Regulation of parthenocarpic fruit set in novel seedless triploid and diploid mandarins

Abstract

Aim of study: Ploidy management is a strategy for obtaining seedless mandarins worldwide. The mechanisms underlying parthenocarpy in triploid genotypes and their response to horticultural management have not been evaluated. This work aimed to analyse how triploidy affects the hormonal and nutritional mechanisms that influence parthenocarpy, and to assess the effectiveness of horticultural practices in enhancing fruit set in novel high-quality triploid and diploid mandarin hybrids. Area of study: Salto, Uruguay (31° 21’ S, 57° 45’ W). Material and methods: Three hybrids of mandarins were selected from the Uruguayan Citrus Breeding Programme: the low-parthenocarpic ability triploid ‘F4P2’ and diploid ‘F2P3’, and the high-parthenocarpic ability diploid ‘B30’. Main results: In the low-parthenocarpic ability triploid and diploid cultivars, ovary gibberellin 1 (GA1) diminished one week post-anthesis, and soluble carbohydrates did not increase during the first stage of fruit development, consistent with their parents. The high-parthenocarpic ‘B30’ diploid exhibited a two-week delay in the ovary GA1 reduction, leading to increased carbohydrate levels and fruit set. Under non-cross-pollination, gibberellic acid (GA3) sprayed at full bloom, combined with trunk girdling 30 days later, increased ovary GA1 concentration and fruit set in the two low-parthenocarpic ability mandarins, irrespective of ploidy level, with no effect in the high-parthenocarpic ‘B30’ hybrid. Research highlights: The physiological mechanisms underlying parthenocarpic ability and the response to horticultural practices were linked to the ovary GA1, independent of ploidy level, and likely represent heritable traits. This work provides tools to improve the agronomic performance of the emerging array of novel triploid Citrus cultivars released by the worldwide breeding programmes.