Influence of plant age on calcium stimulated ammonium absorption by radish and onion

Abstract

The efficient use of N for crop production is important because N is normally the most expensive fertilizer input. Past research has suggested that Ca⁺⁺ can be used to stimulate NH₄+ absorption by plants. The importance of plant growth stage in relation to this phenomenon has not been examined previously. The objectives of this study were to examine Ca⁺⁺ ‐ stimulated NH₄ ⁺ absorption and to examine the effect of Ca⁺⁺ concentration on N content and growth in plant tops, bulbs and roots at different growth stages. Ammonium absorption experiments were conducted in the greenhouse in 4‐L pots containing 3.5 kg of calcareous Gila sandy loam (Typic Torrifluvents) (CEC <1 cMol kg⁻¹). Plants (Radish, Raphanus sativas L., and onion, Allium cepa L.) were grown with a uniform nutrient solution (1/2 strength nutrient solution, all N as NO₃) to the desired growth stage at which time the soil was leached with deionized water. Afterwards, the soils were fertilized with 1/2 strength nutrient solutions (5 mol m⁻³ NH₄) with Ca⁺⁺: NH₄ ⁺ molar ratios of 0, 0.25, 0.50, 1.00, and 2.00 for a period of 30 h. As Ca⁺⁺ concentration increased, NH₄ ⁺ absorption and plant growth increases were greatest with young seedlings. In the intermediate and mature growth stages, Ca⁺⁺ stimulated ¹⁵NH₄ ⁺ absorption was less rapid than in the earlier growth stages but frequently exhibited a different response (i.e., altered metabolite translocation) to the added Ca⁺⁺ ‐ concentration. However, at the intermediate and mature growth stages significantly increased N contents and plant growth also were noted in most cases. The Ca⁺⁺ ‐ increased N content in leaves and bulbs of the older plants had much less ¹⁵N suggesting that the newly absorbed ¹⁵NH₄ ⁺ was being deposited in the roots replacing older N forms that were then translocated to the bulbs or leaves. Thus, increasing Ca⁺⁺ appeared to have anadditional function of increasing the mobility of metabolites (dry matter) from the roots. Since more above‐ground plant products were produced with the same amount of N, plant N use efficiency was increased.