Nodule Growth and Nitrogen Fixation of Selected Soybean Cultivars under Different Soil Water Regimes

Abstract

Sensitivity of N2 fixation to soil-drying has been considered as a major limiting factor to improving soybean yield. Seven soybean cultivars derived from various parents—namely, Nakhon Sawan 1 (NS-1), SJ.4, Sukhothai 1 (ST-1), Sukhothai 2, (ST-2), Sukhothai 3 (ST-3), Chiang Mai 60 (CM-60) and Chakkrabandhu 1 (CK-1) were evaluated for their nodulation and growth and yield in the first experiment. They were derived from various parents and their maturity differed from 90 to 110 d. Cultivars NS-1, ST-2 and ST-3 were selected to examine their nodule growth and N2 fixation capability
under different water regimes in the second experiment. Nitrogenease activity (NA) of the nodulated roots of soybean increased with an increase in nodule dry matter (NDM) until 15 d after vegetative stage 4 (15 DAV4; R4) and then declined at 22 DAV4 (R5) under the well-watered regime. Under a water regime with progressive soil drying, the NA of the nodulated roots significantly decreased by 17.73% of the control from 7 DAV4 (R3). Among the soybean cultivars, NS-1 showed higher nodulation and N2 fixation capacity under both water regimes. The NA of nodulated roots severely diminished by 87.10% of control at 12 DAV4 (R4) under progressive soil drying and did not recover well after rewatering; nodule growth was similarly affected. There was a greater and significant correlation between NDM and other observed physiological parameters at the R4 stage. NDM is the most fundamental parameter for selecting better soybean cultivars for N2 fixation under both water regimes. The soil plant analysis development (SPAD:M-502) reading showed a highly significant correlation (r=0.84) with NA and other physiological parameters. SPAD readings could be used as a rapid assessment technique for the identification of potential cultivars with greater N2 fixation for both water regimes by measuring the
accumulated total N in the shoot biomass at harvest.