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.
Delayed cotton (Gossypium spp.) planting generally reduces lint yield because it reduces the length of the growing season. Planting cotton too early also can reduce lint yield. Most growers believe that reduced yield with early planting is caused by reduced stand. Previous research suggested that low temperature during germination and early seedling growth may reduce lint yield regardless of stand. This study attempts to clarify low temperature effects on lint yield by partitioning cotton lint yield reduction from early planting into the effects from reduced stand and other factors. Soils were Typic Torrifluvents. Thermic Torrilluvents, and Anthropic Torrifluvents. Upland cotton (Gossypium hirsutum L.) had significantly (P < 0.05) reduced lint yield from early planting in five tests. American pima cotton (G. barbadense L.) grown in four of the tests had smaller, nonsignificant lint yield decreases from early planting in three of the tests and higher lint yield from early planting in the fourth test. Early planted cotton generally was shorter. A comparison of upland cotton lint yields, using adjustment by regression analyses with plant population, indicated that something other than plant population, presumably low temperature, caused approximately 100, 94, 66, 22, and 0 % of the lint reduction from early planting in five analyses. Thus physiological and morphological effects of low temperatures early in the cotton planting season often may contribute as much to reduced lint yield as does reduced stand.
Reliable techniques for identification of drought‐resistant germplasm are essential if new lines and hybrids are to be developed for arid and semiarid areas of the world. This experiment was designed to evaluate the irrigation gradient system as a mechanism for evaluating drought tolerance in sorghum [Sorghum bicolor (L.) Moencb.] in the field. Fifty sorghum hybrids which were developed with A‐lines ‘CK‐60’ and ‘Redlan’ were planted perpendicular to the mainline of the sprinkler irrigation gradient system at Yuma, Ariz. in 1979 and 1980. The soil was a superstition sand (sandy, mixed, hyperthermic Typic Calciorthid) with an average available soil moisture of 3.6 % . Characteristics including panicle rank, anthesis date, plant height, panicle and peduncle length, crown diameter, and plant weight were measured on plants of each hybrid grown in the high and low water areas of the gradient system. The distance from the line source to the last grain‐producing panicle in the low water area of the gradient was also measured on each hybrid. Hybrids developed with CK‐60 were able to produce seed‐producing panicles in drier areas of the gradient compared to hybrids developed with Redlan. The average distance from the sprinkler Line source was 14.4 m for CK‐60 hybrids and 11.3 m for Redlan hybrids over both years. Days to anthesis, panicle and peduncle length of plants in the stressed area of the gradient may be potential indicators of drought resistance. The irrigation gradient system is a useful tool for evaluating drought resistance and the selection of parental lines over a wide range of available soil moisture conditions.
The development of more drought‐resistant sorghum [Sorghum bicolor(L.) Moench.] germplasm depends on a fuller understanding of physiological responses to soil moisture stress. To investigate adaptation to moisture stress, physiological responses of six sorghum hybrids and their respective parental lines were evaluated under two irrigation treatments at Tucson, Ariz., in 1980 and 1981. The soil was a Comoro sandy‐loam (coarse‐loamy, mixed calcarious, thermic, Typic, Torrifluvent). Apparent photosynthesis, transpiration, diffusive resistance, and temperature differential (ambient temperature minus leaf temperature) were measured under field conditions at weekly intervals. Regression analysis was used to compare the physiological performance of the germplasm over a wide range of environmental conditions. Hybrid 4 was superior to its parental lines for all four physiological characteristics. In 1980, this hybrid was superior to the other hybrids in temperature differential (1.03°C) and transpiration (23.4 μg cm−2s−1) and had the highest mean apparent photosynthetic rates (45.9 mg dm−2h−1) and lowest mean diffusive resistance (1.41 cm s−1). Regression analysis is an effective tool that can be used for analyzing the physiological responses across diverse environmental conditions. By regressing the response of individual germplasm against the mean response of the population across all environmental conditions, the slope, mean, and coefficient of correlation were used to evaluate genotype‐environment interactions.
Rapid cotton (Gossypium hirsutum L.) boll development and opening can be beneficial in late season. It can result in increased lint yield, earlier harvest, higher grade, less insecticide use, and reduced storm damage. Research was conducted in the field on a fine loamy, mixed (calcareous), hyperthermic, Anthropic Torrifluvent soil. This study evaluates the effects of water stress in late season (early irrigation termination) and pink bollworm [Pectinophora gossypiella (Saunders)] infestation on time required for cotton bolls to develop from flower to open boll in the field. Water stress speeded boll opening. Bolls from stressed cotton were obtained from field tests with five dates of final irrigation. They opened an average of 17 days earlier than bolls of unstressed cotton in late season. Boll period averages ranged from 41 to 76 days depending upon the degree of water stress and time of year. On droughty (shallow) soil [fine loamy, mixed (calcareous) hyperthermic Anthropic Torrifluvent], water stress effects were significant, but smaller. Delayed boll opening may explain reduced lint yield which at times is associated with late final irrigation. Pink bollworm infested bolls opened about 3 days earlier than non‐infested bolls in two tests with deep soil, but only 0.5 days earlier in a test on droughty soil. Boll period population resembled a normal distribution around the mean until 93% of the bolls were open. For a given day of flowering, 80 % of the bolls (from 10 to 90 %) opened within a 8‐ to 11‐day period.
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