A prediction model to determine the cross-pollination ability of Citrus spp.

Abstract

Due to a greater emphasis having been placed on seedlessness by citrus consumers, a need developed in the southern African citrus industry to avoid unwanted cross-pollination and hence seediness. There was a paucity of knowledge upon which to base sound decisions in order to ensure optimal orchard layout to achieve the objective of the production of seedless or "commercially seedless" fruit. In vitro pollen viability of all locally available citrus cultivars/selections was determined and pollen of cultivars with strong and weak pollination potential were used for in vivo pollen tube growth and seed content studies of 'Nules Clementine' mandarin and 'Delta Valencia' orange. Data from the in vitro assays were rated and categorized into 'pollen germination potential categories', while the relationship between these categories and seed content of 'Nules Clementine' was determined by regression analysis. The development of the 'pollen germination potential categories' provides an additional facet to the evaluation and development of new citrus cultivars, providing a means to determine the pollination potential at an early stage of that cultivar's development. Citrus cultivars with a pollen germination percentage exceeding 2 % are likely to set too many seeds for the export of 'Nules Clementine' mandarins according to South African export requirements. In addition to the two previously known pollen sterile cultivars, viz. 'Navel' orange and 'Satsuma' mandarin, 'Star Ruby' grapefruit and 'Delta Valencia' orange have been identified in this study as pollen sterile cultivars that can be used as buffers to avoid cross-pollination between two pollen fertile cultivars. Furthermore, 'Delta Valencia' is also ovule sterile. An initial model is presented whereby the seed content of cross-pollinated 'Nules Clementine' fruit can be predicted (r2 = 0.9192) by determining the in vitro pollen viability of the cross-pollinating cultivar/selection and be calculated as follows: 2.619 x pollen germination category of compatible cultivars - 2.449.