Effects of the environment on shoot survival were studied in winter wheat cv. Avalon grown in microplots at a density of 247 plants m-². The incident radiation and mean temperature were altered during one of three periods of between 14 and 29 d duration, the first (P1) starting at the end of tiller production and the last (P3) finishing near the end of the tiller death phase, about three weeks before anthesis. Plants were given temperature and radiation treatments in growth rooms in two experiments and extra light outdoors in a third experiment: they were at other times grown outdoors. Increasing radiation by between 60 and 100 per cent during P1 had negligible effects on shoot number; during P2 it always delayed tiller death but increased final ear number in only one experiment; during P3 it consistently increased ear number by up to 100m-². Increased radiation always increased crop dry weight immediately after treatment but only sometimes did this increase persist to maturity. Grain dry weight was increased by treatment during P3 of one experiment. Increasing the temperature by 4°C decreased shoot number, usually only temporarily, by hastening death of some tillers. Warmer temperatures decreased crop growth after, but not during, treatment and decreased grain dry weight. Radiation and temperature rarely interacted. Variation in grain yield within and between experiments related well to variation in number of grains m-², which in turn related to variation in ear dry weight at anthesis.
Agronomic characteristics were studied in an R plant population, derived by self-pollination from seventy regenerant plants (R ), derived from immature embryo culture of Norstar, an important Canadian winter wheat cultivar. A total of 1,800 R space planted progeny, representing all 70 regenerant families, were analyzed in field trials during 1985. Significant variation compared to Norstar was observed, among families, for heading date, plant height, hundred kernel weight, tiller number, seed yield and fertility. Variation was also significant within families for all dependent variables except plant height and hundred kernel weight. While much of the variation observed was of negative value compared with the Norstar control, some lines exceeded the control for most of the variables examined. Mitotic and meiotic analyses revealed that at least 65% of the total population appeared cytologically normal. Some, but not most variants were associated with cytogenetic abnormalities.