Varying leaf-to-fruit ratios affect branch growth and dieback, with little to no effect on photosynthesis, carbohydrate or mineral pools, in different canopy positions of field-grown coffee trees

Abstract

Heavy bearing is a typical phenomenon on unshaded coffee ( Coffea arabica L.) trees and limits both the production and retention of leaves, leading to branch dieback, and, thus, results in a strong biennial bearing pattern. The major goals of this study were to investigate the physiological mechanisms that may be associated with the leaf-to-fruit ratio (LFR), branch dieback, biennial production and the relationships between carbohydrate and mineral fluctuations and branch dieback in coffee plants. The trees were grown in north–south-oriented hedgerows under conditions of full sunlight. Leaves and plagiotropic branches from the upper and lower strata of the east- and west-facing sides of the hedgerow were examined. A strong biennial pattern of coffee production was observed over three harvests. Overall, the east face of the hedgerow produced a more sellable crop than the west face, and this was associated with more light availability for the east-facing branches. The branch growth rate was higher with an increasing LFR during 2006–2007, regardless of the canopy position, and no compensatory increase in the photosynthetic rate was found in response to a decreasing LFR. Due to a relatively low fruit yield in 2007–2008, there was no branch dieback. The extent of branch dieback increased dramatically with decreasing LFR and was probably not closely related to changes in the concentrations of carbohydrates, amino acids and minerals. The extent of branch dieback was apparently unrelated to the differences in the photosynthetic rates per unit leaf area, carbon isotope composition, or oxidative stress, as was assessed by the electrolyte leakage from the leaf tissues. We discuss these responses in terms of the relative lack of branch autonomy in coffee trees.