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

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A (n)) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A (n) spot measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A (n) by low light availability (photon flux density, PFD) and stomatal conductance (g (s)). 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 A (n) were related to PFD and g (s) in order to calculate envelope functions which were used to establish PFD or g (s) limitations to A (n). Under the growing conditions of the present trial, mean leaf A (n) remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by similar to 20% at 19% GI. Limitation to A (n) due to g (s) was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A (n) by PFD and g (s) in response to shade explain the absence of a decrease in coffee leaf A (n) with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.