Zinc Nutrition of Fruit Crops

Zinc (Zn) deficiency is widespread throughout the world causing economic losses on a number of crops. Despite the fact that much information was generated during the last 20 years on Zn soil chemistry and its inorganic phase equilibrium, the mechanism controlling the amount of free Zn+2 present in the soil solution is not yet completely understood. This information is critical for the development of effective techniques of supplying Zn through the soil. As Zn moves very slowly through the soil, however, and a large portion of fruit tree root system occupies deep soil layers, foliar sprays with Zn are generally more effective than soil treatments in alleviating Zn deficiency symptoms. That is why many extension specialists recommend this approach. In view of the poor mobility of foliar-absorbed Zn in plants, however, we may need to reexamine this approach. Zinc foliar sprays may be effective in controlling Zn deficiency in leaves, but not in alleviating Zn deficiency in roots or subsequent flushes of growth. Also, the conditions under which fruit trees are most likely to respond to corrective Zn treatments are not well understood and the critical periods for Zn supply to assure optimal fruit set, fruit growth, and high fruit external and internal quality are not well defined. Field studies on fruit trees suggest that Zn deficiency must be quite severe to make the application of this element economically justifiable. In well-controlled greenhouse studies, however, growth responses were realized on plants only mildly affected by Zn deficiency. If considerable field variability may explain this discrepancy in the data, then future field research must use improved methodologies to properly quantify the impact of various levels of Zn deficiency on tree growth, fruit yield, and fruit quality.