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Phloem unloading following reactivation in predarkened mature maize zea mays cultivar prior leaves



Phloem unloading following reactivation in predarkened mature maize zea mays cultivar prior leaves



Planta (Heidelberg) 161(2): 113-119



Mature leaf blades of 48-h predarkened maize plants were excised, and treated apically as the source (light, normal air) and basally as the sink (light or dark, air without CO2). After providing the source portion with 14CO2, the sink portions were harvested after 2, 7 or 14 h by freezing with liquid N, grinding and freeze-drying. Extracts, fractionated by ion-exchanged resins into neutral, basic and acid fractions, were chromatographed on thin cellulose layers and autoradiographed. Identification of labeled compounds was carried out by co-chromatography with authentic labeled substances. Activities of enzymes pertaining to the metabolism of sucrose were checked. Results show that the source supplies sucrose to the sink, where it is unloaded and metabolized by acid invertase (EC 3.2.1.26) in both the light and the dark. Starch appearing in the sink only in the light, after 7 h of re-illumination, yields labeled glucose upon hydrolysis. Although sucrose-phosphate synthetase (EC 2.4.1.14) is active in sinks and in isolated vascular-bundle fragments, whether sucrose unloaded from sieve tubes is metabolized by a method other than inversion remains questionable. Sucrose synthetase (EC 2.4.1.13) was inactive. The main metabolite of unloaded sucrose is G-6-P, giving access to the glycolytic pathway. The main difference between the sinks in the light and the dark is the lack of labeled glycine and serine in the dark. In the light decarboxylation of glycine yields CO2, which is recycled photosynthetically.

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