Fruit Mineral Uptake and Augmentation in the Orchard: Postharvest Quality Implications
It has long been recognized that mineral cations including calcium (Ca2+), magnesium (Mg2+), potassium (K+) and ammonium (NH4+) play some role in the development of postharvest disorders (Ferguson and Watkins, 1989). Bitter pit in apple and cork spot in pears are largely treated in commercial fruit orchards by applying preharvest calcium sprays to fruit. While these disorders have been studied for more than a hundred years, their exact cause and their complete and predictable control have eluded researchers. This may be partially explained by the complex relationships between minerals in the fruit and the differential uptake and transport of cations in the tree. This paper reviews some of the evidence of the involvement of Ca, Mg, and K in the development of bitter pit and cork spot, and the differential uptake and transport of these minerals within the tree and fruit. It is hoped that this will explain the lack of complete control of these disorders with calcium sprays, and how the management of other nutrients may influence quality.
More than 60 years ago, Delong (1936) associated low fruit Ca in apples with bitter pit. Since then, low Ca has been related to many other fruit disorders including cork spot of apples and pears, alfalfa greening of pears, internal breakdown, cracking, water care and others (Swietlik and Faust, 1984). Several authors have related disorders to the ratio of Ca to other nutrients in the tree. It has been demonstrated that the Ca/N ratio is more closely related to the occurrence of bitter pit than the level of Ca alone (Shear, 1974). Others have demonstrated the importance of Ca/K and Ca/Mg ratios in postharvest quality (Faust and Shear, 1968; Sharples, 1980; Wills and Scott, 1981). The evidence supporting the role of ratios of Ca:Mg:K in bitter pit development is further strengthened by the findings that K and Mg foliar sprays increased the incidence of bitter pit in apples (Witney et al., 1991; Yogaratnam and Johnson, 1982).
In the field, the only practical methods of reducing the risks of bitter pit development are by managing tree nutrition and vigor, and maximizing Ca levels in the fruit (Terblanche et al., 1980). The latter is however very difficult to achieve because of the restricted uptake and movement of Ca to the fruit and within fruit tissue.
Uptake of Ca, Mg, and K
In most fruit growing regions of Washington, soil levels of Ca, Mg and K are not limiting to production. Potassium and Mg are both taken up actively from the soil along the length of young roots, particularly by root hairs (Clarkson et al., 1968). Both of these nutrients can move to leaves and fruit via the phloem and are transported in all directions between plant organs in response to metabolic demand. In contrast, Ca uptake is restricted to the ends of very young growing roots and is a passive process dependent on water movement into the xylem (wood). Calcium movement into roots is restricted in all but this very limited region just behind the root tip.
Transport of Ca within the tree is largely restricted to the xylem, with almost no transport in the phloem (Mengel and Kirkby, 1982). As a consequence of this confinement, Ca movement is essentially in one direction toward plant tissues using water. Thus, most Ca is deposited in leaves, and relatively little routed to fruit. Calcium is not transported from leaf to leaf, or from leaves to fruit and other tissues.
Foliar Absorption and Redistribution of Ca, MG, and K
Just as Ca uptake is restricted in roots, so is foliar uptake extremely limited. Calcium sprays are applied to fruit orchards in Washington routinely to reduce the incidence of bitter pit, cork spot and alfalfa greening. Three to 12 sprays are applied per season at a rate of 4 to 12 pounds per acre (Peryea, 1997). In spite of this, calcium related postharvest disorders have not been entirely eliminated. This is easily explained by the very limited uptake of calcium into fruit. Fruit and leaves have evolved to specifically resist water loss. The same barriers in leaves and fruit to water loss prevent uptake of water by above ground plant organs. Leaves and fruit are covered with relatively thick cuticle consisting of cutin and embedded waxes, which are relatively impermeable by water.
Calcium uptake into plant tissue is restricted to areas of water uptake and so any barrier to water is also a barrier to calcium. Some spray material probably enters through channels and breaks in the cutile, and also the stomates (pores) on leaves and young fruit (Franke, 1967). Once in plant tissues, Ca is restricted to the cell wall and intercellular spaces, with very small amounts entering through the cell membrane into the cell cytoplasm. Redistribution of Ca is negligible, and penetration into tissues is extremely limited. Any part of the fruit not physically contacted with the Ca spray (i.e., in the spray shadow) will not benefit from the spray.
Potassium and Mg foliar sprays, on the other hand, are actively taken up by plant tissues. Like calcium these nutrients are also restricted by the cuticle and must enter tissues through channels, breaks, or stomates. However, in contrast to Ca, once in tissues Mg and K are rapidly assimilated, transported and redistributed to areas of greatest metabolic demand (Casagnes et al., 1969). Even parts of the fruit not physically contacted with a Mg or K spray will rapidly increase in concentration as the tissues redistribute these nutrients.
Using the above reasoning, it may be a misconception that an extra Ca spray or two will neutralize the potential adverse effects of a foliar Mg or K spray. Where bitter pit, cork spot, or alfalfa greening are a concern, the use of foliar Mg and K containing nutrient sprays should be used with extreme caution. The gain in fruit color attained by using a Mg or K foliar spray has been shown to be relatively small, while the risk of poor fruit quality is relatively high.
Clarkson, D.T.,J. Sanderson and R.S. Russel. 1968. Ion uptake and root age. Nature (London) 220:805-806.
DeLong, W.A. 1936. Variations in the chief ash constituents of apples affected with blotchy cork. Plant Physiol. 11:453-456.
Faust, M. and C.B. Shear. 1968. Corking disorders of apples: A physiological and biochemical review Bot. Rev. 34:441-469.
Ferguson. I.B. and C.B. Watkins. 1989. Bitter pit in apple fruit. Horticultural Reviews. 11:289-355.
Franke, W. 1967. Mechanism of foliar penetration of solutions. Annu. Rev. Plant Physiol. 18:281-300.
Cassagnes, P., J. Magny, P. Azalbert and J. Caries. 1969. Investigations into the accumulation of minerals during the growth of apple fruit (French). C.R. Acad. Sci. (Paris) 269:708-711.
Mengel, E.K. and E.A. Kirkby. 1982. Calcium. In: Principles of plant nutrition. International Potash Institute, Bern. pp. 437-454.
Peryea, F. 1997. Plant nutrition. In: S. Roberts (ed.). 1997 Crop protection guide for tree fruits in Washington. Washington State University, Pullman. pp. 55-60.
Sharples, R.O. 1980. The influence of orchard nutrition on the storage quality of apples and pears grown in the United Kingdom. In: D. Atkinson, J.E. Jackon, R.O. Sharples and W.M. Waller (eds.) Mineral Nutrition of Fruit Trees. Butterworths, London. pp. 17-28.
Shear, C.B. 1974. Interaction of calcium and nitrogen and time of calcium availability in relation to the development of apple disorders. In: J. Wehrman (ed.) Proc. 7th Intern. Colloq. Plant Anal. Fert. Problems. German Soc. Plant Nutrition, Hanover. pp. 427-436.
Swietlik, D. and M. Faust. 1984. Foliar nutrition of fruit crops. Horticultural Reviews. 6:287-355.
Terblanche, J.H., K.H. Gurgen and I. Hesebeck. 1980. An integrated approach to orchard nutrition and bitter pit control. In: D. Atkinson, J.E. Jackson, R.O. Sharples and W.M. Waller (eds.). Mineral nutrition of fruit trees. Butterworths, London. pp. 71-82.
Wills, R.B.H. and K.J. Scott. 1981. Studies on the relationship between minerals and the development of storage breakdown in apples. Austral. J. Agr. Res. 32:331-338.
Witney, G.W, M.M. Kushad and J.A. Barden. 1991. Induction of bitter pit in apple Scientia Hort. 47:173-176.
Yogaratnam, N. and D.S. Johnson. 1982. The application of foliar sprays containing nitrogen, magensium, zinc and boron to apple trees. II. Effects on the mineral composition and quality of the fruit. J. Hort. Sci. 57:159-164.
Area Extension Faculty, Cooperative Extension Washington State University,
400 Washington Street, Wenatchee, WA 98801
13th Annual Postharvest Conference