Bulb development in onion (Allium cepa L.). III. Effects of the size of adjacent plants, shading by neutral and leaf filters, irrigation and nitrogen regime and the relationship between the red : far-red spectral ratio in the canopy and leaf area index
1986
Mondal, M.F. | Brewster, J.L. | Morris, G.E.L. | Butler, H.A.
Field experiments were done to investigate why onion crops with a high leaf area index (LAI) initiate bulb scales and mature sooner than those with a low LAI. When small plants were grown either mixed with large plants or in pure stands, those in the mixed stands initiated bulb scales earlier than those in pure stands. The timing of bulb development therefore depended on the size of neighbouring plants and the LAI of the stand and was not simply determined by the size or age of individual plants. Shading plots with neutral filters which caused no spectral changes slightly accelerated bulb development compared to unshaded plots but shading by a canopy of climbing pea and bean leaves had a larger effect. Lack of irrigation advanced bulb maturity but in the highly irrigated treatments plots of high plant density still initiated bulb scales and matured before those of low density. Quantitative relationships between the change in R : FR and either LAI or total radiation interception were determined for onion leaf canopies. It is suggested that the decrease in the red to far-red spectral ratio (R : FR) as LAI increases may be the cause of the earlier bulb scale initiation and maturity that occurs as LAI increases.
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