Assessing optimal nitrate/ ammonium- ratios in baby-leaf lettuce to enhance the heat stress tolerance under elevated CO(2) conditions

In recent years, the interest on baby-leaf lettuce has grown steadily, because it is richer in bioactive compounds than other traditional vegetables. However, the quality of lettuce is being increasingly affected by climate change. It is very rare for a climatic effect to occur in isolation. Even then, a large body of work has only focused on the effect of isolated heat stress, fertilization, and elevated CO(2), on morphological, physiological and biochemical parameters. Thus, very few works have focused on how the combination of several of these factors can affect these parameters. For first time, the present work studied the combined effect derived from the application of two different levels of CO(2) (400 and 1000 ppm of CO(2)), four different NO(3)(-)/ NH(4)(+) ratios (100/0 (T-I), 100/0 before the short-term heat stress and finally without NO(3)(-) (T-II), 80/20 (T-III) and 50/50 (T-IV)), and a short-term heat stress (25 and 43°C), on some physiological and quality parameters (dry biomass, photosynthetic parameters, pigments content, lipid peroxidation and total soluble proteins content) of baby-leaf lettuce cv Derbi. Additionally, a comparison of that combined effect of all these parameters between inner and outer leaves was also performed. The results obtained indicated that the interaction between the nutrient solution containing a 50/50 NO(3)(-)/ NH(4)(+) ratio and a high concentration of CO(2) (1000 ppm) improved the biomass, photosynthesis, intercellular/external CO(2) concentration ratio (ci/ca), stomatal conductance (gs), evapotranspiration (E) and lipid peroxidation, and protein content in this baby-leaf lettuce. The results obtained in this work lead us to conclude that this existing interaction between the NO(3)(-)/ NH(4)(+) ratio and the elevated CO(2) concentration may be considered as a new strategy for making baby-leaf lettuce more resistant to heat stress, in other words, stronger against the ever more frequent heat waves.