| Abstract: | In a further analysis of the changing pattern of developmentinduced in Vicia faba by varying the density, it has been foundthat a reduction from a high to a low density has little influenceon the subsequent development unless such thinning is delayeduntil the flowering phase. By this time, save for widely spacedplants, the level of self shading within the population hasbecome marked. In fact, at high densities (55–65 plants/metre2)during the early-ripening phase the light intensity at groundlevel may fall to 0.03 daylight while a considerable proportionof the plant—up to 38 per cent.—may receive lessthan o-1 daylight. At low densities (11-I2 plants/metre2) theminimum intensity at ground level is 0.14 daylight and lessthan 3 per cent, of the shoot is subjected to o-1 daylight.In pot experiments, using a range of screens, it was establishedthat the compensation point is about o-I daylight. Thus, asthe density is increased the light gradient between the apexand the base becomes progressively steeper and the proportionof 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 theinflorescences or leaves were removed over different sectionsof the stem or various parts of the shoot shaded and detailedrecords made of development, particularly of flower- and pod-production.Removal of the upper leaves or shading the apex primarily increasesthe rate of pod abscission after the flowers have set. Partialremoval of the inflorescences, especially at the lower nodes,has an opposite effect, while decapitation, though it augmentsthe percentage of flowers which produce immature pods, subsequentlycauses fewer pods to reach maturity. Shading of the lower nodesreduces at these nodes the number of mature pods but may resultin more pods maturing at the upper nodes. It is concluded that when the light gradient is such as to restrictthe internal supplies of substrates the growth of those organswith the least competitive ability, e.g. the newly formed pods,is arrested. It is at this phase that the factors controllingabscission come into play and that abscission is dependent upona balance between the levels of auxins and the production ofan abscission factor. |
