Alternative pathway respiration is associated with ammonium ion sensitivity in spinach and pea plants

Spinach and pea plants were grown in hydroponic culture with nitrate orammonium salts as the nitrogen source. Dry matter accumulation andphotosynthetic rate declined in spinach plants fed with ammonium salts, whereasthey did not change in pea plants compared with nitrate-fed plants. Measurementof organic nitrogen and free amino acid content showed that ammonium ions wereassimilated in shoots in spinach plants and in roots in pea plants. Ammoniumionnutrition led to a decline in starch content in both species. Organic acidsincreased in roots of pea plants fed with ammonium ions whereas they declinedinspinach plants. In both species ammonium ions increased root respiration ratebut the contribution of both routes (cytochromic and alternative pathway) tothis increase was different depending on the species. In spinach plants,ammonium ions increased the cytochromic path and decreased the alternativepathway, whereas in pea plants both routes were stimulated mainly through thealternative pathway. The differences in the sensitivity to ammonium ionsbetweenboth species are discussed in terms of differences in the availability of Cskeletons and energy, which could be due in part to differences in the capacityto stimulate the alternative pathway.