The Contents of Auxins and Cytokinins in Pea Internodes as Related to the Growth of Lateral Buds

Abstract

Hormonal control exerted by the apical leaves of different age over the outgrowth of the 1st and 2nd lateral buds was studied in pea seedlings. The endogenous phytohormones in the 1st and in the 2nd internodes and basipetal transport ofindole-3-acetic acid (IAA) and cytokinins were examined in 8-10-day-old plants. Three kinds of plants were under study, completely intact ones and plants that had been either decapitated or partially defoliated on the 7th day of growth. In the internodes, decapitation and partial defoliation led to the following changes (as compared with intact plants): slightly decreased levels of abscisic acid (ABA), unchanged levels of gibberellins (GAs), lowered levels of IAA (up to 2-fold), and significantly enhanced levels of zeatin/zeatin riboside (Z/ZR) (up to 5-fold). The extent of lateral bud inhibition corresponded well to the IAA to Z/ZR ratio in the subtending internode, thus supporting the concept of apical dominance control by the ratio of auxin to cytokinin in stem. The results evidence that apical leaves, the source of basipetally transported IAA, up-regulate IAA contents in internodes, but down-regulate Z/ZR content, which is most likely determined by IAA-induced inhibition of xylem transport of cytokinins from the root (Bangerth, 1994). We found that the estimated levels of ZR phloem transport inversely correlated with the levels of IAA transport. It was proposed that phloem ZR is evidently a xylem ZR reexported via leaves; the latter was shown to be regulated by IAA (Bangerth, 1994). It was found, however, that irrespective of the amount of IAA transported from leaves to roots, the older leafwas responsible for the lower rate of phloem basipetal transport of ZR and lower level of Z/ZR in internodes. These data argue that the apical leaves can additionally regulate the cytokinin status of stem tissues through reexport ofZR depending on the leaf age. Both IAA and Z/ZR contents in internodes were found to be affected by the age and the position of internodes themselves. Possible roles of endogenous ABA and GAs in regulation of IAA and Z/ZR contents in internode tissues are discussed.