Light controls plant transpiration and the fixation of atmospheric carbon dioxide by the leaves of healthy trees/vines. The photosynthetic process is important since, for example, more than 90 percent of the total dry matter produced by apple trees originates from photosynthesis by leaves. Biological yield has been increased in many crops by enhancing seasonal light interception.
In general, maximising the interception of incoming sunlight, in an orchard depends on tree/vine spacing, tree/vine dimensions (shape, height), alley width, row orientation as well as leaf area index (ratio between the total area of all leaves per tree to the land area allocated to the tree) and the length of the growing season. Light distribution within tree/vine canopies is critical to actual yield. For example, optimum yields of apple are obtained at about 60-70 percent light interception.
At HortResearch we have found that as leaf area increases excessively, the increases in light interception or potential yield become not only less but in many cases decrease. Our research work indicates that adequate light distribution within the tree/vine canopy is important to secure high fruit quality since shade causes a reduction in fruit weight and symptoms of fruit immaturity such as decreased fruit colour, fruit dry matter, fruit soluble solids and increased fruit firmness.Shade does, however, reduce the incidence of apple skin blemishes such as russet and sunburn.
Several methods and instruments have been developed at HortResearch to describe total tree light interception or microclimate within tree canopies under field conditions (e.g. the 'Whirligig' radiometer, the multiple PAR-sensors 'contraption', the point grid). We have produced several modelling approaches to estimate total light interception of various orchard designs or tree/vine forms and its distribution within the canopies. Such computer models may provide a tool to design the optimum orchard system that consistently produces high yields of top quality fruit.