Quantitative evaluation of some biochemical traits participation to drought resistance in quinoa

Under oxidative stress conditions, the quantity of some biochemical traits, including antioxidants, increases in the plant. Now this question is raised that incremental change in these traits can always increase drought resistance (higher biomass) in drought-stressed conditions? In this regard, conflicting results have been published, which are mostly based on their correlation coefficient. The purpose of this study was a deeper contemplation into these complexities in quinoa; so, the nonlinear, synergistic, and antagonistic relationships of several biochemical traits with biomass were investigated, using multiple nonlinear regression based on stepwise selection. Finally, the resulting regression function was maximized (optimized). The split-plot experiment based on a randomized complete block design was carried out in three replications and two locations (Damghan and Dibaj). Factors were deficit irrigation levels [control (Supplying 100% of the plant's water requirements), moderate deficit irrigation (70%), and severe deficit irrigation (40%)] in the main plots and sodium nitrophenolate spraying levels [the control (spraying tap water on the plant), foliar spraying at the stem elongation stage and foliar spraying at the flowering stage] in the subplots. The data of biomass (dependent variable; Y or drought resistance) and several regressors (superoxide dismutase, anthocyanin, soluble sugars, proline, carotenoid, superoxide dismutase, ascorbate-peroxidase, and catalase; X or independent variables), obtained under severe deficit irrigation conditions, were used for the present analysis. The results of the regression analysis showed that the relation of carotenoid, catalase, anthocyanin, and ascorbate peroxidase with drought resistance was quadratic, however, the mentioned relationship was very different. Over low concentrations (activities), the effect of catalase, anthocyanin, and ascorbate peroxidase was statistically not significant (with increasing concentration, drought resistance showed no change), but over higher concentrations, their impact was decreasing; the effect of ascorbate peroxidase appeared to be increasing just over its higher concentrations. On the contrary, over both low and high concentrations, the effect of carotenoid was positive; the positive effect of higher carotenoid concentrations tended to be slightly stronger than that of low concentrations. Proline and catalase had the highest effect on drought resistance when their increase was considered simultaneously (synergistic relationship). The effect of soluble sugars was negative; particularly, the simultaneous increase in soluble sugars and proline concentrations led to a sharp decrease in drought resistance (antagonistic relationship); hence, it seems that the simultaneous increase of these two traits should not be used for breeding purposes. The results regarding optimization (maximization) of the regression function indicated that a combination of 600 Unit SOD.g-1FW superoxide dismutase, 8.58 μmol ascorbate min-1.mg-1FW ascorbate peroxidase, 2.28 μmol H2O2.min-1FW catalase, 6.65 mg.g-1FW soluble sugars, 2.15 mg.g-1FW anthocyanin, 6.79 mg.g-1FW proline, and 18.8 mg.g-1FW carotenoids could be led to an increase in biomass by 9% higher than the maximum observed value of biomass.