Reversible or irreversible change of molecular dynamics of water in pea seedlings exposed to heat stress

Influences of heat stress on epicotyls of pea (Pisum sativum L.) plants were investigated by using non-destructive method. NMR spin-lattice relaxation time (T ₁), indicating molecular dynamics of water, was determined in tall type, cv. Alaska and dwarf type, cv. Progress No. 9 seedlings grown under light condition. In no-elongation zone for both fresh cultivars a rapid decrease in T ₁ occurred in the tissues exposed to 20-30°C while rapid increase followed by gradual decrease occurred in T ₁ for the tissues exposed to 20-40°C during a heat-cool cycle. On the other hand, in heat-denatured dead tissues, T ₁ linearly corresponded. Further, when pea plants were heated for 5 h at 20-30°C or 20-40°C thermal hysteresis changes in T ₁ corresponding temperature in the subsequent heat-cool cycles was notably different. Epicotyls exposed to 30°C treatments did not show temperature dependency while those of the tissues exposed to 40°C indicated temperature dependency. After the subsequent heat-cool cycles, the former was alive, the latter indicated tissue necrosis. Therefore, the change in T ₁ of epicotyls observed within 1 h was considered a fast adaptation or mortality in cells to heat stress, and a temperature dependency of T ₁ in the subsequent heat-cool cycles clearly reflected tissue viability in pea epicotyls. In conclusion, T ₁ on thermal response can be used as an indicator of reversible or irreversible injury in an intact plant.