IS MAXIMIZING ORCHARD LIGHT INTERCEPTION ALWAYS THE BEST CHOICE?

The productivity of modern high density planting orchards is a function of their light interception. This principle is one of the fundamental bases of orchard productivity and, although it has been demonstrated mostly for apple, it is considered to hold for all temperate fruit crops. Based on this premise, orchard systems have evolved towards maximizing the amount of light intercepted and its distribution within the canopy. As a result, many training systems based on thin, shallow canopies, which expose most leaves to high light levels, have been developed. Because of the very high light levels impinging on the leaves in some fruit growing areas of the world, cultural practices have had to be adopted (i.e. shading nets, white sprays, evaporative cooling, etc.), to reduce sunburn damages. High temperature conditions are known to induce sunburn damages to the leaves and fruit, but little research has been done on the detrimental effects of these conditions on tree productivity. When leaves of C3 plants are exposed to high light intensities, their photosynthetic capacity may be exceeded, which triggers a set of alternative biochemical pathways, which protect the photosynthetic apparatus from oxidative damages. These pathways include the water-water cycle, the downregulation of Photosystem II and increased photorespiration; under most circumstances, they effectively protect the leaf and the fruit from the photoinhibitory damages, although photorespiration represents a net loss of fixed carbon. However, under low or high temperatures, high light, or water stress conditions, the leaf capacity to cope may be exceeded, and photoinhibitory damages may be incurred. This paper discusses the hypothesis that in some fruit growing regions of the world, there may occur conditions of excessively high light levels, which might result in reduced productivity of the trees. If that were the case, in these areas of the world, research would be needed on how to optimize the light interception profiles of the canopies, in order to minimize the detrimental effects of excessive light exposure, while maintaining the highest productivity.