Targeting Nitrogen Fixation Secondary Traits for Cowpea Selection in Low Phosphorus Soil.
Biological nitrogen fixation (BNF) potentials of legumes including cowpea is limited in heavily weathered acid soils that prevail in the humid forest zone (HFZ) of Cameroon, since phosphorus (P) an element important in nodulation, is generally deficient. Fifty cowpea germplasm including ten Cameroon landraces were evaluated in three environments: low phosphorus (LP; 0 mg P kg-1 soil), high phosphorus (HP; 30 mg P kg-1 soil as KH2PO4) and high P and nitrogen (high P and N; 30 mg P and 90 mg N kg-1 soil as (NH4)2SO4). The objective was to identify nitrogen fixation secondary traits that correlate highly with nitrogen fixed in low P environments. Based on soil bulk density, 4.3kg of soil was used per pot in the screen house at the International Institute of Tropical Agriculture (IITA), Cameroon. A two factorial Randomized complete block design was used in two replicates. Each variety had two pots per replicate, one for nodulation and the other for yield parameter measurements. Data collection was done on twenty morphological traits comprising phenological, nitrogen fixation (N-fixed) and secondary traits, yield and yield component traits. Contrast analysis indicated there was significant differences among the genotypes and environments (p<0.05). Under low P environment, thirteen out of the 20 traits were correlated with three principal components (PC) that explained 55.77% of the variation. All three traits related to yield were not correlated with these PCs. Pearson correlation studies indicated the secondary trait shoot nitrogen content (shoot N), was highly and positively correlated with N-fixed in low P environments. Shoot N was considered the best nitrogen fixation secondary trait and can be used for selection of high N2-fixing genotypes in low P soils.
Keywords - Cowpea, low phosphorus, nitrogen fixation traits,selection, shoot nitrogen.