ABSTRACT
Sources of Ammonium for Plants The simplest and least expensive means of obtaining reduced ammonium in plants is probably through direct assimilation by roots. However, ammonium ions normally constitute only a very small portion of the mineral nitrogen content of soil in temperate regions since the nitrifying micro-organisms oxidise them to nitrates (see Chapter 1). In temperate zones, an exclusively ammonia-based nutrition is very often unfavourable for the growth and metabolism of plants. In cultivated areas of temperate regions, reduction of nitrates represents the main pathway for mineral nitrogen assimilation in plants, which is not aided by symbiotic nitrogen fixation. No doubt energy is saved on the reduction of nitrates (15-16 ATP/NO3) but ammonia assimilation by roots results in the appearance of an ammoniacal syndrome (described in Chapter 4). Nitrate reduction is brought about by two enzymes,
nitrate reductase and nitrite reductase, whose characteristics have been described in Chapter 3. This process results in the formation of ammonium in leaves and/or roots (Sechley et ah, 1992).
