Effect of micronutrients, phosphorous and chelator to iron ratio on growth, chlorosis and nutrition of apple seedlings

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Abstract
Apple (Malus domestica) seedlings were exposed to a 2x2x2 factorial arrangement of treatments; phosphate level, micronutrient (Mn, Cu, Zn, Mo and B) level and chelator to Fe ratio level, in solution cultures to assess the effects on growth, development of Fe chlorosis and overall nutrition. Phosphorous was applied at 20 (low) or 400 (high) uM levels, micronutrients at low or high levels and either a 1 to 1 or 10 to 1 ratio of chelator to Fe concentration. After a growth period of 34 days, those seedlings grown under conditions of high chelator to Fe ratio, independent of P or micronutrient level were normal. Under conditions of low chelator to Fe ratio, independent of P level, the high micronutrient level caused chlorosis. Leaf Fe was inversely proportional to the severity of chlorosis. Chlorosis was absent when apple seedling leaf Fe was above 30 ug/g (dry wt. basis). The transport of root Fe to the tops was apparently facilitated when the chelator to Fe ratio was high. Under low chelator to Fe ratio conditions with high micronutrients (independent of P level) excessive levels of Mn, Zn and Cu were detected in the roots. These high Cu levels may have been a causative factor in reducing Fe uptake per se and/or caused a more direct toxic effect on the roots. Overall the effect of elevated micronutrients (particularly Cu) on Fe chlorosis was more pronounced than the reduced availability of Fe by P. However, under low chelator to Fe conditions and high micronutrient supply, increased solution P tended to decrease the role of root Cu in chlorosis development.