Leaf Margin Chlorosis in Alfalfa

Potassium deficiency in alfalfa is usually recognized as a spot chlorosis in older leaves. This symptom has been observed in the western United States in irrigated alfalfa growing on alkaline (i.e., calcareous) soils. A second kind of K-deficiency symptom also has been observed in the region. It appears as a chlorotic leaf margin where the white tissue is localized toward the leaf tips. As the K-deficiency severity increases, the chlorotic margin extends toward the leaf base. Chlorotic spot and margin symptoms occur on separate plants growing in proximity. The soils involved are low in available K and are neither saline nor sodic. Alfalfa is known to absorb varying amounts of Na, depending on K soil availability. The physiological role of Na in alfalfa is not well understood, but it is believed that K relations of this plant cannot be fully described if Na is ignored. The purpose of this study was to seek an explanation of the causes and effects of different visual expressions of K deficiency in alfalfa through an analysis of K-Na plant relations. An experiment was conducted on a calcareous, low-K soil where both K-deficiency symptoms were present. Sodium uptake was extremely high in chlorotic margin plants. Sodium concentrations of whole aboveground plants were: marginal chlorosis, 0.22 mol/kg; spot chlorosis and also K-fertilized plants, 0.05 mol/kg. Observed K/Na ratios were 0.60 in chlorotic margin plants, 3.81 in chlorotic spot plants, and 5.60 in K-fertilized plants. Sodium concentration of the chlorotic margin plants was higher in leaves than stems (0.368 and 0.099 mol/kg, respectively), though in the chlorotic spot plants the opposite was true (0.010 and 0.036 mol Na/kg, respectively). Potassium fertilization eliminated both symptoms, increased herbage yield, and resulted in an increase in K and a decrease in Na in plants. A model was proposed to explain the chlorotic margin symptom of K deficiency in alfalfa. The two different K-deficiency symptoms, together with associated differences in K-Na concentrations and partitioning within the plant, are believed to reflect the genetic diversity of alfalfa.