Uptake and Distribution of Boron in Coconut and Paurotis Palms

Natural distribution patterns of boron (B) among leaves within a canopy, among leaflets within a leaf, and within single leaflets were determined for coconut palm (Cocos nucifera L.) and within leaves for paurotis palm [Acoelorrhaphe wrightii (Griseb. & H. Wendl.) Becc.]. Leaf B concentrations did not vary significantly among leaves within the canopy or among leaflets within a single leaf for coconut palm, but basal leaflets of paurotis palm had higher B concentrations than central leaflets. Boron concentrations were significantly higher toward the tips of individual leaflets in both species. Application of Solubor to the soil significantly increased leaf B concentrations in all leaves of coconut palm after 2 months as well as in new leaves produced up to 6 months later. Application of Solubor as a leaf axil drench was much less effective in increasing foliar B concentrations than soil treatment. Boron deficiency is a common and widespread disorder of palms throughout the world (Broschat, 2007a; Corrado et al., 1992; Elliott et al., 2004; Kamalakshiamma and Shanavas, 2002). Boron deficiency can be extremely transient, affecting developing leaves for as little as a day or 2 before normal growth resumes (Broschat, 2007a). Under these conditions, the effects of a temporary deficiency only become visible when the affected developing leaf emerges 4 or more months after the deficiency occurred. Palms may experience multiple alternating periods of B sufficiency and deficiency during the time that it takes for the first affected leaf to emerge. Thus, visual deficiency symptoms are an indication that a temporary B deficiency has occurred before leaf emergence but provide no clues as to the current B status of the palm (Rajaratnam, 1973). Similarly, leaf analysis for B content may not always be a good indicator of current B status (Oertli, 1994). Boron deficiency can also be chronic, affecting a series of successive leaves as they develop. As an immobile element, B deficiency causes leaflet fusion and malformation, truncation, and reduction in the size of newly emerging leaves (Broschat, 2007a; Corrado et al., 1992; Kamalakshiamma and Shanavas, 2002). These symptoms could be confused with deficiencies of other micronutrients such as manganese, zinc, or copper, herbicide toxicities, or even bud rot diseases (Broschat, 2007b; Elliott et al., 2004). Where visual deficiency symptoms are insufficient to diagnose chronic B deficiency, leaf nutrient analysis can be useful (Mills and Jones, 1996). An understanding of natural B distribution patterns within palm canopies and leaves is necessary to determine which leaves should be sampled for analysis (Rajaratnam, 1973). Such studies have been done for other nutrient elements in coconut palm (Amalu et al., 1988; Broschat, 1997), Canary Island date palm (Phoenix canariensis Chabaud) (Broschat, 1997), and edible date palm (P. dactylifera L.) (Reuther, 1948), but B distribution has only been examined in African oil palm (Elaeis guineensis Jacq.) (Rajaratnam, 1972a). Because Broschat (1997) found that some elements may be mobile in one palm species, but immobile in another, the results obtained for African oil palm may not apply to coconut palm. Furthermore, these four species are pinnate-leaved palms. There is no published information on B distribution within the leaves of any palmate-leaved palm species, which may differ substantially from pinnate-leaved species. There is very little published research on the treatment for B-deficient palms. Soil applications of 40–700 g of borax (Na2B4O7 10H2O) per palm per year have been suggested by Dickey (1977), Kamalakshiamma and Shanavas (2002), and von Uexkull and Fairhurst (1991). Some of these authors also suggest applying borates in the leaf axils, but they provide no data on the relative effectiveness of these two methods. Rajaratnam (1972b) studied the uptake of B applied to the soil and leaf axils in African oil palm, but equivalent studies have not been published for coconut palm. The objectives of this study were 1) to document the natural B distribution patterns in leaves of coconut palm, a pinnate-leaved species and paurotis palm, a palmate-leaved species; and 2) to determine the effects of soil and leaf axil-applied B on the B content of coconut palm leaves of various ages over time. Materials and Methods A block of ‘Fiji Dwarf ’ coconut palms 15 years old and 3–6 m in overall height was used for these experiments. The palms were spaced 6 m apart and were growing in a Margate fine sand soil (siliceous, hyperthermic, Mollic Psammaquent) with a pH of 6.1 and cation exchange capacity of 3.1 mEq/ 100 g in Davie, FL (lat. 26 4#56$ N, long. 8