Wheat plants grown under non-stress conditions at a dayhight temperature of 18/13�C under glasshouse conditions from head emergence to maturity showed a maximum accumulation of water-soluble, non-structural carbohydrates 20-25 days after anthesis. This storage was largely as fructans with the timing and amount of storage and mobilisation varying between cultivars, although the maximum concentration (fructose equivalents per unit stem fresh weight) was similar in all cultivars. The main storage in the culm was located in the lower part of the peduncle enclosed by the flag leaf sheath, in the penultimate internode and for one cultivar also in the flag leaf sheath. 14CO2 pulse-chase studies showed that there was a considerable delay in the incorporation of flag leaf assimilates into stem fructans, a delay probably associated with transfer and metabolic processes in the stem itself. At anthesis, when soluble carbohydrates were rapidly accumulating in the culm, the level of activity of sucrose synthase (SS) in the penultimate internode was much greater than that of sucrose phosphate synthase (SPS). The activity of SS declined rapidly as active storage ceased. This pattern was the reverse of that found in the leaf where SPS, rather than SS, was initially high and declined towards maturity. These changes are discussed in relation to the possible role of sucrose synthesising enzymes, particularly SS, in the accumulation and mobilisation of stem reserves in wheat.

Triticum aestivum (wheat) plants grown at a day : night temperature of 18 : 13mC from anthesis were held as well watered controls, or subject to either a mild (large pot volume) or a more severe (small pot volume) water stress by withholding water from the time of anthesis. Extracts from the peduncle (enclosed by the flag leaf sheath) and the penultimate internode were prepared to determine the activities of fructan exohydrolase and acid invertase and to assess the level of hexose sugars, sucrose and fructans. Measurements were made of ear and individual grain weights and stem fresh weight and dry weight. Plant water relations at the time of each sampling were determined as the flag leaf water potential and the water content of individual organs. Water stress resulted in a shorter duration of kernel filling, smaller kernels at maturity and an earlier loss of stem weight. There was an increase in stem fructose and a fall in fructan level that preceded the loss of dry matter associated with water stress. Coincident with the early fall in fructan content under water stress there was a rise in both fructan exohydrolase and acid invertase in the internodes of stressed plants. This correlation suggests that the conversion of fructans to fructose might have resulted from enzyme induction associated with water stress, but as this conversion occurs before the major export of reserves from the stem it might be only indirectly related to changes in the demand for reserves.

J, 1996, Sucrose accumulaeion in sweet sorghum stem internodes in relation tO ' growth, Sweet sorghum (Sorghum bicoior L, Moench) stems of different cultivars (NK 405, Keller and Tracy) reveal a different pattem of sucrose accumn]ation with respect to intemodal sugar content and distribution. The onset of sucrose storage is not necessarily associated with the reproductive stage of the plant, as was hitherto assumed, but obviously occurs after cessation of internodal elongation as was postulated for the sugarcane stem. For at least two of the three cultivars, ripening is an internode to internode process beginning at the lowermost culm parts. Intensive growth of the intemodes, combined with a high hexose content in stem parenchyma, shows a strong positive correlation (r > 0,94') to the activity of sucrose synthase (SaSy; EC 2,4,13), but not !0 invertase (EC 3,2,1,26) which is not present as soluble (neutral .and acid) or cell wallbound, salt-extractable enzyme in the three cultivars investigated. Sucrose synthase measured in sucrose cleavage and synthesis direction reveals divergent activity rates and sensitivity towards exogenously applied Mg"* ions and pH, SuSy activity' is connected to the increase of internodal sucrose content in so far as (I) its decline is a prerequisite for the onset of sucrose accumulation 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 all internodes. Only source leaves of sorghum show a considerable SPS activity, but neither stem nor leaf SPS reveal a positive correlation to the increase of internodal sucrose content. Sucrose phosphate phosphatase (SPP; EC 3.1.3,24) amounts to only 24-30% of the respective SPS activity bul follows the same distribution pattern. None of the enzymes under study proves to be responsible for the extent of sucrose storage in the stem, so other phenomena such as transport processes within the stem tissue require further investigation.

(This paper is dedicated to Professor Ludwig Bergmann on the occasion of his 70th birthday) Wheat plants were grown at a day\night temperature of 18\13 mC under glasshouse conditions. Twenty-two d after anthesis, one set of plants was shaded to 50 % of the normal photon fluence rate, another was ' degrained ' by selective spikelet removal which left only the grains in the five central spikelets ; a further set was left as control. Individual plants were harvested at days 22, 30 or 42 after anthesis. Extracts from the peduncle and the penultimate internode were prepared to determine the activities of sucrose phosphate synthase, sucrose synthase, fructan exohydrolase and acid invertase, and to assess the concentration of hexose sugars, sucrose and fructans. Measurements were also made of ear and individual grain weights, and stem f. wt and d. wt. There was a decline in the amount of fructans with time, more pronounced in ' shaded ' (source-limited) than in control plants. By contrast, in ' degrained ' (sink-limited) plants, the amount of fructans in the stem initially rose, then decreased, with a concomitant increase in the amount of fructose. The shifts in sugar content of the wheat culm reflected both the sink demand of the ear and source activity. The activity of fructan exohydrolase correlated with the carbohydrate changes. Under limited photosynthate assimilation, the mobilization of fructans from the internodes towards the ear was related to an increase in this enzyme, whereas the other enzymes played a less direct role in the mobilization of fructan reserves from the wheat stem.