Evidence from leaf thermoluminescence for a decrease of the [NADPH plus ATP] energetic potential in cold-sensitive Pisum sativum L. varieties upon hardening at 5 degrees C

Two cold-sensitive and three cold-tolerant varieties of pea were grown at 1 7 degrees/14 degrees C then cold-hardened at 5 degrees C. Chlorophyll therm oluminescence emission was recorded from 0 degrees C to 70 degrees C on fre shly excised leaflets after one, two or three flashes. Two flashes induced the strongest overall thermoluminescence emission, which could be decompose d into two components: (1) a band peaking at 28 degrees C corresponding to S2S3/Q(B)(-) recombination (B-band); (2) a sharp band peaking at about 45 d egrees C corresponding to a heat-induced reverse electron transfer towards the secondary quinone accepters Q(B) in those centers initially in the non- luminescent state S2S3/Q(B) (AG-band). The intensity of the AG-band was app roximately 1.6 that of the B-band in all unhardened pea plants. Upon harden ing, this ratio remained unchanged in the three cold-tolerant varieties, fa lling below 1 in the two cold-sensitive ones. The intensity of the AG-band has been previously shown to reflect the [NADPH + ATP] potential, and we co nclude that this potential is decreased at 5 degrees C in cold-sensitive va rieties. This phenomenon can be explained by the well-documented limitation of orthophosphate availability under cold conditions.