VEGETATIVE GROWTH ANALYSIS OF A DROUGHT-RESISTANT SOYBEAN PLANT INTRODUCTION

Although soybean [Glycine mar L. (Merr.)] Plant Introduction 416937 (PI) is known to maintain turgidity under desiccating conditions during podfilling, the mechanism(s) underlying this drought hardiness is uncertain. The objective of the present study was to determine whether PI develops drought-resistant morphology during early vegetative growth. 'Forrest' was chosen as the control genotype since it is a commonly grown cultivar of similar phenology. A randomized complete block design with eight replications of each genotype was employed. Field conditions, including water, were optimized so that near maximal vegetative growth could be obtained. During 1988 to 1990, plants were excavated from the upper 38 cm of soil at 7- to 14-d intervals. The PI's drought hardiness did not appear to depend upon reduced leaf area development, since annual estimates of its limiting growth values were 25% greater than those of Forrest. At maximum vegetative development, PI was characterized by having a small number of very large leaves, while Forrest had a large number of comparatively small leaves. At most harvests, PI's root mass, root volume, and relative surface area exceeded that of Forrest, and the relative growth rate of PI roots declined more slowly than that of Forrest, resulting in a 24% greater limiting root mass. PI may have a potential yielding disadvantage in that it develops a limited number of branch nodes and is susceptible to lodging. The PI's utility in breeding programs may reside in its potential to increase morphological diversity in soybean.