Alfalfa seeding root characteristics under complex saline-alkali stress

Ecological environment and agricultural sustainability in present days China have been threatened by soil salinity. Plant roots have been the first to feel adverse soil stress conditions through different physiological processes. Study of alfalfa response to salt stress has mainly focused on the above-ground system or on the growth and physiological response to single salt stress. Alfalfa root response to complex saline-alkali stress has been rarely reported. The primary aim of this study was to determine the response of alfalfa root to saline-alkali conditions. The study also explored alfalfa growth and adaptation characteristics under complex saline-alkali stress conditions and provided theoretical bases for alfalfa cultivation in saline-alkali field. The Medicago sativa L. cv. "Gannong No.3" was investigated under complex saline-alkali stress conditions at seeding growth stage. 20 different alkali-saline stress conditions simulated saline-alkali soil conditions obtained by mixing two neutral (NaCl and Na₂SO₄) and alkaline (NaHCO₃ and Na₂SO₃) salts in different proportions A (NaCl︰Na₂SO₄ in 1︰1 ratio), B (NaCl︰Na₂SO₃ in 1︰1 ratio), C (NaCl︰Na₂SO₄︰NaHCO₃︰Na₂SO₃ in 1︰1︰1︰1 ratio), D (Na₂SO₄︰Na₂SO₃ in 1︰1 ratio) and E (NaHCO₃︰Na₂SO₃ in 1︰1 ratio) and concentrations (25 mmol·L^-1, 50 mmol·L^-1, 100 mmol·L^-1 and 150 mmol·L^-1). To understand alfalfa root growth characteristics in mixed salt stress conditions, total root length, root surface area, average root diameter, root volume and root tip number were analyzed. The results showed that salt concentration was the main driving factor of alfalfa root growth. There was obvious negative effect of the saline-alkali conditions on root tip number and insignificant effect on average root diameter. As treatment concentration increased, total root length and root surface area initially increased followed by a decrease. At low concentration of ≤50 mmol·L^-1, total root length under A (25 mmol·L^-1) and B (50 mmol·L ^-1) treatments respectively increased by 50.7% and 37.9% over CK (0 mmol·L ^-1). At a concentration of 150 mmol·L ^-1, total root length under treatments A, B, C, D and E respectively dropped by 26.6%, 37.7%, 51.6%, 37.0% and 55.7% compared with CK; all of which were significantly lower than the CK. Root surface area under treatments A, C and D increased at 25 mmol·L^-1 concentration by 21.1%, 43.4% and 12.7% over CK. At 150 mmol·L ^-1 concentration, root surface area under treatment E dropped by 49.6% over CK. This suggested that low saline-alkali stress (≤50 mmol·L^-1) conditions promoted or insignificantly influenced alfalfa seedling root growth. However, high saline-alkali stress (≥50 mmol·L ^-1) conditions inhibited alfalfa seedling root growth. Increasing alkaline salt under high salt concentration increased alfalfa root growth inhibition.