The effect of light on the formation of lignified tissue in Cucurbita pepo.

Light promotes the formation of lignified clements in Cucurbita popo. The nature of the response and the region perceptive of the light stimulus were determined. The effect of removal of the apex and its replacement with IAA, and the effects of IAA and hydrogen peroxide on the formation of lingnified clements in hypocotyl segments in culture, were studied. A light formed inhibitor of IAA-oxidase activity was detected, and investigations made of the effects of variations in light treatment, and treatment with gibberellic acid, on the level of this inhibitor and the amount of lignified tissue formed. Tosts for peroxidase distribution were carried out, and the effects of red and blue light on the formation of lignified tissue were studied. Experiments show that IAA has marked effects on the formation of lignified tissue. IAA appears, on the one hand, to initiate and promote the differentiation of prospective lignified clements, and yet, at the same time to inhibit lignification. It seems likely that light affects the phase of lignification, probably as a result of induced peroxide genesis, and possibly as a result (in some material at least) of the formation of an inhibitor of IAA-oxidase. The presence of a specific inhibitor of this kind may lead to an increase in the level of peroxidase free to function in lignin synthesis. It seems likely, therefore, that light promotion of the formation of lignified elements is connected with auxin metabolism, but whether light affects all the phases in the formation of mature elements that are affected by auxin, it is impossible to say. The problem is far from solution, but the main difficulties have been clarified. The literature pertaining to the biosynthesis of lignin is reviewed.