De novo arginine biosynthesis in leaves of phosphorus-deficient citrus and poncirus species

Young, fully expanded leaves from 7-month-old P-deficient citrus rootstock seedlings had levels of nonprotein arginine that were 10- to 50-fold greater than those from P-sufficient control plants. Arginine content of the protein fraction increased 2- to 4-fold in P-deficient leaves. Total arginine content, which averaged 72 ± 6 micromoles per gram dry weight of P-sufficient leaf tissue (mean ± se, n = the four rootstocks) was 207, 308, 241, and 178 micromoles in P-deficient leaves from Citrus limon cv rough lemon, Poncirus trifoliata × C. sinensis cv Carrizo citrange and cv Troyer citrange, and P. trifoliata cv Australian trifoliate orange, respectively. For each rootstock, the accumulation of arginine paralleled an increase in the activity of the pathway for the de novo biosynthesis of arginine. The ratio of the nanomoles NaH¹⁴CO₃ incorporated into the combined pool of arginine plus urea per gram fresh weight intact leaf tissue during a 3-hour labeling period for P-deficient to P-sufficient plants was 91:34, 49:11, 35:11, and 52:41, respectively. When P-deficient plants were supplied with P, incorporation of NaH¹⁴CO₃ into arginine plus urea was reduced to the level observed for the P-sufficient control plants of the same age and arginine ceased to accumulate. Arginase and arginine decarboxylase activity were either unaffected or slightly increased during phosphorus deficiency. Taken together, these results provide strong evidence that arginine accumulation during phosphorus deficiency is due to increased activity of the de novo arginine biosynthetic pathway.