The period for which pollen grains remain viable is highly variable and to some extent depends on the prevailing temperature and humidityThe pistil is developed and ovules reach maturity before the pollen grains are ripe. ... hybrid seed production as soon as tomato hybrid cultivars gained popularity, ... ordinary conditions of storage pollen viability can hardly be retained .
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this answer is incomplete and copied from NCERT class 12th.. i am not satisfied by this answer !