Effects of Night-break Treatment on Leaf Area and Leaf Dry Weight in Callistephus chinensis

Abstract

Plants of Callistephus chinensis were grown in eight hours of daylight per day and received either uninterrupted dark periods or dark periods interrupted by one hour of low-intensity light. The growth in area and the accumulation of dry matter was followed for leaves at different heights of insertion on the stem. Light interruption treatment caused more rapid surface expansion of individual leaves. The rate of accumulation of dry matter was also increased but less in proportion to the increase in area so that the area per unit weight of leaf material, i.e. the specific leaf area, was greater. The final areas and dry weights of leaves were also considerably greater than in the uninterrupted night treatment. When plants were transferred from uninterrupted to interrupted nights, a similar response was obtained from those leaves which began to expand rapidly after the commencement of the treatment. The reciprocal transfer into uninterrupted nights immediately retarded the rate of leaf expansion and final leaf areas were less. The rate of accumulation of dry matter was not affected so rapidly and the area per unit weight of leaf material fell below that recorded from leaves which had always received uninterrupted nights. Within any one treatment and on any one occasion the specific leaf area was almost constant, regardless of height of insertion and therefore also of leaf size and degree of development. It is postulated that a promotor of leaf growth is produced in the light interruption treatment and that this regulator may increase cell expansion. Transfer to uninterrupted nights may halt the synthesis of this regulator and may also lead to the production of an inhibitor of leaf expansion. Finally, it is concluded from a survey of the literature, that, in general, where a response to photoperiod exists, leaf area is greater in long days than in short days.