Iron deficiency and chlorosis in orchard and vineyard ecosystems

Abstract

Several perennial, deciduous, as well as evergreen fruit crops develop symptoms of iron deficiency—interveinal chlorosis of apical leaves—when cultivated in calcareous and alkaline soils. Under these conditions fruit yield and quality is depressed in the current year and fruit buds poorly develop for following year fruiting. This paper reviews the main fundamental and applied aspects of iron (Fe) nutrition of deciduous fruit crops and grapevine and discusses the possible development of sustainable Fe nutrition management in orchard and vineyard ecosystems. Cultivated grapevines and most deciduous fruit trees are made up of two separate genotypes the cultivar and the rootstock, providing the root system to the tree. The effect of the rootstock on scion tolerance of Fe chlorosis is discussed in terms of biochemical responses of the roots to acquire iron from the soil. Symptoms of iron chlorosis in orchards and vineyards are usually more frequent in spring when shoot growth is rapid and bicarbonate concentration in the soil solution buffers soil pH in the rhizosphere and root apoplast. Since the solubility of Fe-oxides is pH dependent, under alkaline and calcareous soils inorganic Fe availability is far below that required to satisfy plant demand, so major role on Fe nutrition of trees is likely played by the iron chelated by microbial siderophores, chelated by phytosiderophores (released into the soil by graminaceous species) and complexed by organic matter. As most fruit tree species belong to Strategy I-based plants (which do not produce phytosiderophores in their roots) Fe uptake is preceded by a reduction step from Fe 3+ to Fe 2+. The role of ferric chelate reductase and proton pump activities in Fe uptake and the possible adoption of these measurements for screening procedure in selecting Fe chlorosis tolerant rootstocks are discussed. In a chlorotic leaf the existence of Fe pools which are somehow inactivated has been demonstrated, suggesting that part of the Fe coming from the roots does not pass the leaf plasmamembrane and may be confined to the apoplast; the reasons and the importance for inactivation of Fe in the apoplast are discussed. The use of Fe chlorosis tolerant genotypes as rootstocks in orchards and vineyards represents a reliable solution to prevent iron chlorosis; in some species, however, available Fe chlorosis resistant rootstocks are not very attractive from an agronomic point of view since they often induce excessive growth of the scion and reduce fruit yields. As most fruit tree crops and grapes are high value commodities, in many countries growers are often willing to apply synthetic Fe chelates to cure or to prevent the occurrence of Fe deficiency. The application of iron chelates does not represent a sustainable way to prevent or cure iron chlorosis because of to their costs and of the environmental risks associated with their use. Since Fe chelates were introduced, little research on alternative means for controlling the chlorosis has been performed. Sustainable management of Fe nutrition in orchards and vineyards should include all genetical and agronomical means in order to naturally enhance Fe availability in the soil and in the plant. Special attention should be given to soil analysis and to prevention measures carried out before planting. Alternatives to iron chelates are being developed and in the future they should be included into the routine practices of managing fruit trees and grapevine under Integrated Production and Organic Farming.