The contents of auxins and cytokinins in pea internodes as related to the growth of lateral buds

Hormonal control exerted by the apical leaves of different: age over the ou tgrowth of the 1st and 2nd lateral buds was studied in pea seedlings. The e ndogenous phytohormones in the 1st and in the 2nd internodes and basipetal transport of indole-3-acetic acid (IAA) and cytokinins were examined in 8-1 0-day-old plants. Three kinds of plants were under study, completely intact ones and plants that had been either decapitated or partially defoliated o n the 7th day of growth. In the internodes, decapitation and partial defoli ation led to the following changes las compared with intact plants): slight ly decreased levels of abscisic acid (ABA), unchanged levels of gibberellin s (GAs), lowered levels of IAA (up to 2-fold), and significantly enhanced l evels of zeatin/zeatin riboside (Z/ZR) (up to 5-fold). The extent of latera l bud inhibition corresponded well to the IAA to Z/ZR ratio in the subtendi ng internode, thus supporting the concept of apical dominance control by th e ratio of auxin to cytokinin in stem. The results evidence that apical lea ves, the source of basipetally transported IAA, up-regulate IAA contents in internodes, but down-regulate Z/ZR content, which is most likely determine d 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 tha t phloem ZR is evidently a xylem ZR reexported via leaves; the latter was s hown to be regulated by IAA (Bangerth, 1994). It was found, however, that i rrespective of the amount of IAA transported from leaves to roots, the olde r leaf was responsible for the lower rate of phloem basipetal transport of ZR and lower lever of Z/ZR in internodes. These data argue that the apical leaves can additionally regulate the cytokinin status of stem tissues throu gh reexport of ZR 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 interno des themselves, possible roles of endogenous ABA and GAs in regulation of I AA and Z/ZR contents in internode tissues are discussed.