Vaknin, Y., Department of Pomology, University of California Davis, Davis, CA 95616, United States, Department of Agronomy and Natural Resources, Institute of Field and Garden Crops, Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel

Immature trees of pistachio (Pistacia vera L.), a dioecious species, exhibit major cropping problems; in particular, insufficient male bloom and low yields of poor quality nuts. The aims of this research were to elucidate the extent of pollen limitation and factors that affect pollination and fruit-set during this precarious period. The experiment was conducted in a 4 year-old, 57 ha orchard, in the southwest of the San Joaquin Valley, California. Female trees of the cultivar 'Kerman' and trees of the male cultivar 'Peters' were interplanted at a ratio of 1:24. Bloom phenology was recorded visually. Supplementary pollination treatments, using electrostatic and non-electrostatic methods, were applied to the female trees and compared to open-pollination. Pollen density in the air was analysed during the application of the treatments. The following results were found: male and female bloom completely overlapped, although pollen availability became a limiting factor early in the flowering period; supplementary pollination significantly increased pollen flow, while pollen levels were extremely low in open-pollination; supplementary pollination, and electrostatic pollination in particular, significantly increased pollen germination on the stigma, and reduced the level of blank nuts. In conclusion, pollination was found to be a limiting factor in immature pistachio trees. It is suggested that, in some cases, electrostatic pollination could supplement natural pollination, thereby ensuring higher yields with better nut quality. However, poor levels of pollen alone could not account for all the differences in cropping between immature and mature pistachio trees.