The incidence of parthenocarpy and seed abortion as causes of blank seed production in the dioecious, wind-pollinated shrub Pistacia lentiscus were examined in a natural population in the Doñana area, Huelva province, Spain. Fruits with empty seeds resulted either from parthenocarpy, in absenee of pollination, or from embryo abortion, and in both cases the fruit with empty seed rarely matured but was retained on the plant. Average % of viable seeds were 19.89 ±15.82% (N= 29 plants), and 43.38 ± 11.90% (N= 32 plants) for 1981 and 1982, respectively. During 1981, with a severe drought, most fruit abscission was caused by embryo abortion, with a small incidence of parthenocarpy. Increased rainfall during the flowering period of 1982 most likely impaired the conditions for wind transport of the pollen, and parthenocarpy was the main cause of fruit abscission. The highest rates of fruit shedding were recorded for most plants just after fruit set, well before the fruits had attained their full size. After adjusting for the effects of crop size variation, the number of viable seeds produced was positively correlated with plant size. The number of aborted seeds and parthenocarpic fruits was independent of plant size, being correlated only with crop size. The phenological uncoupling with the flowering of males and increased distance to males, caused by the neighborhood of other females, also contributed to reduced seed set. The role of parthenocarpy and seed abortion as regulators of fruit number is discussed in connection with the constraints derived by the wind-pollination syndrome; their implications for the endozoochorous dispersal of seeds by birds are also considered.

Photo: Ripe fruits of Pistacia lentiscus. The seed is shown split open; from left to right: parthenocarpic fruit, with the seed only with vestigial remains; aborted seed (note the blackish embryo aborted); ripe fruit with seed including a fully expanded, viable, embryo.