Hormonal Profile in Ovaries of Mandarin Varieties with Differing Reproductive Behaviour

The endogenous levels of 13 gibberellins (GAs), three cytokinins (CKs), abscisic acid (ABA), indole-3-acetic acid (IAA) and jasmonic acid (JA) were analyzed in naturally pollinated ovaries of three mandarin cultivars selected for their different capacity to produce seeds and their differing parthenocarpic ability. The varieties compared were Murcott (pollen self-compatible, highly seeded), Moncada (self-incompatible, moderately seeded), and Moncalina (pollen sterile, seedless), obtained from Moncada by bud γ-irradiation. As expected, the 13-hydroxylation pathway was predominant in ovaries and our results further indicate that cultivar differences exist in GA metabolism. The active gibberellin GA₁ levels in ovaries seems to be related with presence of fertilized ovules and, therefore, with the ability to produce the seeds of a variety. Sterility gamma irradiation arrested the biosynthesis of GA₁ and its precursor GA₁₉ in Moncalina ovaries if compared to Moncada. The productive efficiency of the studied cultivars also indicated that fruit set depends strongly on the GA₁ level achieved by ovaries, which is also closely related with carbohydrate content. The study of the expression of gibberellin-oxidase genes showed that the pollination/fecundation process enhances GA20ox2 and GA3ox1 activities in naturally pollinated Murcott and Moncada ovaries compared with unpollinated Murcott and Moncalina, respectively. GA2ox1 expression was lower in the ovaries of the highly seeded cultivar Murcott than in those of Moncada or Moncalina. Unpollinated Murcott ovaries contained much lower levels of GA₁ and IAA than the naturally pollinated ovaries of this cultivar. Conversely, unpollinated ovaries, which exhibited 100 % abscission, had more ABA and JA contents. Cytokinin activity seemed constitutive and independent of pollination/fecundation. However, trans-zeatin (t-Z) and 2-isopentenyl adenine (2-IP) concentrations were higher in Murcott ovaries than in Moncada/Moncalina.