RESUMO
Effects of putrescine (Put) and high temperature (HT) on the responses of wheat (Triticum aestivum L.) seedlings or developing grains were studied in six wheat cultivars (PBW 343, C 273, C 306, PBW 550, PBW 621 and HD 2967) raised in the laboratory (25°C or 32°C) in Petri dishes for seven days or in the field under normal (November) and late (December) planting conditions. High temperature significantly reduced root/shoot lengths and dry weights of all the six cultivars, whereas Put ameliorated this effect by increasing these attributes. C 306 and C 273 showed higher root/shoot lengths and dry weights over PBW 550, PBW 621. High temperature significantly increased the activities of ascorbate peroxidase, guaiacol peroxidase, superoxide dismutase and catalase in correspondence with an increase in membrane injury index (MII) and thiobarbituric acid reactive substances (TBARs) and H2O2 contents. Application of Put under HT further elevated activities of these enzymes whereas MII and contents of TBARs, H2O2 were reduced. In fact, Put treated seedlings performed better in terms of antioxidant response under HT than seedlings in absence. Various morpho-physiological parameters like plant height, peduncle length, 1000 grain weight and the number of grains per year were decreased in the late sown crop. However, Put application under HT stress increased these parameters. Overall, it appears that upregulation of the antioxidant system by Put in wheat cultivars contributes to the better protection and membrane integrity in the root, shoot and grains against HT through reduced MII and TBARs contents.
RESUMO
Stripe rust (Puccinia striiformis f.sp. tritici) is the most devastating disease of wheat (Triticum aestivum L.) accounting huge economical losses to the industry worldwide. HD 2329 was a widely grown wheat cultivar which had become highly susceptible to stripe rust and was used to understand the biochemical aspects of the host pathogen interaction through characterization of superoxide dismutase (SOD). In the present study, two types of SOD, ionically or covalently bound to the particulate fraction were found in the stripe rust infected and uninfected wheat leaves of susceptible cultivar HD 2329. Cell walls of leaves contained a high level of SOD, of which 41-44% was extractable by 2 M NaCl and 10-13% by 0.5% EDTA in infected and uninfected leaves. The NaCl-released SOD constituted the predominant fraction. It exhibited maximum activity at pH 9.0, had a Km value of 1.82-2.51 for uninfected and 1.77-2.37 mM for infected, respectively with pyrogallol as the substrate, and a Vmax of 9.55-21.4 and 12.4-24.1 A min-1g-1FW. A temperature optimum of 20oC was observed for SOD of both uninfected and infected leaves. SOD showed differential response to metal ions, suggesting their distinctive nature. Inhibition of wall bound SOD by iodine and its partial regeneration of activity by mercaptoethanol suggested the involvement of cysteine in active site of the enzyme. These two forms showed greater differences with respect to thermodynamic properties like energy of activation (Ea) and enthalpy change (H), while entropy change (S) and free energy change (G) were similar. The results further showed that pathogen infection of the leaves of susceptible wheat cultivar induced a decrease in the SOD activity and kinetics which might be critical during the response of plant cells to the infection.