The Influence of Day Length on Seed Coat Colour, an Index of Water Permeability, of the Desert Annual Ononis Sicula Guss.

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Abstract
Depending on the time of the rains, Ononis sicula usually germinates either in late autumn or in early spring and dies in early summer. The seed coat colour typical of the very slowly imbibing seeds produced in long days has earlier been found to be yellow, whilst the faster and fastest imbibing seeds produced in short days were green and brown. These colours may be used as indicators of the permeability of the seed coats. Transfer of plants from one day length to another at successive stages of seed maturation showed that the critical period for the development of the ultimate seed coat colour is restricted to the last eight days preceding full maturity, when the seed pod has already reached its final size but has not yet turned from green to yellow. A change from short to long days even at this stage results in seeds the same as those produced under long days throughout; a change from long to short days results chiefly in typical greenish and brown short-day seeds although 1.5-20% remain yellow. The survival value of this mechanism in desert conditions is demonstrated with reference to field observations.

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