Germination of pear pollen markedly improved when boric acid was added to the medium. The pollen was more sensitive to boron in water than in 10 % sugar solution. Supplying weak solutions of boron to pear branches before flowering resulted in a good germination of the pollen in sugar solution without boron. Spraying apple-trees and pear-trees with boron solutions had no significant effect on fruit set, despite the pollen sensitivity to boron in vitro.The amount of boric acid in the medium determined percentage germination (saturation curve) and length of pollen tubes. Boron was not required until the tubes protruded. Temperature and sugar concentration both showed optimum values for germination. The effect of boron on germination rate was positively correlated with temperature and negatively with sugar concentration. An increase of 10°C roughly doubled this influence on germination rate. Germination was positively correlated with pollen grain number per drop, whether or not boron was present. So the mutual stimulation of the pollen was more effective with boron or with pollen from rather boron-rich branches. Pollen extracts in 10 % sugar solution with or without boron stimulated germination (also of other species).Both apple and pear pollen (and those of many other species) remained viable longest at both low humidity and temperature. In deep-freeze longevity was more than 2 years at -180°. Viability remained unchanged. Longevity would probably have been indefinite.
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