The uptake of different formulations of foliar applied micronutrients by Citrus sinensis (L.) Osbeck cv. Valencia

Abstract

Perennial fruit crops like citrus can benefit from foliar nutrition since deep roots can deplete the soil and soil amendments other than fertigation are not easily applied without damaging roots. Increases in production costs are urging farmers to be more cost effective in supplying their crops with the necessary micronutrients. Different formulations (amino acid, chelated or inorganic complexes) for micronutrients exist and, when used as foliar sprays, more information is needed on: i) the most effective and cost effective formulation for uptake in citrus, ii) the highest concentrations for each formulation for the most efficient application and iii) the influence of contact time between micronutrient and citrus leaves on nutrient uptake. Plant species, organ types and developmental stages of the plant organ may also result in differences in cuticle ultrastructure that influences the uptake of foliar applied chemicals. Potted ‘Midknight’ trees on Carizzo Citrange rootstock were used to evaluate the uptake of foliar applied micronutrients. All experiments were done in a greenhouse at the University of Pretoria. For the main experiment different formulations, concentrations and times of sampling for foliar applied manganese (Mn), zinc (Zn), copper (Cu), iron (Fe), boron (B) and molybdenum (Mo) were evaluated. The treated leaves, as well as the leaf directly above and below the treated leaves were sampled. The most effective and cost effective formulation and concentration were determined by applying B: H3BO3 (2X) and Mo: Na2MoO4.2H2O (4X) as a multiple of the recommended concentration of the FSSA (Fertiliser Society of South Africa) (2003) and Mn: MnSO4 (4X), Zn: ZnSO4 (4X), Cu: CuSO4 (4X) and Fe: FeSO4 (0.5X) as a multiple of the recommended concentration of the manufacturer of the amino acids used. The multiple concentrations are given in parenthesis. The optimal times of sampling after application to determine the time at which maximum uptake occurred for the different elements were: Cu – 24 h, Mo & B – 48 h, Mn & Zn – 96 h, Fe – 192 h. The translocation of Mn, B and Mo occurred to the leaf directly above and below the treated leaf, while Cu, Zn and Fe did not translocate readily, indicating that multiple seasonal applications may be necessary. A scanning electron microscopic study (SEM) was done on leaf surfaces of lemon, grapefruit, mandarin, navel and orange trees. The physical appearance of the wax on the leaf surfaces was visually inspected and no considerable differences between the different citrus species, for the same age leaves, were found that may affect penetration of foliar applied micronutrients. Mature leaves had larger amounts of surface wax than young leaves. Cuticle thicknesses for the different citrus species differed among the leaf age and the abaxial and adaxial leaf surfaces. ‘Bahianinha’ navel, ‘Satsuma’ mandarin and ‘Star Ruby’ grapefruit leaves had thicker cuticles than Valencia leaves in most cases. ‘Washington’ navel and ‘Genoa’ lemon leaves had thinner cuticles than that of Valencia. Cuticle thickness may therefore have an influence on differences in the uptake of foliar applied products between citrus species.