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The ability of mepiquat chloride and chlormequat growth retardants to modify grain yield components and stem length in two contrasting spring barley cultivars was evaluated in 2 years of small scale field experiments in central southern England. Interactions with plant population density and sowing date were studied.The mid-tall cv. Koru was more responsive than the shorter high tillering ' erectoides' cv. Goldmarker. Early treatments of mepiquat chloride (applied before the start of main shoot stem elongation) on Koru raised grain yield/plant in both years. Early treatments with chlormequat and later applications (during main shoot stem elongation) of either retardant were largely ineffective. Yield increases were achieved by raising the number of spikes/plant along with the number of grains/spike and the above-ground biomass in certain later-formed, lower ranking shoots.A medium plant population density (250 plants/m 2 ) allowed the best response to early treatment with mepiquat chloride, as did a normally-timed sowing in contrast to a late one. Retardant-induced improvements in yield components in Koru were positively correlated with plant height, the result of over-compensation in the length of upper internodes.It appears that effective treatments reduced the dominance of the main shoot during tillering, allowing greater initiation and survival of florets (grain sites) in smaller shoots.

Fourteen cultivars of north temperate, Australian, Mexican and Rhodesian origins, were grown in eight treatments: 2-day-lengths (10 h and 14 h) x two temperature regimes (18/13 "C and 25/20 "C) x two seed vernalization treatments (unvernalized and 28 days at 1-2 "C). Numbers of days to 50 yo ear emergence, leaf numbers and spikelet numbers of the main shoots were recorded.The north temperate cultivars were the most sensitive to daylength:in long days their mean spikelet number was 15 and they headed in 40-50 days, while in short days they had a mean of 24 spikelets and failed to head in IOO days. The Rhodesian cultivars were the least sensitive, but nevertheless headed 20-30 days earlier and had three to five fewer spikelets per ear in long days. The effects of vernalization on the tropical cultivars were related to maturity class: they ranged from negligible in early cultivars such as Sonora 64 and Devuli to decreases of 20 days to heading and seven spikelets in late cultivars such as Mexico 120 and Cajeme 71 in long days.The effects of temperature varied with cultivar and with vernalization treatment. Early cultivars and vernalized late cultivars headed earlier and had fewer spikelets in the warmer regimes while unvernalized late cultivars tended to have more spikelets and headed later. It is suggested that some vernalization of these late cultivars took place in the cooler regime.The significance of the results for the understanding of the physiological basis of the adaptation of wheat cultivars to different climatic zones is discussed.