Co-application of gibberellin and copper sulphate reduces number of seeds as innovative technique for alleviating alternate bearing in Balady mandarin

Seeds are regarded as a source of gibberellins (GA) which promote fruit set, fruit growth and plant growth. However, trees with more-seeded fruits tend to alternate bearing (AB) due to high gibberellins supplies. So, this investigation aimed to reduce number of seeds via spraying with 1000 ppm GA or copper sulphate (0.1% CuSO₄) or 1000 ppm GA + 0.1% CuSO₄ twice during flowering as a new technique to alleviate AB in Balady mandarin tree. Generally, individual spray of GA or CuSO₄ reduced number of seeds per fruit and AB. Also, the combined application of GA + CuSO₄ recorded the lowest significant number of seeds by 35.48% & 37.22% and fruit set by 27.6% & 25%, meanwhile, it recorded the highest increase percentage of fruit weight (32.2% & 37.2%) and marketable yield (17.7% & 28.17%) compared to the control in the two On year seasons. In addition, this treatment recorded the lowest concentration of endogenous GA in leaves of mandarin trees (7.6 & 7 µg/g F.W.) with the highest reduction percentage of 16.48% & 22.2% in comparison to the control (9.1 & 9 µg/g F.W.) in both seasons, respectively. Thus, this treatment reduced endogenous GA levels by 16.48% & 22.2% and AB index by 55.4% & 50.7% in both seasons. Furthermore, in the following two Off seasons, GA + CuSO₄ improved fruit weight by 2.4% & 16.7%, number of fruits by 76.7% & 97.7%, and yield by 58.3% & 107.7%, in both seasons, respectively. Accordingly, GA + CuSO₄ proved to be efficient treatment which effectively reduced endogenous GA in leaves of mandarin trees, produced low-seeded fruits (high quality fruits), reduced AB index and improved On year yield via increasing fruit weight and the following Off year yield via increasing fruits number per tree.