Vol. 80 No: 6

Title:
Seasonal fluctuations in nutrients and carbohydrates in pecan leaves and stems

Authors:
TEHRYUNG KIM and HAZEL Y.N WETZSTEIN

pp: 681-688

Abstract:
An integrated study of changes in carbohydrates and nutrients in pecan was conducted and related to developmental changes during a growing season. Perennial woody plants exhibit marked seasonal changes in nutrient content, carbohydrate metabolism, and organ development. Information on nutrient changes may provide insight into seasonal demands and requirements and be useful in developing fertilisation regimes for this crop. Leaf and stem tissues collected at full bloom, leaf maturation, full cotyledon expansion, fruit maturation, and leaf abscission were analysed for starch, specific soluble sugars, and 11 other macro- and micro- nutrients. During leaf expansion, foliar levels of fructose and glucose declined from 12 mg g^-1 DW to 5 mg g^-1 DW and from 7 mg g^-1 DW to 3 mg g^-1 DW, respectively. After fruit maturation, stems exhibited an increase in sucrose, from 15 mg g^-1 DW to 37 mg g^-1 DW, and a marked decrease in starch, from 59 mg g^-1 DW to 8 mg g^-1 DW. Seasonal variations in elemental content indicated developmental stage-related nutrient movement. During leaf development, all nutrients accumulated in leaves, with more than a 70% increase in N, K, Ca, Mg, S, B and Fe content. During fruit development, the foliar content of many nutrients decreased, suggesting fruit are a strong sink for essential elements, as well as carbohydrates. However, nutrient changes differed with the stage of fruit development. For example, foliar concentrations of Mg and Zn decreased by 23% and 24%, respectively during cotyledon formation; while P, K, S, B, Cu and Mn levels decreased during reserve accumulation by 4, 15, 7, 12, 8 and 10%, respectively. During the period leading-up to leaf abscission, leaf nutrient resorption was observed with decreased contents of N (6%), P (14%), K (7%), Ca (7%), Mg (10%) and S (3%). Pecan leaf expansion and fruit development are critical periods, when nutrient movement can be appreciable. Maintaining adequate fertility levels during developmental periods of high nutrient demand may be important to optimise tree function. Management practices that promote late-season leaf retention would prevent nutrient losses associated with premature defoliation.

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