Canopy light and plant health

Abstract

In order to achieve elevated yields per unit area, plants must be grown at high density, but increasing crop density and LAI may have negative effects on plant resistance to pests and diseases that are reminiscent of the effects of sunlight deprivation on human health. There is now ample evidence that light, and light signals associated with open space, are positive regulators of plant defense against a broad spectrum of enemies via mechanisms triggered by specific photoreceptors for UV-B and R:FR (Fig. 5). From an evolutionary perspective, this beneficial effect of light might reflect the activity of an optimization strategy that distributes limited resources between growth and defense as a function of the risk of competition that the plant senses using its photoreceptors (Ballaré, 2009). Whether the plant?s solution to this dilemma could be manipulated in species of economic interest to reduce pesticide loads without greatly forfeiting crop yields requires further investigation. The molecular mechanisms that mediate the effects of photoreceptor signals modulating the expression of plant defenses are beginning to be elucidated. Understanding these mechanisms may allow us to manipulate planting density and canopy structure to optimize light penetration for improved crop health. In addition, this understanding will provide key functional information for the design of crop varieties that maintain elevated levels of defense even at high planting density. In this regard, the rapid growth that we have witnessed in the last few years in the field of regulation of plant immunity suggests that, in the not very distant future, we will be able to identify targets for biotechnological manipulation to improve crop health at high LAI. These strategies may help us design agroecosystems that safely deliver healthy products to meet the nutritional demands of humankind in the following decades.