across a temperature rang of 20-30 °C. Maximum stomatal conductance (Gmax) also scaled positively with Ss. Although Gmax observed during daily time courses, and stomatal conductances during thetamax measurements declined in response to seasonally decreasing soil water contents, thetamax was insensitive to prolonged water stress, and was not strongly correlated with stomatal conductances during its estimation. These results suggest that photorespiration was an important electron sink when intercellular CO2 concentration was low because of closed stomata. Given that xanthophyll cycle pool size (VAZ, sum of violaxanthin, antheraxanthin, and zeaxanthin) may play an important role in dissipation of excess excitation energy, the response of VAZ to fluctuation light and temperature provided another possible explanation for the stable thetamax. Yanthophyll cycle carotenoids per total leaf chlorophyll (VAZ/Chl) scaled positively with integrated light and negatively with daily minimum air temperature, whereas the correlation between VAZ/Chl and irradiance was best with integrated light averaged over 3 days preceding foliar sampling. We conclude that the potential capacity for electron transport is determined by long-term acclimation of theta to certain canopy light conditions, and that the rapid adjustment of the capacity for excitation energy dissipation plays a significant part in the stabilization of this potential capacity. Sustained high capacity of photosynthetic electron transport during stress periods provides an explanation for the instantaneous response of theta to short-term weather fluctuations, but also indicates that theta restricts potential carbon gain under conditions of water limitation less than does stomatal conductance.