Studies On The Role Of Photorespiration And The Antioxidative Metabolism During Drought Stress In Desert Plant Reaumuria Soongorica

The great controversy about the photoprotective function of photorespiration under drought stress exists for many years.Some workers suggest that photorespiration can protect the photosynthetic apparatus against photodamage.The other workers believe that photorespiration is not important for photoprotection.In the present study,resurrection plant Reaumuria soongorica(Pall.)Maxim.,an extreme xeric shrub and a dominant species in add and semi-add areas,was used to investigate photoprotective function of photorespiration and effects of photorespiration on metabolic matter by application of an inhibitor of photorespiration,isonicotinic acid hydrazide(INH)under different levels of drought stress.In addition,it is questionable whether POD-CAT-SODs system plays a protector role against oxidative stress in plants.Therefore,the roles of antioxidative systems(POD-CAT-SODs)during the progressing drought stress were also studied. The main results were as follows:1.Diurnal patterns of gas exchange and chlorophyll(Ch1)fluorescence parameters of photosystem 2(PS2)were analyzed in two-year-old R.soongorica. Results displayed that drought induced the decreases of net photosynthetical rate(P_N) and the maximal photochemical efficiency(F_v/F_m)and actual photochemical efficiency(ΔF/F_m')of PS2,which indicated that drought led to photoinhibition in R. soongorica.The rate of photorespiration was estimated by combined measurement of gas exchange and Ch1 fluorescence.The rate of photorespiration(P_R)increased with the increasing drought stress(DS),as suggested that drought stress resulted in the obvious increases of photorespiration rate in R.Soongorica.2.F_v/F_m in INH-sprayed plants was lower than that in normal plants under moderate DS,but no significant difference was observed under severe DS.P_R,P_N,J_T, J_c and J_o in INH-sprayed plants was lower than that in normal plants under moderate DS.These results showed that photorespiration is a key pathway consuming excess electrons.H₂O₂ content in INH-sprayed plants was significantly lower than that in normal plants under severe DS.The data here indicated that high photorespiration promoted H₂O₂ accumulation.3.Total chlorophyll and soluble protein contents in INH-sprayed plants were lower than those in normal plants under moderate DS,but those were inverse under severe DS.Nitrate reductase activity,free amino acid and proline contents in INH-sprayed plants were lower than those in normal plants under all levels of DS. Results presented here indicated that photorespiration in R.soongorica decreased degradation of chlorophyll and increased protein accumulation.Photorespiration was strongly related to nitrogen metabolism.4.Antioxidative system's responses were examined in the different phases of drought stress and during recovery in three-year-old R.soongorica.Drought stress was imposed during 100 consecutive days and re-watering after 16,72 and 100 days. The concentration of H₂O₂,malondialdehyde(MDA)and superoxide dismutases (SODs)activity were elevated significantly with progressing drought stress. Guaiacol peroxidase(POD)and catalase(CAT)activities increased markedly in the early phase of drought and decreased significantly with further drought stress continuation,and POD activity was unable to recover after re-watering.Ascorbate (AsA),reduced glutathione(GSH),ascorbate peroxidase(APX)and glutathione reductase(GR)activities declined in the initial stages of drought process,elevated significantly with further increasing water deficit progression and recovered after re-watering.These results indicate that:1)iron SODs removing superoxide anion is very effective during the whole drought stress;2)CAT scavenges H₂O₂ in the early phase of drought while ascorbate-glutathione cycle scavenges H₂O₂ in further increasing drought stress;and 3)POD doesn't contribute to protect against oxidative damage caused by H₂O₂ under drought stress.Taken together,photorespiration played a different role under different DS: photorespiration in R.soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS,whereas it accelerated H₂O₂ accumulation markedly and induced the leaf abscission under severe DS.The results of antioxdative systems showed:SODs-CAT played the main role in scanvenging reactive oxygen species in the early phase of drought;in contrast,SODs-AsA-GSH cycle played the main role in further increasing drought stress.From both above results,we suggested that higher photorespiration and SODs activities induced the excess H₂O₂ accumulation under severe DS,which resulted in dormancy of resurrection plant R.soongorica.