ABSTRACIThe effect of temperature on accumulation of starch was studied in grain slices of wheat (Titicum aestivum cv SUN9E), taken 15 days after anthesis. As compared with pretreatment of such slices at 25°C, pretreatment at 30 or 35C reduced the subsequent conversion of sucrose to starch. In contrast to rice (Oryza sativa cv Calrose), pretreatment of wheat soluble starch synthase in vitro at 30°C or higher temperatures reduced its activity. In zymograms using nondenaturing polyacrylamide gel electrophoresis followed by activity staining, the slowest mirating band represented the most temperature sensitive isozyme. Although preincubation of a soluble enzyme sample in vitro at 25°C did not result in loss of starch synthase activity, it did result in a gradual shift of zymogram banding pattern toward faster migrating species. Pretreatment of isolated starch granules at 40°C increased their bound starch synthase activity. Both soluble and bound enzymes in the grains of whole wheat plants lost activity when the plants were held above 30°C for 30 minutes or longer. Both activities lost from the grains after a 1 hour treatment at 37°C were restored in 1 to 2 days by a return to 21°C. In slices, inactivation of the soluble starch synthase was increased by incubation with 2,4-dinitrophenol. It is tentatively suggested that in vivo heat inactivation of soluble starch synthase may be a direct effect of heat on the enzyme protein and that of bound enzyme an indirect effect involving metabolic factors.In wheat the optimum temperature, of plant culture for dry weight per grain at maturity occurs at a day/night temperature of 15°C/10°C (4). Increasing the temperature of the ear independent of the rest of the plant from 15 to 20°C and 25C has resulted in an increased growth rate of the ear but this was accompanied by earlier senescence and a shorter duration of growth (6). Ear warming at 33°C/25°C (day/night) in comparison with 2 1C/16°C, resulted in a reduction ofgrain growth rate after the 6 d warming period (1). From these data, it would be expected that at a suitably high temperature such a reduction in rate could be observed earlier and analyzed more directly. Also it has been suggested that for maize kernels grown in vitro, the adverse effects of high temperature on starch synthesis during the grain-fill period were due to deficiencies in the synthetic process rather than in the supply of substrate (8)

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Abstract. Starch synthesis by developing wheat endosperm slices incubated in liquid media was more rapid, at optimum concentration, from sucrose as external substrate than from glucose and/or fructose. Fructose inhibited conversion of sucrose or glucose. The results are consistent with the hypothesis that sucrose is not hydrolysed in the apoplast before uptake. Besides a diffusional influx and efflux of labelled sucrose there was a non‐diffusional influx; it was inhibited by dinitrophenol, potassium arsenate, potassium iodide, and parachloromercuribenzene sulphonate (PCMBS). PCMBS inhibited both uptake and conversion of label from 150 molm−314C‐sucrose by 75%. Uptake and conversion of sucrose were stimulated by lowering pH and by fusicoccin, a promoter of proton extrusion. Extracellular solutes like raffinosc and polyethylene glycol stimulated net uptake of label from 14C‐sucrose — the larger molecule being more effective — this being due to a non‐specific inhibition of diffusional efflux. At too high an osmotic concentration such solutes reduced net uptake; the larger the molecule the lower this transitional concentration. In conclusion, wheat endosperm is better equipped to convert apoplastic sucrose rather than the hydrolysis products to starch; active loading of sucrose possibly involves proton co‐transport; and large molecules in the extracellular solution reduce the diffusional elllux of loaded substrate.

As shown below, isolated fenugreek (Trigonella foenum graecum L.) cotyledons respond readily to cytokinins with increased expansion. Similar responses have been described for isolated cotyledons of spinach (4) and radish (5) and for sections of Xanthiwn cotyledons (3). A comparison shows that the fenugreek cotyledons exhibit a higher degree of specificity as they do not respond to GA. The findings may lead to an alternative bioassay of cytokinins. The use of cotyledons has a number of advantages from the point of simplicity in laboratory practice. Also cotyledons exhibiting cytokinin specific responses may be used to investigate the biochemical mechanism of cytokinin action as we report elsewhere (8). Here we describe the expansion of isolated cotyledons as affected by some growth substances, light, nitrogenous compounds, sucrose, and dormancy status. As in other Trifolieae, the attached cotyledons do not merely function as a reserve store, but expand and become green in light.The cotyledons can be separated from the embryonic axis by a cut through the dry seed as the space between these parts is indicated by a groove in the seed exterior. Further manipulationssterilization and washing of cut seeds, their transfer onto wet filter paper in Petri dishes, removal of seed coats 20 hr later, weighing and layering of cotyledons-were done in green safe light. Usually five cotyledons were arranged in a 7-cm Petri dish containing one filter paper disc to which was added 3 ml of test solution containing 1 mg each of mycostatin (nystatin), penicillin, and streptomycin per 50 ml while four to six replicates constituted one treatment. For light treatments the dishes were exposed to continuous incandescent light (30 ft-c near dishes) at 25 C. The dark treatments were enclosed in light tight boxes at the same temperature. CO2 treatment was given by incubating an uncovered Petri dish containing the cut seeds during the initial 20-hr soaking period for 18 hr in a desiccator in which CO2 was released from sodium bicarbonate by hydrochloric acid.To minimize variability due to differences in initial size of cotyledons, results have been expressed as relative increases (%) in fresh weight. Thus, within one experiment equal kinetininduced expansion was observed for samples of seed differing close to 100% in average air dry weight.