Cytokinin dynamics in differently senescing laminae of Phragmites australis plants grown in different habitats

Abstract

Common reed is a cosmopolitan species, which occurs also in South Africa. At a bay of the Baltic Sea stands of tall, large-leafed reed (TR) and stands of short, small-leafed reed (SR) were found. While TR grew where deep soil with much humus and phosphate was available, SR was found growing on shallow soil, which was often of relatively high salinity. As changes in coloration and chlorophyll analyses showed, SR laminae senesced earlier than did TR laminae. The differences in growth and senescence were eliminated by transplanting TR and SR into garden earth. As literature data from other species show, soil components like phosphate and NaCl can influence levels of the antisenescence hormones cytokinins. Our aim was to find correlations between cytokinin levels and the vitality in reed laminae from natural sites. We used ELISAs for cytokinin analysis and completed the investigations by LC-MS determination of 17 cytokinins and cytokinin conjugates in laminae of HR and SR grown near Prague. From July to autumn the content of trans-zeatin, dihydrozeatin and their 9-ribosides decreased in third laminae under panicles at TR on an average to 1.3nmol trans-zeatin-9-riboside equivalents kg−1 fresh mass and at SR to 0.7 (8 sample pairs). In autumn SR laminae had also lower levels of N6-(Δ2-isopentenyl)adenine and its 9-riboside and probably also of trans-zeatin-O-glucoside and cis-zeatin-9-riboside-O-glucoside compared to TR laminae. The putatively inactive or weakly active cis-zeatin compound increased during the summer to very high quantities. After isolation of laminae, the content of cytokinins declined and cytokinin application delayed yellowing. Contrary to some other examples, our results are in concordance with the rule that endogenous cytokinins retard leaf senescence and help thus to explain the senescence differences in TR and SR.