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

Abstract

Pistacia vera 'Kerman' (pistachio nut) typically produces high numbers of seedless or blank fruits. Patterns of vascular transport into fruits and ovules were studied over 3 years by following the movement of disodium fluorescein solution from cut branches into developing fruitlets. Early in the season, vascular conductivity is intact through to the chalazal end of the ovule. Soon afterwards, the percentage of ovules with vascular conductivity through to the chalaza declines, and in a variable fraction of fruits, movement of the fluorochrome solution becomes blocked either at the placenta or in the funiculus. Six to 9 weeks after anthesis there is blockage in 90 (1 year) to 100% (2 years) of fruits. Subsequently, vascular conductivity resumes in 83.3% (3 year mean) of ovules, a percentage that correlates well with the mean percentage of seeded nuts at harvest (77.5%). Ovules from fruits with dysfunctional vascular conduction early in the season are smaller than those with fully functional vascular tissue. At the time conductivity declines, a high percentage of those ovules with blocked vascular movement lack endosperm and appear to be unfertilized; none of the ovules that retain full vascular flow lack endosperm. Pollination using gamma-irradiated pollen (60Co, 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 valuable tool to predict the fate of ovules, and are consistent with the hypothesis that parthenocarpic fruit set may be an important factor in blank nut production in pistachio.