The relationship between light availability and fruit quality of sweet cherries grown on narrow row, planar canopy systems

It is known that productivity is positively correlated with seasonal fractional light interception of a mature tree canopy. Fruit quality is dependent on the irradiance (light transmission) experienced by the tree's reproductive structures. New planting systems for cherry trees, aimed at increasing fruit yields, are being investigated in New Zealand, using designs that increase canopy light interception by reducing inter-row distances. However, spacing rows more closely introduces the risk of reducing light transmission within the canopy and consequently yield and fruit quality. To address this risk of lower light levels, narrow planar configurations were designed. In 2014, 'Sweetheart'(R) sweet cherry trees were planted as part of a larger, replicated experimental block, with 2 m and 1.5 m inter-row spacings, in a replicated design of four blocks. Planted 3 m apart in the row, they consisted of two opposing cordons oriented along the row and positioned slightly above horizontal from the ground. Each cordon had six vertical, minimally branched, fruiting shoots spaced 25 cm apart, with an anticipated maximum height of 3 to 3.5 m. In the 2019/20 season, photosynthetically active radiation (PAR) at four vertical positions within these canopies was measured to better understand light transmission within different planar canopy systems. Where the tree rows were spaced at 1.5 m, average daily light integral (DLI) calculated as light intensity over a day differed by 82% between the upper and lower parts of the canopy. Where the tree rows were spaced at 2 m, the DLI differed by 46%. Measuring quality attributes of fruit from the four vertical regions of fully developed vertical fruit stems in the planar cherry system showed that total soluble solids (TSS) was the only fruit quality characteristic that showed a relationship with light availability, and yield decreased down the canopy, but was significant only in 2 m row spacing.