Enhancement of [¹⁴C]Sucrose Export from Source Leaves of Vicia faba by Gibberellic Acid

Abstract

The effect of gibberellic acid (GA3) on sucrose export from source leaves was studied in broad bean (Viciafaba L.) plants trimmed of all but one source and one sink leaf. GA3 (10 micromolar) applied to the source leaf, enhanced export of ['4qsucrose (generated by '4C02 fixation) to the root and to the sink leaf. Enhanced export was observed with GA treatments as short as 35 minutes. When GA3 was applied 24 hours prior to the '4CO2 pulse, the enhancement of sucrose transport toward the root was abolished but transport toward the upper sink leaf was unchanged. The enhanced sucrose export was not due to increased photosynthetic rate or to changes in the starch/sucrose ratio within the source leaf; rather, GA3 increased the proportion of sucrose exported. After a 10-min exposure to 1'4qGA,, radioactivity was found only in the source leaf. Following a 2 hour exposure to I'4qGA3, radioactivity was distributed along the entire stem and was present in both the roots and sink leaf.Extraction and partitioning of GA metabolites by thin layer chromatography indicated that there was a decline in I'4CjGA3 in the lower stem and root, but not in the upper stem. This pattern of metabolism is consistent with the disappearance of the GA3 effect in the lower stem with time after treatment. We conclude that in the short term, GA3 enhances assimilate export from source leaves by increasing phloem loading. In the long term (24 hours), the effect of GA3 is outside the source leaf. GA3 accumulates in the apical region resulting in enhanced growth and thus greater sink strength. Conversely, GA3 is rapidly metabolized in the lower stem thus attenuating any GA effect. and more direct short term effects on transport and carbon metabolism.Based on the evidence in the literature, it is believed that GA and IAA enhance and ABA inhibits phloem loading of sucrose (4, 5). Gibberellin and kinetin influence assimilate transport at the site of hormone application (11,(13)(14)(15) With the exception of work by Daie et al. (4), most evidence on the hormonal regulation ofphloem loading has been obtained in vitro. In the present study we investigated the effect of GA3 on movement of 14C assimilates from the source leaf to remote sinks (root and young sink leaves) in vivo. The results indicate that a short-term effect ofGA3 may be enhanced phloem loading.The observation is consistent with our combined in vitro and in vivo studies in celery (5), supporting a direct role for GA and IAA in phloem loading. MATERIALS AND METHODSHormones are believed to play an important role in the regulation of assimilate partitioning and translocation in plants (2, 11, 12 and references therein). In the source-path-sink continuum, phloem loading of organic solutes is a crucial step subject to regulation by phytohormones (3,13,16). However, the mechanism by which each hormone influences these processes is not well understood. In general, a distinction must be made between the indirect effect ofhormones through their influence on growth Plant Material. Broad bea...

Regulation of Apoplastic pH in Source Leaves of Vicia faba by Gibberellic Acid

“…GA modification of H+-ATPase is in general agreement with the reported observations that fusicoccin, IAA, and GA3 increase 6 ['4C]sucrose uptake in leaf discs (7,10) and isolated phloem segments (1). However, it is in contrast to the observed GA3 enhancement of the H+-ATPase activity in isolated phloem segments of celery (4).…”

“…GA3 would attenuate wound-induced proton extrusion. Considering data of Daie (4) and Aloni et al (1) and if proton extrusion is not related to a wound response, it is tempting to speculate that GA3 effects in source leaves are related to generating conditions 369 that are optimal for sucrose efflux from mesophyll cells and sucrose influx into the sieve element/companion cells.…”

Regulation of Key Enzymes of Sucrose Biosynthesis in Soybean Leaves

“…It is important to note that the carbon assimilation rate remained unchanged in response to GA treatment (data not shown [6]). The decrease in sucrose levels 2 h following GA treatment could be explained in two ways: the simultaneous enhancement of acid invertase activity 2 h following GA application (6) or the increase in sucrose export from the leaf (1,5).…”

“…GAs and ABA have been reported to influence carbohydrate transport out of leaves of tomato and bean plants (1,5). There have also been indications that, in excised leaf systems of beans and sugar beet, ABA and GA3 modified the activity of SPS (7).…”

Effect of Macronutrients and Gibberellic Acid on Photosynthetic Machinery, Nitrogen-Fixation, Cell Metabolites and Seed Yield of Chickpea (Cicer arietinum L.)

“…As mentioned earlier, it plays role in cell division and cell enlargement [25][26][27][28][29][30] resulting in proper development of under-developed pods especially at the terminal end of branches; P N [31,32] supplying sufficient C skeleton; and membrane permeability and transport of photosynthates [83][84][85] favoring partitioning hence higher values for the yield parameters of treated plants. These results broadly corroborate the findings of, [86], and Shah & Samiullah.…”