EFFECTS OF NITROGEN RATES ON DRY MATTER AND NITROGEN ACCUMULATION IN CITRUS FRUITS AND FRUIT YIELD

Abstract

Fruit growth patterns and nitrogen (N) accumulation in fruit over the growth period were studied on ‘Valencia’, ‘Parson Brown’, ‘Hamlin’, and ‘Sunburst’ cultivars which received either 168, 224, or 280 kgNha−1yr−1 as broadcast applications of N:P:K dry soluble granular fertilizer. Over a 4-year period, fruit yields of all cultivars did not respond to N rates during low production years. The effects of fertilizer rates were non-significant on either fruit diameter, dry weight, or fruit N content. Fruit weight and fruit diameter increased rapidly from June to Aug. or Sept., followed by a slow increase during the remainder of the fruit development period. Fruit size of ‘Sunburst’ was significantly smaller than that of the other three cultivars, which showed no differences in fruit diameter or dry weight. Nitrogen accumulation in the fruit increased rapidly between June and Aug., followed by marginal or no increase during the remainder of the growth period. Thus, most of the N accumulation occurred prior to Aug., for all cultivars. Nitrogen content of mature fruit (fresh weight basis) followed the order: ‘Valencia’ > ‘Parson Brown’=‘Hamlin’ > ‘Sunburst’. Total N content of the fruit at a given fruit yield level followed the order: ‘Sunburst’ = ‘Valencia’ > ‘Parson Brown’ > ‘Hamlin’. This study has clearly demonstrated that adequate availability of N is critical during the initial fruit development and growth stages. Application of N later during the growing period will have minimal impact on the fruit yield. However, it could contribute to N reserve in the tree, which may be utilized for fruit production the following year.