Changes in arginine, PAL activity, and nematode behavior in salinity-stressed citrus

De novo arginine biosynthesis has been described as a response of citrus to a range of stresses. It is often noted that stress in plants enhances susceptibility to herbivory and pathogenic attack. Using a citrus and nematode (Tylenchulus semipenetrans) system, the effects of salinity stress on nematode behavior, amino acids (particularly arginine), and phenylalanine ammonia lyase (PAL) activity was investigated. The hypothesis was tested that under salinity stress, citrus grows more slowly and produces arginine in response to high levels of in vivo ammonia, resulting in lower PAL activity and increased susceptibility to nematode attack. After 30 days of high salinity (0.1 M NaCl), plants exhibited a 38% reduction in growth, 35% reduction in PAL activity, and had 54% higher infection rates. PAL activity was inversely correlated (P < or = 0.05) with salinity level and with increase in arginine concentration.