Foliar application of LBU influences the leaf nitrogen level, vegetative and reproductive performance of 'Blood Red' sweet orange

Winter leaf drop in sweet orange has been considered as the main cause for loosing the tree vigour and yield, which has resultantly almost vanished it from the citrus industry of Punjab-Pakistan. The current study was aimed at determining the optimum time of low-biuret urea (LBU) foliar spray to improve leaf age, fruit set, and yield of 'Blood Red' sweet orange (Citrus sinensis L. Osbeck.) through nitrogen (N) enrichment of over wintering leaves. A 2% solution of LBU was sprayed on to 15 years old 'Blood Red' sweet orange trees to the point of run off, during mid October, November and December along with control sprayed with simple water. The changes in the level of total N and NH₄-N in the leaves were studied after 0, 3 and 6 days of LBU spray and then during February (flushing), March (flowering), and April (fruit set). The maximum total leaf N contents were found in December treatment (3.3%) followed by November (3%) and October (2.8%) compared with control (2.4%). Likewise, maximum NH₄-N was observed in December and November sprayed trees (1.7% each). The LBU treatments did not significantly influence total number of shoots per branch, vegetative shoots (%), generative shoots (%), length of shoot and number of leaves per shoot. All the LBU treatments significantly increased the leaf age along with total number of buds per branch and bud drop intensity (%) compared with control, while flower maleness and flower opening tendency were significantly decreased by all treatments compared with control. There was no significant difference among treatments in case of flower drop, fruit set on current shoots and old shoots. In conclusion, winter application of LBU significantly improved the leaf age, tree vigour and production of 'Blood Red' sweet oranges through improved flowering without any improvement in fruit setting.