INFLUENCE OF PLANT-AGE ON CALCIUM-STIMULATED AMMONIUM ABSORPTION BY RADISH AND ONION

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 (NH4+)-N-15 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 N-15 suggesting that the newly absorbed (NH4+)-N-15 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.