Exogenous glycine betaine and proline play a protective role in heat-stressed barley leaves (Hordeum vulgare L.): A chlorophyll a fluorescence study

Effects of drought and exogenous glycine betaine and proline on Photosystem II (PSII) photochemistry were studied in barley leaves under heat stress induced by exposing them to 45 degrees C for 10 min. Polyphasic fluorescence transient (OJIP) was used to evaluate PSII photochemistry in leaves treated with either glycine betaine or proline, combined or not with heat treatment. A distinct K step in the fluorescence transient OJIP appeared in control leaves, indicating an inactivation of the oxygen evolving complex (OEC). Drought stress and exogenous glycine betaine and proline modified the shape of the OJIP curve of leaves heated at 45 degrees C and the K step was not as pronounced. Increased thermostability of PSII may be associated with the resistance of OEC and increased energy connectivity between PSII units. The thermostability of PSII was also reflected by a lower decrease in maximum quantum yield of primary photochemistry (phi Po F-V/F-M) and performance index (PI). Exogenous application of glycine betaine or proline can play an important role in enhancing plant stress tolerance and may help reduce effects of environmental stresses.