Comparative movement of labelled nitrogen and zinc in 1 ... - INTA

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842 However, such studies could not simulate complete foliar application, in which all leaves, shoots and trunk of the tree are treated, nor quantify the percentage redistribution of Zn and N from over-Winter storage to new growth the following Spring. Furthermore, no previous studies on fruit trees have identified the parts which serve as sites of for Zn deposition over Winter On the other hand, the dynamics of uptake of foliar N by trees has been widely reported (Rosecrance et al., 1998; Tagliavini et al., 1998; Weinbaum, 1988). Thus inclusion of labelled N in the applications allowed us to compare N with Zn, to gain better information on their comparative partitioning to storage organs, their redistribution after the resumption of growth in Spring, and identification of probable source organs for nutrient allocation into new growth. By treating the entire canopy, we were able to quantify the amounts and percentages of labelled N and Zn applied to leaves that were translocated out prior to leaf abscission. Between 45.3 – 49.2% of N, and 6.7 – 7.8% of Zn was recovered in the trees after leaf fall. The 15 N and 68 Zn recovered in new Spring growth had been foliarly applied the previous Autumn, stored during the Winter, and remobilised in the Spring. These data should be considered as the minimum percentage redistribution of N and Zn from storage organs to new growth, because our experiment was terminated 4 weeks after full bloom. We cannot rule out the possibility of additional nutrient redistribution from storage if the experiment had been extended into the growing season . Using 15 N-labelled urea, Tagliavini et al. (1998) demonstrated 58 – 69% absorption of the urea-N intercepted by the canopies of nectarine [Prunus persica (L.) Batsch var. nectarina] trees. We did not analyse senescent leaves, but recognise that additional recovery of foliar-applied N in these leaves was likely, as complete export of urea-N prior to leaf senescence was not anticipated. Other studies have shown N recovery rates in the permanent structure of the tree of 50% or better after an Autumn foliar application of 15 N-labelled urea Foliar application of labelled N and Zn in peach TABLE IV Distribution of labelled N and Zn (mg tree –1 ) among different organs of trees sampled during dormancy (16 January), 2 weeks after full bloom (22 March) and 1 month after full bloom (5 April), and percentage recovery of total isotope applied January 16 March 22 April 5 Organ N Zn N Zn N Zn New Growth – – 13.01 ± 1.78 0.397 ± 0.06 22.30 ± 1.87 0.568 ± 0.069 Shoots 13.58 ± 1.02 a 0.425 ± 0.053 7.56 ± 2.7 0.232 ± 0.072 7.28 ± 0.06 0.296 ± 0.04 Trunk 6.31 ± 1.74 0.107 ± 0.03 4.24 ± 0.94 0.081 ± 0.017 3.32 ± 0.31 0.073 ± 0.022 Roots 37.22 ± 4.02 0.385 ± 0.027 29.38 ± 1.14 0.166 ± 0.028 26.0 ± 5.39 0.079 ± 0.017 Whole Tree 57.11 ± 4.84 0.918 ± 0.074 54.19 ± 5.89 0.876 ± 0.137 59.89 ± 6.58 1.016 ± 0.100 RECOVERY (%) 47.75 ± 4.05 7.10 ± 0.57 45.31 ± 4.93 6.78 ± 1.06 49.24 ± 5.50 7.86 ± 0.78 a Values are means ± SD (n = 5). (Weinbaum, 1988; Rosecrance et al., 1998). In the present study, we were unable to determine how much of the labelled Zn deposited on the leaf surfaces was actually absorbed by the leaves. Nevertheless, 7 – 8% transport is approx. the same percentage translocation as has been reported in other crops (Boaretto et al., 1998; Ferrandon and Chamel, 1988; Zhang and Brown, 1999). Some workers have even reported less than 1% translocation (Boaretto et al., 2002; Volschenk et al., 1999). Presumably, Zn transport levels less than 10% might be considered the maximum that can be achieved with standard sprays of ZnSO4. There are significant barriers to the absorption of foliar-applied Zn. Zn can be adsorbed to the cuticle and/or cell walls (Ferrandon and Chamel, 1988), or sequestered in an insoluble form (Zhang and Brown, 1999) and thus made unavailable for transport. Partitioning of N to the root system in Autumn, for storage during the Winter, is well-documented in perennial trees (Sánchez et al., 1992; Millard, 1996), but no similar data on Zn are available in the literature. Sixty-five percent of the recovered, labelled N was transported to the roots in this experiment (Table V). Although not as much as N, 42% of the recovered 68 Zn was found in dormant roots (Table V), suggesting good mobility of the small amount of leaf-absorbed Zn in peach trees. This contrasts with citrus, where several studies have shown limited movement of foliarly-applied Zn to roots (Swietlik, 2002a, b). In the Spring, trees had remobilised approx. 24% of their 15 N and 45.5% of their 68 Zn for new growth 2 weeks after full bloom. By the end of the experiment, 4 weeks after full bloom, remobilisation had increased to around 38% and 55.9% of tree 15 N and 68 Zn, respectively (Table V). Remobilisation of 15 N and 68 Zn was mainly from roots and shoots. Roots were particularly important for supplying Zn, as 79% of the 68 Zn in roots moved into new growth, compared to only 30% for 15 N (Table IV). Once again, this supports our conclusion that Zn from foliar sprays is reasonably mobile in peach trees. In fact, TABLE V Percentage distribution of total labelled N and total labelled Zn among the different organs during dormancy (16 January), 2 weeks after full bloom (22 March), and 1 month after full bloom (5 April) January 16 March 22 April 5 Organ N Zn Sig. N Zn Sig. N Zn Sig. New Growth – – – 24.0 ± 1.3 45.5 ± 3.9 ** 38.0 ± 2.6 55.9 ± 3.1 ** Shoots 24.0 ± 3.2 a 46.3 ± 3.0 ** b 13.7 ± 3.6 26.2 ± 5.3 ** 12.5 ± 2.1 29.2 ± 3.6 ** Trunk 11.0 ± 2.7 11.7 ± 3.0 NS 7.8 ± 0.9 9.2 ± 1.2 NS 5.7 ± 0.8 7.2 ± 2.0 NS Roots 65.0 ± 2.2 42.0 ± 2.9 ** 54.6 ± 4.9 19.1± 3.5 ** 43.8 ± 4.3 7.7 ± 1.3 ** Whole Tree 100.0 100.0 100.0 100.0 100.0 100.0 a Values are means ± SD (n = 5). b NS, **, non-significant, or significant at P ≤ 0.01, respectively, by paired t test.
in early Spring, Zn appears to be even more readily available than N. The question of Zn mobility in plants is controversial. Early observations suggested that Zn was quite immobile; however, recent studies have demonstrated extensive phloem mobility of Zn in wheat (Garnett and Graham, 2005; Haslett et al., 2001; Page and Feller, 2005). At present, it appears that Zn may exhibit variable mobility (Longnecker and Robson, 1993). Experiments with apple have shown no or little movement of Zn from the leaf receiving the application (Orphanos, 1975); however, wheat experiments have shown extensive mobility throughout the plant (Haslett et al., 2001). Some of these differences may be due to species differences. The degree of mobility could also be influenced by the Zn status of the plant, thus deficient plants would not remobilise Zn well (Longnecker and Robson, 1993); and by the stage of plant development (Millikan et al., 1969). For instance, in wheat, where Zn mobility has been studied extensively, Zn appears to be quite immobile in young growing plants. However, when flowering and seed formation begin in mature plants, Zn mobility is increased. Senescing wheat leaves export more than 50% of their Zn to developing seeds (Garnett and Graham, 2005). One important, practical question is whether a foliar spray of Zn, under the conditions in our experiment, could effectively supply the Zn needs of a growing peach tree in the Spring. By 1 month after bloom, only about 5% of Zn in the new growth was derived from the foliar application in the Autumn. This is insufficient to maintain vigorous growth, if root uptake is limited. However, the concentration of Zn used in our experiment (1 mg ml –1 ) is considerably less than typical recommended rates of 11 – 17 kg ZnSO4 (36% Zn) ha –1 (Johnson and Uriu, 1989). Assuming an application rate of 830 l ha –1 , this range of recommended rates would provide 4 – 6 mg ml –1 Zn. In addition, 68 Zn was applied only to the adaxial sides of the leaves. Thus, with higher concentrations applied both to the adaxial and abaxial leaf surfaces, it is conceivable that 8 – 12 times more Zn could be supplied to the tree. However, even that amount would still only provide 40 – 60% of the total amount in the new growth. Therefore, it appears that BOARETTO, A. E., OLIVEIRA, M. W., MURAOKA, T. and NASCIMENTO FILHO, V. F. (1998). Absorption and translocation of 65 Zn applied to common bean leaves. Journal of Nuclear Agricultural Biology, 27, 225–230. BOARETTO, A. E., BOARETTO, R.M.,MURAOKA, T., NASCIMENTO FILHO, V. F., TIRITAN, C. S. and MOURAO FILHO, F. A. A. (2002). Foliar micronutrient application effects on citrus fruit yield, soil and leaf Zn concentrations and 65 Zn mobilization within the plant. Acta Horticulturae, 594, 203–209. BROWN,P.H.,ZHANG, Q. and BEEDE, B. (1994). Effect of foliar fertilization on zinc nutritional status of pistachio trees. Annual Report of the California Pistachio Industry, Crop Year 93–94. Fresno, CA, USA. 77–80. DEJONG,T. M., DAY,K.R.,DOYLE, J. F. and JOHNSON, R. S. (1994).The Kearney Agricultural Center Perpendicular “V” (KAC-V) orchard system for peaches and nectarines. HortTech, 4, 362–367. E. E. SANCHEZ,S.A.WEINBAUM and R. S. JOHNSON REFERENCES 843 several post-harvest foliar sprays would be required to overcome severe Zn deficiency in peach trees. To fully answer this question, additional research is needed to determine the effect of multiple sprays, solutions of different concentration, and times of application on the efficiency of Zn uptake in mature bearing trees. We conclude that foliar applications of Zn in the Autumn are not very efficient at supplying Zn to peach trees (< 10%), especially when compared to supplying N as Autumn urea sprays (≥ 50%). However, once absorbed by the tree, Zn is quite mobile and is stored throughout the plant, including in the roots. During early Spring growth, this stored Zn is supplied to new shoot growth in greater percentages than N, especially for that portion stored in the roots. Thus, the most significant barrier to improving the efficiency of Zn fertilisation in peach trees is getting adequate Zn into the tree, not its mobility once it is in the tree. New approaches to improve the efficiency of Zn uptake would be useful. Ongoing research is required to determine the quantitative significance of foliar applications of Zn, which have become a standard practice for many peach growers. Since sprays are often applied during leaf senescence in late Autumn (Johnson and Uriu, 1989), many leaves fall to the ground before any efflux of Zn has occurred. Other leaves are induced to abscise within 1 – 2 weeks and may not have sufficient time to export the full 7 – 8% of foliar-applied Zn that may be available for export. Thus, there is a good chance that foliar applications supply substantially less of the Zn needs of growing fruits and shoots than we have estimated above. Indeed, since at least 92 – 93% of the applied Zn falls to the ground with senescing leaves, the greatest effect is probably on increasing soil Zn levels (Boaretto et al., 2002). This could increase the availability of Zn for root uptake, but eventually could lead to problems with heavy metal contamination of the soil. The authors acknowledge the technical assistance of Kevin Klassen and Becky Phene at the Kearney Agricultural Center, and the Interdisciplinary Center of Plasma Mass Spectrometry, specifically Michelle Gras for her expertise in developing and implementing the methodology for Zn isotope determination at UC Davis. FERRANDON, M. and CHAMEL, A. R. (1988). Cuticular retention, foliar absorption and translocation of Fe, Mn and Zn supplied in organic and inorganic form. Journal of Plant Nutrition, 11, 247–263. GARNETT, T. P. and GRAHAM, R. D. (2005). Distribution and remobilization of iron and copper in wheat. Annals of Botany, 95, 817–826. GRAUKE,L.J.,STOREY,J.B.,EMINO, E. R. and REED, D. W. (1982). The influence of leaf surface, leaf age, and humidity on the foliar absorption of zinc from two zinc sources by pecan. HortScience, 17, 474. HASLETT,B.S.,REID, R. J. and RENGEL, Z. (2001). Zinc mobility in wheat: Uptake and distribution of zinc applied to leaves or roots. Annals of Botany, 87, 379–386. HAUCK, R. D. and BREMNER, J. M. (1976). Use of tracers for soil and fertilizer research. Advances in Agronomy, 28, 219–266.
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