Ontogenetic Variation of Four Cytokinins in Soybean Root Pressure Exudate

Abstract

No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction).The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.The roles of cytokinins in plant growth and development are not fully understood, but evidence supports the hypothesis that cytokinins are produced by the root system and transported to the shoot, where they are involved in the regulation of shoot processes (3,22,27). Transport in the xylem sap of materials with cytokininlike activity (determined by bioassay) has been demonstrated in numerous species including sunflower (22), tomato (7), and lupin (8, 9). Cytokinin-like materials have also been reported in the root nodules of Viciafaba (13) nodules, they detected Z2 and a number ofits metabolites throughout the plant.If cytokinins from the root system are involved in regulating shoot growth and function, then their supply to the shoot should fluctuate over the course of development. Seasonal variations of cytokinin-like activity in root pressure exudate have been reported in a number of genera (2, 3, 7-9, 22, 24). In Perilla, there was a 5-fold increase in cytokinin-like activity in root pressure exudate after floral induction (2, 3). Conversely, the root sap of Xanthium plants which had been induced to flower contained less than onethird as much cytokinin-like activity as did the sap of vegetative plants (24).Although cytokinins were not specifically identified as the active regulating component, several studies have shown that roots or factors from the roots are required for maintaining leaf Chl content (18) and photosynthesis (4,5,18,27). In Phaseolus, foliar applications of BA delayed Chl loss and photosynthetic decline associated with senescence (1).In A. glutinosa, cytokinin-like activity in the root nodules increased prior to the onset of nitrogenase activity (acetylene reduction) and declined by the time N2-fixing capacity had been fully established (14). Likewise, Newcomb et al. demonstrated that cytokinin-like activity in pea nodules was greatest early in nodule development and decreased thereafter (20). Henson and Wheeler hypothesized that cytokinins exported from nodules affected N2-fixation through either local action in the nodule or by a remote effect on the shoot (14).Presence of cytokinins in soybean root pressure exudate has not been demonstrated. Therefore, it was not known whether or not these hormones could be involved in the regulation of soybean development. Hence, the objectives of our research were to (a) establish the presence of cytokinins and (b) describe profiles of cytokinin flux in root pressure exudate in relation to the ontogeny of soybean plants grown in the field and in growth chambers. Root pressure exudate was collected and acetylene reduction activity was measured from late in vegetative growth through maturity. Four cytokinin...

Changes in Cytokinin Activities and Mass Spectrometric Analysis of Cytokinins in Root Exudates of Rice Plant (Oryza sativa L.)

“…Such a phenomenon, i.e. a low cytokinin content in exudate at the early reproductive stage, has been observed in rice (23) and soybean (8). After heading, the cytokinin activities declined as leaves senesced and grains filled.…”

“…As such, the exudation rate and cytokinin biosynthesis in the root may be controlled by different systems. In fact, lack of similarities between exudation rate and cytokinin flux has been shown in tomato (3), lupine (4,5), sunflower (18), and soybean (8). Hence, we presented our results as amount of cytokinin per plant per 12 h and not as concentration.…”

Effects of Source/Sink Ratio and Cytokinin Application on Pod Set in Soybean

“…Liu et al (2003) found that the reduced pod-set percentage induced by water defi cit was associated with an increase in ABA content of reproductive structures during a period from 3 to 5 days after anthesis. Endogenous cytokinins detected in exudate showed a maximum concentration during a period from 0 to 9 days after anthesis, when most fl owers are destined to either initiate or abort pods (Heindl et al, 1982;. The application of BA to racemes reduced abortion and increased pod number (Crosby et al, 1981;Carlson et al, 1987;Dyer et al, 1987;Peterson et al, 1990;Mosjidis et al, 1993;Reese et al, 1995;Nagel et al, 2001).…”

“…By reducing this abortion, we might be able to increase pod and seed numbers, thereby increasing the yield (Kokubun, 2004). Many previous studies have focused on two possible factors that might be involved in regulating the abortion; availability of photoassimilate or nutrients (Brevedan et al, 1978;Antos and Wiebold, 1984;Brun and Betts, 1984;Heitholt, 1986aHeitholt, , 1986b and quantity of certain hormones (Huff and Dybing, 1980;Heindl et al, 1982;Spollen et al, 1986aSpollen et al, , 1986bCarlson et al, 1987;Yarrow et al, 1988;Kokubun and Honda, 2000).…”

Cytokinin Biochemistry in Relation to Leaf Senescence

“…They concluded the developing seeds do not accumulate significant proportions of these cytokinins. The principal xylem cytokinins in soybean appear to be ZR and DZR (8). Since the latter is more stable than ZR (12), it is particularly important to assess its translocation pattern and metabolism using the explant system.…”