SUCROSE ACCUMULATION IN SWEET SORGHUM STEM INTERNODES IN RELATION TO GROWTH

Sweet sorghum (Sorghum bicolor L. Moench) stems of different cultivars (NK 405, Keller and Tracy) reveal a different pattern of sucrose accu mulation with respect to internodal sugar content and distribution. Th e onset of sucrose storage is not necessarily associated with the repr oductive stage of the plant, as was hitherto assumed, but obviously oc curs after cessation of internodal elongation as was postulated for th e sugar-cane stem. For at least two of the three cultivars, ripening i s an internode to internode process beginning at the lowermost culm pa rts. Intensive growth of the internodes, combined with a high hexose c ontent in stem parenchyma, shows a strong positive correlation (r grea ter than or equal to 0.94) to the activity of sucrose synthase (SuSy; EC 2.4.13), but not to invertase (EC 3.2.1.26) which is not present as soluble (neutral and acid) or cell wall-bound, salt-extractable enzym e in the three cultivars investigated. Sucrose synthase measured in su crose cleavage and synthesis direction reveals divergent activity rate s and sensitivity towards exogenously applied Mg2+ ions and pH. SuSy a ctivity is connected to the increase of internodal sucrose content in so far as (1) its decline is a prerequisite for the onset of sucrose a ccumulation and (2) it remains at a constant low level during sucrose storage. Sucrose phosphate synthase (SPS; EC 2.4.1.14) activity in the sorghum stem is low compared to SuSy and uniformly distributed over a ll internodes. Only source leaves of sorghum show a considerable SPS a ctivity, but neither stem nor leaf SPS reveal a positive correlation t o the increase of internodal sucrose content. Sucrose phosphate phosph atase (SPP; EC 3.1.3.24) amounts to only 24-30% of the respective SPS activity but follows the same distribution pattern. None of the enzyme s under study proves to be responsible for the extent of sucrose stora ge in the stem, so other phenomena such as transport processes within the stem tissue require further investigation.