Exogenous application of polyamines improves fruit set, yield and quality of sweet oranges

The studies were aimed at improving fruit production and quality of 'Blood Red' sweet orange (Citrus sinensis L. Osbeck.). Aqueous solutions (0.01 mM) of putrescine, spermidine and spermine and the mixture of all three (0.001 mM each) were sprayed on to whole trees during full bloom. The control trees were sprayed with only water. The experimental trees were 12-15 years old, spaced at 7×7 m and were growing under similar cultural conditions. Single trees were taken as experimental plots with three replicates. Polyamines (PAs) significantly increased initial fruit set, yield/tree, and production of grade-I fruit. Maximum fruit set (25.89%) was observed on trees sprayed with spermidine followed by spermine (22.73%) and putrescine (15%) compared with control (10.10%). Putrescine application resulted in the highest fruit yield (729.3 fruit/tree) compared with control (500 fruit/tree). While, the mixture of all three PAs decreased the fruit set (9.723%) and yield (290 fruit/tree) compared with control. Fruit quality, including peel colour, peel smoothness, total soluble solids (TSS), total and non-reducing sugars was significantly improved with PAs compared with control. Maximum TSS (9.93%) was observed with the spermidine treatment compared to the minimum in controls (8.46%). Total sugars were highest (5.53%) with spermidine and non-reducing sugars (2.34%) with spermine sprays respectively, compared to control. Fruit acidity, was minimum (0.86%) with the spermidine treatment compared to control (0.93%), however, the difference was not statistically significant. Spermidine application produced fruit with higher peel color (4.12) and peel smoothness score (3.9) compared with control (2.48 and 2.57 scores respectively). Among the three PAs tested, spermidine proved to be the most effective in improving fruit set, final pack out of grade-I fruit of better quality compared with control.