An Analysis of the Influence of Plant Density on the Growth ofVicia faba

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Abstract
In a further analysis of the changing pattern of development induced in Vicia faba by varying the density, it has been found that a reduction from a high to a low density has little influence on the subsequent development unless such thinning is delayed until the flowering phase. By this time, save for widely spaced plants, the level of self shading within the population has become marked. In fact, at high densities (55–65 plants/metre 2 ) during the early-ripening phase the light intensity at ground level may fall to 0.03 daylight while a considerable proportion of the plant—up to 38 per cent.—may receive less than o-1 daylight. At low densities (11-I2 plants/metre 2 ) the minimum intensity at ground level is 0.14 daylight and less than 3 per cent, of the shoot is subjected to o-1 daylight. In pot experiments, using a range of screens, it was established that the compensation point is about o-I daylight. Thus, as the density is increased the light gradient between the apex and the base becomes progressively steeper and the proportion of the leaves not actively assimilating correspondingly greater. To assess the ways in which such a light gradient operates, experiments were carried out in which either the apex or the inflorescences or leaves were removed over different sections of the stem or various parts of the shoot shaded and detailed records made of development, particularly of flower- and pod-production. Removal of the upper leaves or shading the apex primarily increases the rate of pod abscission after the flowers have set. Partial removal of the inflorescences, especially at the lower nodes, has an opposite effect, while decapitation, though it augments the percentage of flowers which produce immature pods, subsequently causes fewer pods to reach maturity. Shading of the lower nodes reduces at these nodes the number of mature pods but may result in more pods maturing at the upper nodes. It is concluded that when the light gradient is such as to restrict the internal supplies of substrates the growth of those organs with the least competitive ability, e.g. the newly formed pods, is arrested. It is at this phase that the factors controlling abscission come into play and that abscission is dependent upon a balance between the levels of auxins and the production of an abscission factor.