Seedlessness and parthenocarpy in Pistacia vera L. (Anacardiaceae): Temporal changes in patterns of vascular transport to ovules

Pistacia vera 'Kerman' (pistachio nut) typically produces high numbers of s eedless or blank fruits. Patterns of vascular transport into fruits and ovu les were studied over 3 years by following the movement of disodium fluores cein solution from cut branches into developing fruitlets. Early in the sea son, vascular conductivity is intact through to the chalazal end of the ovu le. Soon afterwards, the percentage of ovules with vascular conductivity th rough to the chalaza declines, and in a variable fraction of fruits, moveme nt of the fluorochrome solution becomes blocked either at the placenta or i n the funiculus. Six to 9 weeks after anthesis there is blockage in 90 (1 y ear) to 100% (2 years) of fruits. Subsequently, vascular conductivity resum es in 83.3 % (3 year mean) of ovules, a percentage that correlates well wit h the mean percentage of seeded nuts at harvest (77.5 %). Ovules from fruit s with dysfunctional vascular conduction early in the season are smaller th an those with fully functional vascular tissue. At the time conductivity de clines, a high percentage of those ovules with blocked vascular movement la ck endosperm and appear to be unfertilized; none of the ovules that retain full vascular flow lack endosperm. Pollination using gamma-irradiated polle n (Co-60, 1.0 kGy) led to a nearly three-fold increase in the production of blank nuts. The results indicate that fluorescein transport may be a valua ble tool to predict the fate of ovules, and are consistent with the hypothe sis that parthenocarpic fruit set may be an important factor in blank nut p roduction in pistachio. (C) 1999 Annals of Botany Company.