Changes in free amino acid levels in sour orange leaves in response to cold stress and during recovery from cold stress

In a previous study, we reported that potted sour orange trees recovering from cold stress attracted more Asian citrus psyllids than the control plants continuously kept under warm condition. In parallel studies, cold-treated plants were shown to have relatively increased amounts of ninhydrin positive material during 5-24 h recovery period from cold stress. Here we report on changes in free amino acid levels in sour orange leaves in the 24 h recovery period following the termination of the chilling treatment. Proline was most abundant amino acid and increased in response to chilling treatment; it remained at higher level than initially during the first 5 h recovery period and then declined to initial level after 24 h. In addition, amino acids, such as serine, glutamic acid, glycine, lysine, isoleucine, tyrosine, arginine and histidine, increased due to cold treatment and/or during recovery after cold treatment, while asparagine, phenylalanine, leucine, alanine and threonine either decreased or remained unchanged in response to cold stress and during immediate recovery period. The special pattern with which levels of different free amino acids change in response to cold stress might have important implications on interactions between Asian citrus psyllids and citrus host plants.