Eight soybean [Glycine max (L.) Merrill] varieties in each of four maturity groups were grown to maturity at optimum and deficient soil moisture conditions to determine differences in varietal response to soil stress. A significant variety by stress level interaction on seed yield was detected among varieties in each of four maturity groups, indicating that the effect of soil moisture stress on yield varied among varieties.
Under high moisture stress conditions, the yield of the most stress resistant varieties was reduced about 20%, while the yield of the least stress‐resistant varieties was reduced about 40%. The absolute reduction in yield for the most stress‐sensitive varieties was approximately 1000 kg/ha, while the yield of the least stress‐sensitive varieties was reduced about 200 to 400 kg/ha.
Under conditions of optimum soil moisture, the difference in yield among varieties was large relative to the difference in yield produced under deficient moisture conditions. A low or nonstress soil moisture level permits greater genotypic expression, thereby increasing genotypic variance among varieties. A low soil moisture stress environment appears to be the optimum environment for selecting soybean yield attributes.
Previous studies comparing the accumulated heat unit (AHU) method with the calendar day method for classifying corn (Zea mays L.) hybrids with respect to maturity do not clearly show the superiority of the AHU method over the calendar day method. They appear to have two limitations: (1) the range of climatic conditions under which the studies were conducted was not broad enough to elicit wide variation in the rate of occurrence of plant development and maturity, and (2) the studies were truncated at silking because the time interval from silking to maturity was assumed to be constant.
The objective of the study herein was to determine, under a wide range of climatic conditions, whether AHU methods are superior to the calendar day method for classifying the maturity of corn hybrids and predicting the occurrence of phenological events. For a particular corn hybrid grown under a range of climatic conditions, the number of calendar days between any two phenological events may vary widely but the AHU should be constant.
Four plantings of three corn hybrids were made at weekly intervals at seven widely dispersed locations in Ohio in 1969 and 1970. Calendar days and AHU, calculated by six methods, were determined from planting to tasseling and silking, and from planting to 40, 35, 30, and 25% kernel moisture and black layer formation.
The coefficients of variation of AHU and calendar days to a particular phenological event for each of the three hybrids were determined on the pooled data from the four planting dates and seven locations for each of 2 years.
The differences in coefficients of variation among the six AHU methods were relatively small. However, as a group, the AHU methods were about half as variable as the calendar day method for classifying corn hybrid maturity. The data indicated that the AHU method of classifying corn hybrids was superior to calendar days.
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