PHOTOMORPHOGENIC REGULATION OF REPRODUCTIVE DEVELOPMENT IN GROUNDNUT AND THE SIGNIFICANCE OF NYCTINASTIC LEAF MOVEMENTS

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Summary

Many leguminous species are grown under relatively high plant population densities > 250000 plants ha−1. It is argued that these plants can tolerate high population densities because their nyctinastic leaf movements allow their aerial parts to receive directly the last light of the day, which increases the Pfr level in the plants before the onset of darkness, causing them to develop like unshaded plants. Measurements of irradiance above and below the canopy of a densely planted crop of groundnut (Arachis hypogaea fastigiata Waldron) showed that, during daytime, when the leaf laminae were horizontally orientated, only 10% of the incident light reached the ground, while after sunset, with the leaves in the vertical position, 70 to 90 % of the incident light could penetrate to the ground. Natural light, available to the plants after this time, was capable of inducing the straightening of hypocotyl hooks of dark-germinated seedlings, a typical red light requiring photomorphogenic reaction. Laboratory experiments showed that groundnut plants, receiving daily 5 min red light immediately after the end of the photoperiod, developed significantly more branches and more reproductive organs than plants receiving 10 min far red light. The greater adaptability of two leguminous plants, bush bean (Phaseolus vulgaris L.) and groundnut, to high population density was demonstrated by comparing their performance with that of maize (Zea mays L.), okra (Abelmoschus esculentus Moench), safflower (Carthamus tinctorius L.) and sesame (Sesamum indicum L.) under a high and a low population density. Although crowding caused the plants of all six species to grow taller, with fewer branches and fewer reproductive organs, the harvest index of the two leguminous species remained unaffected by the population density, while that of the remaining four species was significantly reduced by the higher population density.

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