Increased prolificacy and reduced barrenness have been identified as physiological traits in maize (Zea mays L.) hybrids that are tolerant of environmental stresses induced by high plant density. The objective of this study was to investigate ear and kernel formation under a range of plant densities in old and new hybrids. Experiments were carried out during 1987 and 1988 at two locations in Ontario with nine maize hybrids representing three decades of yield improvement in Ontario, grown at 2, 4, 8, 10, and 13 plants m−2. Dry matter was measured at 1 wk presiiking, 3 wk postsilking, and at physiological maturity and plant growth rates calculated for both periods. Grain yield, kernel weight, and kernel number were determined at physiological maturity. Kernels per plant and plant growth rate for the period from 1 wk presilking to 3 wk postsilking could be fitted to a discontinuous function of multiple rectangular hyperbolae; a single rectangular hyperbola was associated with the kernel number on each grain‐bearing ear. The first rectangular hyperbola for each hybrid had a positive x‐axis intercept, which was interpreted as the threshold plant growth rate for ear formation. The analysis showed that grain yield improvement of Ontario maize hybrids was associated with an increase in kernel number per plant and higher plant growth rates from 1 wk presilking to 3 wk postsiiking. The increase in kernels per plant was associated with an increase in ears per plant. The increased prolificacy of recent hybrids appeared to be the combined result of higher rates of plant dry matter accumulation during silking and a greater inherent tendency to exhibit prolificacy. The lower barrenness of recent hybrids at high plant densities was associated with higher plant growth rates, but was not the result of a lower threshold plant growth rate for ear formation.

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Temperatures greater than 27°C, in a growth cabinet, have resulted in floral sterility and yield loss in Brassica napus L. Maximum daily temperatures often exceed this in the major canola growing regions. Our objective was to examine the effects of heat stress during flowering on yield and yield components of B. napus, B. rapa L., and B. juncea (L.) Czernj. & Cosson. Cultivars of the three Brassica species were grown in a split‐plot design in the field for 3 yr at Ottawa, Canada. Three seeding dates were used each year to obtain different levels of heat stress during flowering. The number of flowers on the main raceme and the first and second branch racemes were counted at anthesis, from 10 plants per plot. At maturity, pods from the sampled plants were counted and seed numbers and weight determined. Two rows of the remainder of each plot was harvested for seed yield. A heat stress index was developed on the basis of the accumulation of daily maximum temperatures greater than a threshold temperature. The threshold temperature during flowering, which resulted in seed yield losses, was 29.5°C for all Brassica species. High mean maximum temperature during vegetative development resulted in a reduction in flower number for all Brassica species. Seed yield decreased as heat stress during flowering increased. The reduction in seed yield was primarily due to a reduction in flower number and in the number and size of the seeds produced per flower. Plant breeders should actively select for heat stress tolerance in future canola cultivars.

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