Measurement of oxygen tension changes in the style during pollen tube growth

Using a Clark micro electrode the actual oxygen tension at various places in a style can be measured. Within the unpollinated, ripe style of Hippeastrum a pO2 gradient occurs. In the upper part, possibly with a little increase in the stigmatic branches, a very high oxygen tension is found; in the lower part, about 5 mm from the ovary, a sudden decrease occurs. In the ovary itself the oxygen tension is extremely low. After pollination, passage of the mass of pollen tube tips can be followed by a fairly sharp drop of the oxygen tension, which shifts down the style with progressive growth. This oxygen tension depression seems to be sharply limited to the region of pollen tube tips and the amount of decrease is dependent on the growth rate of the tubes. After passage of the pollen the original high level of oxygen tension is not fully restored in the upper style. This suggests that the metabolic activity of the pollen tube is strongly linked with the metabolic condition in the style. It seems that tubes grow most of their way to the egg under aerobic conditions; only at the base of the style, just before entering into the ovary, does the pollen have to switch over to an anaerobic pathway. For most of the style oxygen tension does not form a tropic gradient for tube growth.