The effects of age and size of seed on seedling vigor, rate of respiration, and 2,3,5‐triphenyl‐2H‐tetrazolium chloride (TTC) reduction by germinating seeds were studied in the laboratory. Wheat seed (Triticum aestivum ssp. compactum (Host) MacKey ‘Elmar’) was used. Stratification was necessary to eliminate the effect of dormancy in the youngest seed class. Significant positive correlations were found between rate of emergence and vigor (ability to emerge from deep seeding), emergence and rate of respiration, and between vigor and both tetrazolium reduction and rate of respiration for seed of different ages. With different seed size classes, significant positive correlations were found between seed weight and tetrazolium reduction, and between shoot weight per plot and tetrazolium reduction. In a preliminary test, varietal differences in seedling vigor, which were largely due to relative coleoptile elongation, were not correlated with TTC reduction. The possible place of TTC reduction as an empirical seedling vigor test and respiration of germinating seeds as a component of vigor are discussed.

Plant populations of American pima cotton (Gossypium barbadenseL.) and upland cotton (G. hirsutumL.) giving maximum lint yields vary widely in reports in the literature. This study attempts to explain the wide variations in plant populations giving maximum lint yield for pima cotton by considering the plant population X plant height interactions with lint yield. Field tests were conducted for 5 yr at four locations on Anthropic Torrifluvents, Torrifluventic Haplostolls, and Typic Torrifluvents soils. Fifteen pima cotton tests with varied plant populations, and with 1.02‐m row spacing, had plant population, plant height, and lint yield measured on each plot. The relationships among yield, population, and height were analyzed by multiple linear regression in each test. In three of the 15 individual test analyses all regression components were significant. A combined analysis of the 15 tests showed a significant negative (P=0.001) plant population × plant height interaction for lint yield. Most of the plant population and plant height treatment means in these tests were within the range of 20 000 to 200 000 plants ha −1and 60 to 190 cm high. Plant population giving maximum yield decreased about 11 000 plants ha −1for each 10‐cm increase in plant height. Cultivars and irrigation treatments did not greatly alter the interaction. Year and geographical location effects were more varied. The variation in plant population producing maximum lint yield for different plant heights was great enough to explain the varied plant populations for maximum lint yield reported in the literature.

Plant population effects on yield and growth of irrigated and non‐irrigated castorbeans (Ricinus communis L.) was tested for 3 years. The optimum plant population of irrigated dwarf‐internode castorbeans was about 58,000 plants per ha. Row spacings from 0.5 m to 1.0 m did not significantly affect yields, if plant population was held constant. The optimum plant population for nonirrigated castorbeans was near or higher than the highest population in our tests. It was clearly higher than the current recommendation of 20,000 plants per ha. Linear regression of yield on plant population was significantly different among cultivars and among tests for the nonirrigated castorbeans. Cultivars with bushy growth and small racemes had greater yield depression from low populations than did more erect cultivars with large racemes. Plant population effect on yield was reduced as the yield level of the nonirrigated tests decreased. Low plant population in nonirrigated tests increased plant height, hypocotyl diameter, length of primary raceme, number of racemes per sequential set, and yield of all sets of racemes. Low population reduced height of the primary raceme from the soil. Population had no effect on nodes to the primary raceme. Oil content of the seed or test weight were not affected by plant population of nonirrigated castorbeans.

No abstract

Improving cotton (Gossypium spp.) heat tolerance (ability to set bolls in high temperature environments) has been an important aspect of the American Pima (Pima) cotton (G. barbadense L.) breeding program for over 25 years. However, heat tolerance is difficult to quantify. This report estimates change in heat tolerance of Pima cotton through yield response over 30 years in six Arizona counties that differ in elevation and mean summer temperatures. Pima cotton/upland cotton (G. hirsutum L.) lint yield percentages were compared for each county by regression with year of production. County mean yield percentages were also compared with elevations. Pima cotton yields as percentages of upland cotton yields increased in five of the six counties over the 30‐year period. Pima lint yield increased from 57 % of upland yield in 1956 to 75 % in 1985 when averaged over the six counties. This increase represents an estimated 206 kg lint ha‐1 in addition to any upland yield increase. Comparison of regression coefficients suggest that nearly 50 % of the 30‐year lint yield increase of Pima cotton at lower elevations was the result of increased tolerance to high temperature in improved cultivars. County 30‐year mean Pima cotton lint yields as percentage of upland cotton yield increased significantly (P < 0.001) from 52.2% to 82.1 % as county elevations increased from 37 m to 1273 m (cooler summer temperatures). These results support reports of greater heat tolerance for upland cotton than Pima cotton, but also show that the difference between species has been reduced substantially by breeding in the last 30 years.