Changes in the Soluble Protein and Free Amino Acid Content of Chill-Sensitive and Chill-Resistant Plants During Chilling and Hardening Treatments

The effects of low temperature (5 °C and 12°C) and drought treatments on leaf soluble protein content and free amino acid content have been investigated in four species, which were ranked according to chilling-sensitivity: pea (chill-resistant), mung bean (highly chill-sensitive), and tomato and french bean (intermediate chilling-sensitivity). Drought treatment caused a 30–40% decrease in protein levels, and in all but the mung bean, a 100–200% increase in free amino acid concentration. Four days chilling at 5°C, 85% r.h. caused leaf water content to decrease by almost 50% in the mung bean, but by only approximately 6–7% in the other three species. During this treatment the leaf soluble protein content decreased in all four species although the decrease was greatest and most rapid in the mung bean, commencing with 8 h of chilling (coinciding closely with the onset of water loss), and decreasing by over 80% after 4 d. In the chill-sensitive species (but not in the pea) the decrease in protein content was accompanied by an increase in free amino acid content. However, on a mgg−1 dry wt. basis, this increase was insufficient to account for all the protein lost. When plants of each species were chilled at 5°C, 100% r.h., water loss was greatly reduced or prevented and there was no significant decrease in leaf soluble protein. It is concluded that the protein decrease which occurred at 5°C, 85% r.h., was a response to water loss and not the direct result of low temperature. However, chilling at 100% r.h. did cause an increase in free amino acid content of the chill-sensitive species, suggesting that this was a direct response to low temperature. Although drought treatment caused a 6–20 fold increase in free proline content in the leaves of the four species examined, chilling (5°C) and chill-hardening (12°C) caused little change in free proline content, indicating that the accumulation of this ‘protective’ amino acid is unlikely to contribute to the effectiveness of the chill-hardening treatment.