Document Type

Thesis - University Access Only

Award Date


Degree Name

Doctor of Philosophy (PhD)

Department / School

Plant Science


Drought stress is often the primary limiting factor and causes economic losses in soybean production in regions along the western fringes of the soybean belt. Breeding for improved water-use efficiency (WUE) and seed yield is therefore a major objective in plant breeding programs for soybean cultivar development in these regions. Measurement of long-season WUE directly under field conditions is difficult, time-consuming and costly. Carbon isotope discrimination (CID) was proposed as a screening tool to select in some crops for long-season WUE because it provides an integrated measurement of genotypic variation in WUE. The degree of discrimination is determined by the ratio of intercellular CO2 to CO2 atmospheric (Ci/Ca), which in tum is theoretically, linearly related to CID and inversely to WUE. To study CID as a potential screening tool for WUE, it is important to understand how different environments affect CID, as well as other physiological and agronomic characters. The objectives of the study were to evaluate the selected physiological characters with CID in soybean cultivars and determine the relationships of CID, relative water content (RWC), osmotic potential ('I's) and stomata} conductance (gs) with seed yield, protein and oil concentrations. Six early maturing soybean cultivars were evaluated in 1995 and 1996 in three irrigated and four non-irrigated field environments, as well as in the greenhouse under stress studies. Significant cultivar variations were observed for most physiological and agronomic characters. The greenhouse study showed that genotype x year interactions was not significant for CID, suggesting that the rankings of soybean cultivars for these characters did not change across years. Osmotic adjustment (OA) was observed among soybean cultivars grown under the greenhouse study (P<0.05), but not in the field study. Protein and oil concentrations were not affected by water treatments. They varied, however, across years and environments. The study showed that the stressed plants had less discrimination against 13C compared to non-stressed plants. A strong significant association was observed between RWC and Ψs in both well-watered (r=0.94**) and stressed treatments (r=0. 72**) in the greenhouse study. There were significant positive correlations between 100-seed weight and days to maturity (r=O. 72**). The data showed that RWC, Ψs and gs did not provide consistent patterns across different environments and years, but cultivar mean rankings for CID were consistent across environments. Carbon isotope discrimination was negatively correlated with seed yield (r=-0. 30**). Lower CID cultivars produced greater seed yield and may confer a better WUE.

Library of Congress Subject Headings

Soybean -- Varieties Soybean -- Drought tolerance Carbon -- Isotopes



Number of Pages



South Dakota State University