Hybrid is produced by crossing between two genetically dissimilar parents. Pollen from male parent (Pollen parent) will pollinate, fertilize, and set seeds in the female (seed parent) to produce F1 hybrid seeds. For the production of a hybrid CROSSING between two parents is important, the crossing process will result in heterosis. In a self-pollinated cross, it is difficult to cross but in cross-pollinated crops, it is easier.
In nature to create genetic variability and for its wider adaptation in different environmental conditions, flowering plants have adopted many mechanisms for cross-pollination. Cross-pollination results in genetic heterogeneity and shows wider adaptations. Flowering plants have evolved a number of devices to encourage cross-pollination. Those mechanisms are;
Decline: Flowers are unisexual. In monoecious plants, male and female flowers are borne on the same plant eg., cucurbits, maize, castor, and coconut. In dioecious plants, male flowers are borne on different plants eg., papaya, cannabis, mulberry.
Dichogamy: Time of anther dehiscence and stigma receptivity are different forcing them for cross-pollination. The time gap between the two may vary from one day to many days. In protandry, anthers dehisce earlier than the stigma receptivity eg; maize, sunflower. In protogyny, stigma becomes receptive earlier than the anther dehisces eg., Pearl millet mirabilis.
Self-incompatibility: self-fertilization is avoided by recognizing the self pollen by the stigma. Eg., Brassica, Petunia, Lilium.
Herkogamy: there is the spatial separation of the anthers and stigma. Their relative position is such that self-fertilization cannot occur. The stigma projects beyond the anthers and therefore pollen cannot land on stigma. Eg., Lucerne's stigma is covered with a waxy film. The stigma does not become receptive until this waxy membrane is broken by the visit of honeybees resulting in cross-pollination.
Male sterility: Absence or atrophy or malformed of male sex organ (functional pollen) in normal bisexual flower. Male sterility is of three types: genetic male sterility, cytoplasm sterility, and cytoplasmic- genetic male sterility.