Water relations, nitrogenase activity and root development of three grain legumes in response to soil water deficits

Abstract

Because of the importance of dinitrogen (N 2) fixation and its relation to leaf photosynthesis and dry-matter accumulation in grain legumes, a field study was conducted in 1984 to compare the relative abilities of three grain legumes to maintain N 2 fixation rates under well-defined soil water deficits imposed in field environments. Measurements of leaf water status, root distribution and nitrogenase activity were determined on peanut ( Arachis hypogaea L.), soybean ( Glycine max L. Merr.), and pigeon pea ( Cajanus cajan L.) throughout the growing season. The three crops were grown in a Kendrick fine sand (loamy, siliceous, hyperthermic family of Arenic Paleudults) and subjected to two water management treatments: 1) an irrigated treatment which was well-watered throughout the growing season: and 2) a water-stressed treatment which was subjected to several naturally-occurring water-deficit periods. During periods of mild and moderate water stress, peanut maintained higher leaf water potentials and stomatal conductance than either soybean or pigeon pea. When well-watered, peanut also maintained considerably higher rates of specific nitrogenase activity than either soybean or pigeon pea. However, nitrogenase activity was reduced in all three crops before visible symptoms of water stress (leaf wilting) were apparent. Peanut maintained nitrogenase activity longer after withholding irrigation than either soybean or pigeon pea. Root distribution measured 68 days after emergence suggested that root length density of peanut at soil depths below 30 cm increased in response to the imposed water deficits. Proliferation of roots in those soil layers as a result of water stress was not observed in either soybean or pigeon pea. The maintenance of higher leaf water-status, stomatal conductance and nitrogenase activity by peanut during soil water deficits compared to the other crops was attributed to greater density of roots in the lower depths of the soil profile.