Studies on Genetic Male-Sterile Soybeans

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Abstract
Soybean germplasm, essentially isogenic except for loci controlling male sterility (ms1) and nodulation (rj1), were developed to study the effects of reproductive development and N source on certain aspects of photosynthesis. Plants were sampled from flowering (77 days after transplanting) until maturity (150 days after transplanting). With all 4 genotypes, net C-exchange rates were highest at flowering and declined thereafter. Photosynthetic rates of the sterile genotypes (nodulated [inoculated with Rhizobium japonicum] and non-nodulated) declined more rapidly than the fertile genotypes, and after 105 days, both sterile genotypes maintained low but relatively constant carbon exchange rates (< 3 CO2/g fresh weight per h). Photosynthetic rates and starch accumulation (difference between afternoon and morning levels) declined with time. The sterile genotypes attained the highest morning starch levels, which reflected reduced starch mobilization. After 92 days, the proportion of photosynthetically fixed C that was partitioning into starch (relative leaf starch accumulation) in the sterile genotypes increased dramatically. In contrast, relative leaf starch accumulation in the fertile genotypes remained relatively constant with time. Throughout the test period, all 4 genotypes maintained leaf sucrose levels between 5 and 15 .mu.mol glucose equivalents/g fresh weight. The activities of sucrose phosphate synthase (SPS) in leaf extracts of the 4 genotypes declined from 77 to 147 days. Nodulated genotypes tended to maintain higher activities (leaf fresh weight basis) than did the non-nodulated genotypes. In general, relative leaf starch accumulation was correlated negatively with the activity of SPS (normalized with leaf net C exchange rate) in leaf extracts for all 4 genotypes during early reproductive development, and for the fertile genotypes at all sampling dates. In contrast, leaf sucrose content was correlated positively with SPS activity during early reproductive development. These results suggested that a direct relation existed between the activity of SPS and starch/sucrose levels in soybean leaves. However, the interaction between these processes also may be influenced by other factors, particularly when leaf photosynthetic rates and plant demand for assimilates is low, as in the sterile genotypes.