Heritability of N2 fixation traits, and phenotypic and genotypic correlations between N2 fixation traits with drought resistance traits and yield in peanut
Drought stress reduces growth and yield in peanut (Arachis hypogaea L.) and also reduces nitrogen fixation (NF). Peanut production in drought prone areas should be enhanced by the development of cultivars that can fix more nitrogen (N) under drought conditions. The aims of this study were to estimate heritability for NF and to estimate phenotypic and genotypic correlations among traits related to NF with drought-resistance traits and yield under well-watered and drought conditions. A total of 140 lines in the F4:7 and F4:8 generations derived from four crosses, parental lines, and a non-nodulating line as a non-N2-fixing reference plant were evaluated in the dry seasons 2005/2006 and 2006/2007. These lines were evaluated in rhizobium inoculated soil without N fertilizer under field capacity (FC) and 2/3 available soil water (AW). Data were recorded for specific leaf area (SLA), SPAD chlorophyll meter reading (SCMR), nodule number (NN), nodule dry weight (NDW), shoot dry weight (SDW), pod dry weight (PDW), total dry weight (TDW), harvest index (HI), and NF. Broad-sense heritability (h2b) and narrow-sense heritability (h2n) estimates for NF under FC and 2/3 AW were in the same ranges (h2b = 0.84 to 0.98 and h2n = 0.29 to 0.39). Positive relationships between NF under FC and 2/3 AW (r = 0.73, P ≤ 0.01) indicated that selection for the lines that fixed high N under well-watered conditions should produce lines that fixed high N under drought conditions. Selection for NF under drought conditions might be more effective in improving yield because of a higher correlation between NF and PDW (rG = 0.43, P ≤ 0.01) under drought conditions than under FC (rG = 0.13). The use of SCMR and SLA as surrogate traits for NF would be less effective than direct selection because of weak correlations between these traits and NF.