Fruit set and development: Pre-requisites and enhancement in temperate fruit crops

Fruit set is the transformation of ovary to a rapidly growing young fruit which is initiated after successful pollination and fertilization. The various prerequisites for good fruit set are the development of flower buds, certain temperature range for good pollination, pollen tube growth, fertilization and relatively high level of photosynthates for developing fruit. Three general categories of limited fruit set recognised may be attributed to limited pollination, limited nutrients and precocious abscission of flowers (Verma and Jindal, 1985). Pollination is an essential process for fruit set, fruit growth, fruit quality, and seed set of temperate fruits mostly apple cultivars. The first step of successful pollination is the transfer of pollen to the stigmatic surface (typically vectored by bees) followed by an adhesion of pollen grains to the papilla cells of the stigmatic surface (Dresselhaus and Franklin-Tong, 2013). Once pollen reaches the base of the style, one sperm nucleus fuses with the egg cell to produce the embryo (resulting in seed set), and the other fuses with the two polar nuclei to produce the endosperm. Pollen source and temperature have a tremendous influence on the rate of pollen tube growth. Almost all apple cultivars are reported to be either self-incompatible or semi-compatible and require cross-pollination to set fruit in marketable quantities (Garratt et al., 2014). For commercial production, at least two cross-compatible cultivars with synchronous flowering are recommended (Banday and Sharma, 2010). There is significant decline in fruit set with increasing distance between pollinizer and main cultivars, and pollinizers should be planted at proper ratio i.e. 11%, 15%, 25% and 33% for effective pollination (Pandith, 2014). Pollen source is considered to be one of the most influential factors affecting the fruit set. Pollen density on the stigmatic surface is positively correlated with pollen tube growth and fruit growth rate (Zhang et al., 2010). Fruit set improvement can be increased by growth regulators like auxins, cytokinins and gibberellins. This effect can be obtained by natural hormones extracted from the plant itself or by exogenous application of synthetic substances. Other substances like thidiazuron, CPPU (Petri et al., 2010), Prohexadione calcium (P-Ca) (Greene, 2008), and some micronutrients, especially the boron (Lee et al., 2009) increases fruit set especially in apple. Apple seeds, during their development in the fruit, produce a sequence of different types of hormones, the appearance and disappearance of which is linked with successive stages in the development of the endosperm and embryo. Fruits with no seeds or with only a low seed content are not normally able to survive this competition (Bramlage et al., 1990).