The relative increase in final kernel weight (SR, %) with an artificial reduction of about 80 % in the number of grains per spike was assumed to estimate the degree of post-anthesis assimilate or source limitation to grain weight in wheat. This assumption was supported by comparisons with other treatments designed to alter the ratio of source to grain number. SR was determined in crops grown under irrigation and high fertility in north-west Mexico over a 5 year period, in order to examine the effect of environment (year, sowing date) and cultivar on source limitation. Some old tall cultivars and many modern short cultivars of wheat (Triticum aestivum L., T. turgidum L.) and triticale ( X Tritosecale Wittmack) were studied.For modern cultivars, kernel weight always appeared to be limited by source, SR averaging 21 % over all years for eight such cultivars. For a given cultivar, environmental effects were considerable, being partly explained by a positive relationship between SR and mean temperature after anthesis. Each year SR was significantly affected by cultivar and, despite cultivar by year interaction, showed some consistency between years. Old bread wheats and triticales usually showed low source limitation (SR < 10 %), whereas most modern cultivars, regardless of species, showed higher source limitation (SR up to 50%) and higher grain yields, probably because of higher grain numbers per sq metre. Within this latter group, there was no relationship between SR and grain yield. The kernel weight attained as a result of grain number reduction, defined here as potential kernel weight, was a stable cultivar trait useful in understanding yield variation.

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There is a need to unravel mechanisms controlling elongation and submergence tolerance in rice (Oryza sativa L.) and to develop technology for improving rice for deepwater regions. Because of the reported involvement of ethylene in stem internode elongation of deepwater rice, we examined the relationship of seedling elongation to ethylene production. Fourteen rice cultivars/breeding lines with widely ranging differences in elongating capacity in deepwater were used in these studies. Ethylene production by intact 20 to 27‐day‐old seedlings was determined by submergence in 122 ✕ 6‐cm glass cylinders and related to leaf blade/sheath elongation. The floating rice, ‘TCA 177’, elongated largely by the leaf sheath supporting the youngest leaf. The elongating capacity of the rice seedling resided in the top two leaf blades and their sheaths. Elongation decreased with the duration of submergence and after 72 h little increase occurred. A 24‐h submergence enhanced the capacity of the seedlings to produce ethylene and the production was highest during the first hour after removal from water. Complete submergence caused greater production of ethylene than partial submergence. The capacity for ethylene production and elongation peaked at 24 to 36 h of complete submergence and then declined. Ethylene production following submergence generally followed the order: floating rice > deepwater rice > elongating modern cultivar > nonelongating modern cultivar. Elongation index correlated well (r = 0.88**, significant at the 0.01 probability level) with ethylene producing capacity of seedlings in a population of 14 rice cultivars/breeding lines. The results show that ethylene production by young rice seedlings, following complete submergence, is a good measure of their elongating ability.

Enhanced ethylene‐producing capacity of young seedlings of floating and deepwater rice (Oryza sativa L.) during submergence has been related to increased seedling elongation via leaf sheath/leaf blade. In this study we present evidence for modulation of elongation by hormones. Floating rice cuitivars ‘TCA 4’, ‘TCA 177’, and ‘TCA 212’ were used. Effects of various hormones and chemicals on elongation and ethylene‐producing capacity were determined during complete submergence of seedlings in 122‐ ✕ 6‐cm glass cylinders. Enhancement in ethylene‐producing capacity by submergence was completely suppressed by 0.05 mM aminoethoxyvinylglycine (AVG), indicating that the enhancement depended on the synthesis of 1‐ aminocyclopropane‐l‐carboxylic acid (ACC). Submergence‐induced elongation was partially inhibited by 0.05 mM AVG in 24 h and reversed to some extent by 0.1 mM ACC or 0.05 mM ethephon (2‐chlorethane phosphonic acid). Increase in ethylene‐producing capacity during submergence was also inhibited by 1 mM Co2+, which had no effect on seedling elongation. Gibberellin (GA) biosynthesis inhibitor, tetcyclacis [5‐(4‐chlorophenyl)‐3,4,5,9,10‐pentaazatetracyclo‐5,4,102.6, 08.11 dodeca‐3, 9‐diene] (TCY), strongly inhibited the increase in seedling elongation of TCA 177 without affecting ethylene‐ producing capacity. The effect of TCY on elongation was reversed by 0.1 mM GA4+7 (gibberellin A4 + A7). Abscisic acid (ABA) inhibited the ethylene‐producing capacity and the effect was reversed by kinetin. Application of GA4+7 plus kinetin caused a 60% reversal of ABA inhibited elongation. Nonsubmerged floating (TCA 177) and deepwater (‘BR 118‐3B‐17’) rice elongated to a greater extent than the nonelongating types when sprayed with 0.1 mM GA4+7. Thus, GA may influence elongation more directly than other hormones.

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