Limitation of coffee leaf photosynthesis by stomatal conductance and light availability under different shade levels

Abstract

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (An) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee An "spot" measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to An by low light availability (photon flux density, PFD) and stomatal conductance (gs). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of An were related to PFD and gs in order to calculate envelope functions which were used to establish PFD or gs limitations to An. Under the growing conditions of the present trial, mean leaf An remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to An due to gs was strong in full sun and decreasing with increasing shade levels. On the other hand, limitatio