In this study,the gas exchange,chlorophyll fluorescence and antioxidant activity in tall fescue cultivar,Houndog 5 ,were investigated under long-term high temperature stress. The results showed that the net photosynthetic rate (Pn),chlorophyll content,apparent quantum yield (AQY),the maximum carboxylation rate of Rubisco (V c,max ),the maximum rate of RuBP regeneration (J max ) and carboylation efficiency (CE) were significantly decreased after treatment with high temperature for 30 d. High temperature also decreased the efficiency of excitation capture by open PSⅡ reaction centers (F’v/F’m) and the proportion of open PSII reaction centers,thereby resulting in an increased electron transport flow (ETR). The non-photochemical quenching (NPQ) and energy-dependent quenching (qE) of the tall fescue were significantly decreased,and the excess energy of PSII reaction centers (Ex) was largely increased after long-term high temperature stress. Moreover,high temperature stress significantly decreased the SOD,POD,APX and GR activities,and increased the MDA content. These results indicated that the mechanism of excitation energy dissipation and antioxidant systems were injured,so that the excessive excitation energy could not be dissipated and reactive oxygen species could not be scavenged efficiently.
In order to thoroughly understand the mechanism Of drought resistance in plants at DNA level, the DNA damage of two ecotypes of reeds (Phragmites communis T.) stressed by PEG 6000 was analyzed by means of fluorescence analysis of DNA unwinding (FADU). The results showed that the residual double strand DNA percentages (dsDNA%) in dune reed (DR) were significantly higher than those in swamp reed (SR) treated with either 20% or 30% PEG 6000. This meant that the DNA of DR was less damaged in comparison with SR. Similarly, DR resisted DNA damage more strongly than SR as reactive oxygen species (ROS) increased by adding ROS producers diethyldithio carbamate (DDC), H2O2 and Fe2+ of different concentrations. Meanwhile, treating PEG stressed SR with ROS scavengers such as dimethyl sulphoxide (DMSO) and ascorbic acid (Vc) resulted in the reduction of DNA damage, suggesting that ROS could cause DNA damage. In addition, the DNA repair for water-stressed reeds indicated that DR repaired DNA damage much faster and more completely. This might be the first indication that drought stress led to DNA damage in plants and that drought resistance of plants was closely related to DNA damage and repair.
