Effect of pollen load size and source (self, outcross) on seed and fruit production in highbush blueberry cv. 'Bluecrop' (Vaccinium corymbosum; Ericaceae)

Reproductive fitness of a plant is ultimately determined by both number and quality of seed offspring. This is determined by sexual selection of polle n microspores and ovules during pollination and fertilization. These proces ses may include pollen competition and seed abortion, which reduce the numb er of microspores and ovules available for final seed production. Thus. eve n an excess of pollen microspores to ovules does nor result in fertile seed s equal to ovule number. We investigated pollen requirements of highbush bl ueberry (Vaccinium corymbosum cultivar 'Bluecrop') for maximal seed product ion and how fertile seed number translates into fruit quality, since fruit quality would ultimately determine the dispersal of its offspring. We demon strate that individual blueberry flowers with a mean of 106 ovules reach th eir maximum fruit set and mass and minimum time to ripen when 125 outcross pollen tetrads pollinate a flower, compared to 10 or 25. Three hundred tetr ads resulted in the increase of fertile seeds, but did not result in a furt her increase of fruit mass or fruit set, or decrease in time to ripen. We a lso examined the effect of pure and mixed loads Of Self and outcross pollen (25 and 125 tetrads), and found no differences in fertile seed number, fru it mass, or percentage fruit set when pollen lends were either 25 self or o utcross pollen tetrads, although number of days to ripen was significantly shorter by 8 d with 25 outcross tetrads. When the pollen load of 125 tetrad s consisted of self or a 50:50 mixture of self and outcross pollen, fruit m ass, days to ripen, and percentage fruit set were not different from lends of 125 outcross pollen. In addition, a pollen load of 25 outcross tetrads r esulted in fertile seed number and fruit quality in between that of 25 self , and 125 self 125 mixed, or 125 outcross tetrads. Large, small, and flat s eed types were identified, and only large seeds (length = 1.7 mm) were fert ile. These results improve our understanding of pollen load size and source requirements of a crop plant and the limits to pollen transfer when transl ated to fruit growth.