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SUMMARY The effects of foliar and root applied 6‐benzylamino‐9‐(tetrahydropyran‐2‐yl)‐9H‐purine (PBA), 6‐benzylamino‐purine (BAP), indole‐3‐acetic acid (IAA) or gibberellic acid (GA3) on tiller bud growth were examined in cereal seedlings. Generally IAA and GA3 inhibited tiller bud growth whilst the synthetic cytokinins both markedly promoted growth. Some observations on stem and root growth were also recorded. The results are discussed with respect to the diversion of both assimilates and cytokinins and appear to favour the view that tiller bud development is controlled by a competition for assimilates with the roots and main shoot.

The post‐emergence herbicide isopropyl (±)‐2‐[N‐(3‐chloro‐4‐fluorophenyl)benzamido]propionate (flamprop‐isopropyl) showed good activity against wild oat with selectivity in barley. The basis for activity and selectivity was similar to that previously established for benzoylprop‐ethyl, and found to be dependent on its rate of degradation to the biologically active acid flamprop. Flamprop stunted the growth of the oat by inhibiting cell elongation and showed a relatively high rate of movement in the phloem, approximately five times that of benzoylprop. Selectivity of flamprop‐isopropyl was dependent on its relative rate of hydrolysis and the subsequent detoxication of the acid to inactive conjugates. However, although the relative rate of de‐esterification of flamprop‐isopropyl was lower than that of benzoylprop‐ethyl similar quantities of the parent ester gave comparable effects on oat. The inherent activity of flamprop is approximately twice that of benzoylprop. The effect of flamprop‐isopropyl was best seen when the compound was applied during growth stages when the crop could offer the most effective competition to the wild oat. Throughout a range of growth stages the rate of hydrolysis of flamprop‐isopropyl was higher in oat than in barley. The metabolism of the compound was not markedly affected when the plants were under stress.

The mechanism of temperature regulation of endosperm development has been investigated by studying the effects of two temperature regimes on starch deposition and starch synthase activity during grain development in two cultivars of wheat. Most of the starch synthase activity was present throughout development as the granule-bound form using ADPglucose as the principal substrate. That starch synthase may be a rate-limiting enzyme for accumulation of starch, and hence dry weight, is suggested by: (1) rates are proportionately less in the cultivar with the lower final endosperm dry weight; (2) at elevated temperatures when starch content and dry weight are reduced, starch synthase activity falls; (3) the rate of starch deposition calculated to be possible from measured rates of starch synthase activity is close to the observed rate of starch deposition. On the other hand, it was concluded that it is not lack of starch synthase activity that causes termination of starch deposition, since activity is maintained well after starch deposition has ceased. Using the same two wheat cultivars, grown as detached ears in liquid culture, the effects of reduced endosperm water content, induced by the presence of polyethylene glycol in the culture medium, were investigated. Endosperm starch synthase activity was unaffected but ADPglucose pyrophosphorylase activity was greatly reduced, suggesting a possible role in the termination of starch synthesis.