RE-GREENING OF LYCHEE (LITCHI CHINENSISSONN.) LEAVES WITH FOLIAR APPLICATIONS OF IRON SULFATE AND WEAK ACIDS

Abstract

Lychee (Litchi chinensis Sonn. cv. ‘Mauritius’) trees growing in calcareous soils are prone to iron (Fe) deficiency, typically prevented by soil application of chelated Fe, which is very expensive compared to iron sulfate (FeSO₄). Therefore, the effects of foliar applications of weak acids (ascorbic or dilute sulfuric acid), weak acids plus FeSO₄, or a soil drench of chelated iron (Fe-EDDHA) on leaf chlorophyll index, leaf ferrous iron (Fe²⁺) and total Fe concentrations, leaf gas exchange and plant dry weight were determined for 2-year-old lychee trees growing in calcareous soil in containers. Treatments were: foliar applications of ascorbic acid in deionized (DI) water plus surfactant, sulfuric acid in DI water plus surfactant, ascorbic acid in DI water plus FeSO₄ plus surfactant, sulfuric acid in DI water plus FeSO₄ plus surfactant, FeSO₄ in DI water plus surfactant, FeSO₄ in DI water with no surfactant, FeSO₄ in well water plus surfactant, sulfuric acid plus FeSO₄ in well water plus surfactant, surfactant in DI water. An additional treatment was chelated iron (Fe-EDDHA) applied to the soil. The organosilicone compound Freeway® was used as the surfactant. Well water was used instead of DI water in two treatments because fertilizers are often mixed with water from a well that typically has a pH much higher than that of DI water. Foliar treatments were applied at 4-week intervals for 24 weeks and chelated iron was applied to the soil at the time of first foliar treatments and 8 and 16 weeks later. There was generally no difference in leaf chlorophyll index among plants receiving chelated iron as a soil drench or foliar treatments containing FeSO₄ plus surfactant with or without organic acid. Foliar applications of acid with no FeSO₄ resulted in a lower leaf chlorophyll index than soil-applied Fe-EDDHA or any of the foliar treatments containing FeSO₄ plus surfactant. Leaf total Fe and Fe²⁺ concentrations were generally highest for plants in the soil-applied Fe-EDDHA treatment and all foliar application treatments containing FeSO₄ plus surfactant. Net CO₂ assimilation (A_net), stomatal conductance (g_s), transpiration (Tr) and leaf, stem, root and total plant dry weights were highest for plants treated with soil-applied Fe-EDDHA or foliarly applied FeSO₄ with no surfactant. Further studies indicated that the surfactant Freeway® inhibits leaf gas exchange and plant growth. The results indicate that foliar applications of FeSO₄ are almost as effective as soil applications of chelated Fe for eliminating or preventing Fe deficiency symptoms of lychee trees growing in calcareous soil. While the addition of a surfactant to the foliar spray mixture appears to be necessary for preventing Fe deficiency, care must be taken when selecting a surfactant for foliar applications of FeSO₄ to be sure that there are no negative effects on leaf gas exchange and plant growth.