Growth and nodulation characteristics of subclover ( Trifolium subterraneum L.) and Rhizobium leguminosarum BV. Trifolii at different soil water potentials

Abstract

Optimal pasture productivity in western Oregon and California requires annual self-seeding legumes such as subclover ( Trifolium subterraneum L.) to reestablish upon the onset of Fall rains. During the drought years of the 1980s, Fall rains were delayed and of low intensity resulting in seedling germination and primary development occurring at low soil water potential values. The objectives of this work were: (1) to characterize subclover nodulation and seedling development in a silty-clay loam of the Abiqua series (Cumulic Ultic Haploxeroll) maintained at water potential values of −0.03 MPa and below, and (2) to determine if inter-strain variation in growth at low water potential might influence which members of the resident soil population of Rhizobium leguminosarum bv. trifolu nodulated subclover. The majority (78%) of “Nangeela” subclover seeds germinated and underwent early seedling development at a soil water potential between −1.1 and −1.3 mPa. Although root growth was significant at all incremental increases in soil water potential ⩾ 1.3 mPa, nodulation was not detected until soil water potential reached −0.8 mPa, and significant numbers of nodules did not develop until soil water potential was −0.6 mPa. Shoot growth lagged behind root growth until a soil water potential of −0.4 mPa was established. Serotype AS6 was the dominant nodule occupant regardless of water potential. Isolates of major and minor nodule-occupying serotypes could not undergo cell division at water potential values ⩽ 0.8 mPa. Despite this modest lower water potential limit of growth, isolates recovered their reproductive capacity rapidly when soil water potential was raised instantaneously from potentials as low as −18 to −0.03 mPa