Effect of applied nitrogen on seedling growth and cotyledon protein utilization of effective and ineffective nodulating alfalfa

Abstract

Cotyledons arc an important nitrogen (N) source for germination and early seedling development of alfalfa (Medicago sativa L.). The objectives of these studies were to determine changes in cotyledon N or protein concentration during alfalfa seedling growth and to ascertain the influence of N availability on seedling development and cotyledon protein use. Greenhouse‐grown seedlings of effective and ineffective ‘Saranac’ were grown in sand with either minus‐N nutrient solution, or a nutrient solution containing 20 mM ammonium nitrate as an N source. Shoots, cotyledons, and roots were sampled between 3 and 43 days after planting, and N or protein concentrations and dry wts. determined. Cotyledon protein or N declined rapidly in both effective and ineffective Saranac between 3 and 15 days after planting, but was more extensive for ineffective Saranac 19 days after planting and later. Nitrogen per seedling increased in effective Saranac between 15 and 22 days after planting indicating that dinitrogen fixation had begun. At 22 days after planting root and shoot N of effective Saranac increased whereas cotyledon N continued to decline. For effective Saranac applied N did not alter the decline in cotyledon protein concentration, but cotyledon protein per seedling was slightly greater with applied N because of greater cotyledon mass per seedling. N application to ineffective Saranac resulted in responses similar to those of effective Saranac, whereas more extensive N and protein losses occurred in cotyledons of ineffective Saranac seedlings without N. Soil N application and nodule effectivity enhanced seedling development of alfalfa, but did not prevent the rapid decline in cotyledon N and protein.