The Relationship between the Growth of the Primary Leaf and of the Coleoptile in Seedlings ofAvenaandTriticum

Abstract

Partial inhibition of extension growth of the primary leaf occurs when whole Triticum seedlings are immersed in aerated solutions of IAA but is replaced by growth promotion when sucrose is added to the external solution. In seedlings in which the coleoptile has been excised, IAA increases the growth of the leaf both with and without additional sucrose. Inhibition of the leaf by moderate concentrations of IAA no longer occurs when the seedling is detached from the endosperm. Sucrose added to the external solution raised the percentage elongation of the coleoptile almost to the level of that attained in intact seedlings without additional carbohydrate. It also enabled the leaf to show a positive growth response with IAA. The results indicate that in intact seedlings treated with IAA the growth of the primary leaf is markedly diminished owing to diversion of carbohydrate to the coleoptile if the growth of the latter is promoted as a result of the treatment. When the competition of the coleoptile for carbohydrate is diminished or eliminated, acceleration of the growth of the primary leaf by IAA becomes apparent. In addition to the endogenous rhythm, with a period close to 24 hours, induced in the growth-rate of the coleoptile when seedlings of Avena are transferred from red light to darkness, a similar rhythm, with a slightly longer period, is induced in the growth-rate of the primary leaf. This rhythm persists in elongating leaves so long as they remain within the coleoptile. It can be recorded for at least 100 hours in deseeded seedlings. When intact seedlings of Avena are immersed for one hour in relatively high concentrations of IAA and then transferred to distilled water for 18 hours, the elongation of the coleoptile is greater and the inhibition of the leaf is less than when they are transferred to humid air. Sections of the leaf of Triticum showed a slight increase in elongation in concentrations of IAA up to 5 mg./l., but no evidence was obtained that sections of leaf and coleoptile exert any.influence on each other's elongation when floated together on solutions of IAA.