Exogenous Applications of Plant Growth Regulators Influence the Reproductive Growth of Citrus Sinensis Osbeck Cv. Blood Red

To study the influence of exogenous applications of plant growth regulators on the reproductive behaviour of low bearing sweet orange (Citrus sinensis Osbeck) trees, three separate experiments were conducted on twelve years old ‘Blood Red’ Sweet orange trees budded on Rough Lemon (Citrus jambheri L.) root stock. In the first experiment, trees were sprayed with 20 mg L^-1 2, 4-D and GA₃ alone or in combination at mid bloom (MB) stage, whilst in the second and third experiments 20 mg L^-1 2, 4-D and GA₃ alone or in combination were sprayed at MB + 6 weeks after MB, and at MB + 22 and 28 weeks after MB stages, respectively. A single tree was selected as an experimental unit and each treatment was replicated four times. Data regarding the flowering intensity, flower drop, fruit set, fruit drop and fruit harvest percentages (%) were collected and analyzed statistically. In all experiments exogenous application of 20 mg L^-1 2, 4-D and GA₃ alone or in combination to Blood Red sweet orange trees reduced the flower drop % and increased the fruit set % as compared to untreated trees. Application 2, 4-D and GA₃ alone or in combination at MB did not affect the fruit drop % and fruit harvest % in contrast to untreated trees. The trees sprayed with 20 mg L^-1 GA₃ alone or in combination with 2, 4-D at MB + 22 and 28 weeks after MB exhibited highest reduction in the fruit drop % compared to control trees. In conclusions application GA₃ (20 mg L^-1) alone or in combination of 2, 4-D (20 mg L^-1) at MB + 22 and 28 weeks after MB can be used effectively to increase the fruit set and reduce the fruit drop in ‘Blood Red’ sweet oranges.