Fruit growth model, thermal requirements and fruit size determinants in papaya cultivars grown under subtropical conditions

Abstract Papaya is a tropical fruit crop that produces large-sized berries of high nutritional value. Due to climatic restrains, papaya cultivation in Europe is restricted to limited areas of South East Spain where, even so, it must be grown under protected cultivation in greenhouses to be profitable. The knowledge of papaya fruit growth dynamic and its thermal requirements measured in Growing Degree Days (GDD) from bloom to ripening are essential to predict fruit size and harvest dates. Having this in mind, the objective of this work was to establish the best model describing papaya fruit growth and GDD required for fruit ripening in four cultivars selected for their contrasting vigor and fruit size. The results show that papaya fruits grow according to a simple sigmoid curve. For the sake of simplicity and the good fitting obtained, we recommend Gompertz equation for describing papaya fruit growth. Seasonal changes in the time required from flowering to harvest were detected. Fruits setting in autumn required 8 months to mature, while the fruits setting in spring reached ripening after only 5 months. Heat units required for fruit ripening ranged between 1293 GDD for ‘Siluet’ and 1488 GDD for ‘Red Lady’. Its calculation was revealed as a more useful tool to predict harvest dates than counting calendar days. Initial fruit size was found to be a clear determinant of fruit size at harvest; thus, small fruitlets remained proportionally small as ripening, while large fruitlets reached premium sizes at harvest.