Abstract
To explore the variations in symbiotic N2 fixation and water use efficiency in cowpea, this study evaluated 25 USDA cowpea genotypes subjected to drought under field conditions at two locations (Kpachi and Woribogu) in the Northern region of Ghana. The 15N and 13C natural abundance techniques were respectively used to assess N2 fixation and water use efficiency. The test genotypes elicited high symbiotic dependence in association with indigenous rhizobia, deriving between 55% and 98% of their N requirements from symbiosis. Consequently, the amounts of N-fixed by the genotypes showed remarkable variations, with values ranging from 37 kg·N-fixed·ha-1 to 337 kg·N-fixed·ha-1. Most genotypes elicited contrasting symbiotic performance between locations, a finding that highlights the effect of complex host/soil microbiome compatibility on the efficiency of the cowpea-rhizobia symbiosis. The test genotypes showed marked variations in water use efficiency, with most of the genotypes recording higher δ13C values when planted at Kpachi. Despite the high symbiotic dependence, the grain yield of the test cowpeas was low due to the imposed drought, and ranged from 56 kg/ha to 556 kg/ha at Kpachi, and 143 kg/ha to 748 kg/ha at Woribogu. The fact that some genotypes could grow and produce grain yields of 627 - 748 kg/ha under drought imposition is an important trait that could be tapped for further improvement of cowpea. These findings highlight the importance of the cowpea-rhizobia symbiosis and enhanced water relations in the crop’s wider adaptation to adverse edaphoclimatic conditions.
Highlights
In most parts of the world, crop yields on smallholder farms are often limited by low soil fertility resulting from continuous cropping with minimal or no fertilizer application [1]
The present study explored the symbiotic N nutrition, C assimilation and water use efficiency among USDA cowpea genotypes subjected to drought under field conditions
The findings revealed marked variations in N2 fixation, with genotypes deriving between 55.4% to 97.7% of their N requirements from symbiosis, corresponding to N-fixed values between 36.7 kg/ha to 336.6 kg/ha
Summary
In most parts of the world, crop yields on smallholder farms are often limited by low soil fertility resulting from continuous cropping with minimal or no fertilizer application [1] Besides their importance as human foods, grain legumes such as cowpea can improve overall soil nutrient status due to their ability to fix atmospheric N2 in association with rhizobia in the soil [2] [3]. Of the many techniques used to estimate legume symbiotic performance, the 15N natural abundance has so far been useful in quantifying N2 fixation in field-grown legumes [8] [9]. This technique requires the estimation of the isotopic fractionation associated with N2 fixation in the legume (so called B value), and the selection of reference plants to estimate the soil N uptake by the legume [10] [11]
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