Shoot structure and photosynthetic efficiency along the light gradient in a Scots pine canopy

We examined the effects of structural and physiological acclimation on the photosynthetic efficiency of Scots pine (Pinus sylvestris L.) shoots. We es timated daily light interception (DLI) and photosynthesis (DPHOT) of a numb er of sample shoots situated at different positions in the canopy. Photosyn thetic efficiency (epsilon) was defined as the ratio of DPHOT to the potent ial daily light interception (DLIref) defined as the photosynthetically act ive radiation (PAR) intercepted per unit area of a sphere at the shoot loca tion. To calculate DLIref, DLI and DPHOT, the radiation field surrounding a shoot in the canopy was first modeled using simulated directional distribu tions of incoming PAR on a clear and an overcast day, and estimates of cano py gap fraction in different directions provided by hemispherical photograp hs. A model of shoot geometry and measured data on shoot structure and phot osynthetic parameters were used to simulate the distribution of PAR irradia nce on the needle surface area of the shoot. Photosynthetic efficiency (epsilon) was separated into light-interception e fficiency(epsilon (1) = DLI/DLIref),) and conversion efficiency (epsilon (P HOT) = DPHOT/DLI). This allowed us to quantify separately the effect of str uctural acclimation on the efficiency of photosynthetic light capture (epsi lon (1)), and the effect of physiological acclimation on conversion efficie ncy (epsilon (PHOT)). The value of epsilon increased from the top to the bottom of the canopy. Th e increase was largely explained by structural acclimation (higher epsilon (1)) of the shade shoots. The value of epsilon (PHOT) of shade foliage was similar to that of sun foliage. Given these efficiencies, the clear-day val ue of DPHOT for a sun shoot transferred to shade was only half that of a sh ade shoot at its original position. The method presented here provides a to ol for quantitatively estimating the role of acclimation in total canopy ph otosynthesis.