THE POLLEN-TUBE ACTIVATED ABSCISSION ZONE IN THE STIGMA OF PEARL-MILLET - STRUCTURAL AND PHYSIOLOGICAL-ASPECTS

Abscission of the stigmas of pearl millet (Pennisetum typhoides (Burma n) Stapf et Hubbard) after pollination results from the activation of a specialized zone of tissue near the base. This abscission zone compr ises a well-defined stretch of the basal column made up of thin-walled cells lacking a cuticularized epidermis. The penetration of the first few pollen tubes following a compatible pollination results in loss o f turgor throughout the tissue, followed by loss of adhesion between t he cells and ultimately disruption of the whole zone. Pectolytic enzym es are released concomitantly, but the cellulosic skeletons of the cel ls persist. Numerous calcium oxalate druses, identified cytochemically and by energy-dispersive X-ray analysis, are present in the cells of the abscission zone; these undergo dissolution after activation of the zone, while simultaneously the druse population in the stigma stump a nd the upper ovary wall increases. The barrier formed by the degenerat ing abscission zone tissues excludes pollen tubes arriving after the i nitial activation and prevents the penetration of pathogens such as er got and so has a prophylactic role. We suggest that the collapsed absc ission zone forms not only a physical barrier but also a chemical one. Dissolution products of the numerous calcium oxalate druses during th e breakdown of the tissue are likely to raise the calcium ion concentr ations beyond the optimum for pollen-tube growth, thereby preventing f urther penetration. Inhibited tubes swell at the tips and may show gro wth disorientation before arrest, responses similar to those seen in p earl millet tubes in culture when exposed to higher than normal calciu m ion concentration.