A method is described by which the changes in shape that accompany hydration of Brassica pollen grains may be rapidly measured. Using this technique it has proved possible to chart the small amount of hydration that takes place on anthesis, in addition to the response of pollen to a range of relative humidities in vitro and in vivo. Such measurements, together with pollen transfer experiments, indicate that under normal field conditions self-pollen undergoes a brief period of hydration followed by some loss of water and that, in the course of this hydration, many pollen grains are inhibited from further growth. Raised levels of atmospheric water cause a variety of responses in self-pollen, ranging from tube growth through the pistil to the ovary, to tubes inhibited at the stigmatic surface, accompanied by the formation of callose. Surprisingly, compatible cross-pollen is also affected by high humidity, often developing extended tubes that are incapable of penetrating the stigmatic cuticle. The development of stigmatic callose is also stimulated by these tubes, as also occurs when other members of the Cruciferae are induced to germinate on Brassica stigmas by high levels of atmospheric water. This formation of callose in response to challenge by all types of pollen tube suggests models for the self-incompatibility response in Brassica that involve a direct linkage between S (incompatibility) gene products and the formation of callose may require some reexamination. Close study of the operation of the self-incompatibility system in a number of individuals has revealed all aspects of the response to be heavily dependent on the particular S genes possessed by the plant.

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