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

Two cold-sensitive and three cold-tolerant varieties of pea were grown at 17°/14 °C then cold-hardened at 5°C. Chlorophyll thermoluminescence emission was recorded from 0°C to 70°C on freshly excised leaflets after one, two or three flashes. Two flashes induced the strongest overall thermoluminescence emission, which could be decomposed into two components: (1) a band peaking at 28°C corresponding to S2S3/QB− recombination (B-band); (2) a sharp band peaking at about 45°C corresponding to a heat-induced reverse electron transfer towards the secondary quinone acceptors QB in those centers initially in the non-luminescent state S2S3/QB (AG-band). The intensity of the AG-band was approximately 1.6 that of the B-band in all unhardened pea plants. Upon hardening, this ratio remained unchanged in the three cold-tolerant varieties, falling 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 conclude that this potential is decreased at 5°C in cold-sensitive varieties. This phenomenon can be explained by the well-documented limitation of orthophosphate availability under cold conditions.