lettuce

Lettuce (Lactuca sativa) represents a major horticultural crop in Iran and worldwide. Lettuce requires a high amount of water to grow well and is adversely affected by drought. Regarding the reports of the positive effect of seaweed extract on increasing the resistance of plants to abiotic stresses, the present study aimed to evaluate the effect of the seaweed Sargassum angustifolium extract on growth and yield indices of lettuce under drought stress. This study was conducted as a factorial experiment with three treatments of algae extracts concentrations and two treatments of drought stress, and non-stress treatment with three replications. To evaluate the effects of algae extract on lettuce under drought stress conditions, a combination of morphological and physiological characteristics including plant height and dry weight, photosynthetic pigment content, and antioxidant activities were measured. Results showed that treatment of seaweed extract significantly increased plant resistance to drought stress and improved morphological and physiological indices of lettuce (p< 0.05). The best results were obtained in the 1.5 g/l treatment of seaweed extract.

IIn this study, the effects of paraxanthin on lettuce were investigated. Paraxanthin is a group of purine alkaloids, secondary metabolites of plants. This compound is made up of the caffeine metabolism found in plants such as tea, coffee and cocoa. There have been previous reports of paraxanthine antihypertensive properties.Three different concentrations of paraxanthine as 10, 50 and 100 µg/ml were treated on lettuce seedlings and then some physiological and biochemical parameters were measured at 7 leave stage of the plants. The results showed that seed germination, radicle and plumule growth of lettuce plant decreased at different concentrations of the paraxanthine in a dose dependent manner. At the concentration of 100µg/ml of paraxanthine, all of the growth parameters completely inhibited. Some measured parameters such as wet and dry weights of the shoots, photosynthetic rate, photochemical efficiency of photosystem II, leaf relative content and total protein decreased in the paraxanthine treated plants compared to the control group. However, some other parameters such as proline content, hydrogen peroxide and the activity of polyphenol oxidase, ascorbate peroxidase, catalase and protease in treated plants increased. On the other hand, a significant change in electrophoretic pattern of the plant leaf proteins were observed. As some electrophoretic bands in treated plants with 100µg/ml of paraxanthine removed in comparison than control group. The present study demonstrated that the paraxanthine as an allelochemical causes some physiological and biochemical responses in the lettuce which are much similar to those appears under abiotic stress.

The experiment was conducted in factorial test based on a completely randomized design with three replications in greenhouse of Isfahan University of Technology of Iran. Lettuce seedlings were exposed separately to the exhaust emissions in three different exposure times (10, 20 and 30 days) and after the exposure time, their morphological, physiological and anatomical traits were measured and compared with morphological, physiological and anatomical traits of control plants. Results obtained in lettuce, showed significant increases in traits including catalase, ascorbate peroxidase, guaiacol peroxidase, proline, electrolyte leakage, relative water content and number of stomata and significant decreases in photosynthetic factors, leaf area, height, aerial fresh and dry weight and length of stomata in lettuce plants which were exposed to gases for 30 days, compared to controls. Significant increase in the photosynthetic rate, stomatal conductance, CO2 intracellular concentration and leaf area of lettuce seedlings were exposed to emissions for 10 days, most likely, indicate the nutritional role of output gases from generator in the short period time of exposure. It was observed that the anatomical features of contaminated samples have been changed by gases in 30-days exposure plants compared to others. The results showed that the exposure of lettuce plants to gases, especially 30-days exposure, caused morphological, physiological and anatomical reactions due to contaminants that could be considered these reactions as the adaptation of these plants to the stress of contamination, in order to adapt or to deal with stress conditions and thus to survive the plant under pollution stress conditions.

As natural plant growth stimulators, amino acids are widely used to improve the yield and quality of crops. Change in enzymes activities of N assimilation (NR, NiR and GS), residual nitrate (NO3-), soluble protein content, and yield of spinach and lettuce plants were investigated under replacing 20% nitrate-N in the nutrient solution by L-glycine and blood meal amino acids. Seeds of the mentioned plants were sown in soilless medium for two weeks in the growth chamber. Seedlings in double leaf stage were transferred into the pots. After 30 days from transplanting, 20% of nitrate in the nutrient solution was replaced by L-glycine and a mixture of amino acids extracted from blood meal. Compared with the full nitrate treatment, amino acids replacement caused decrease in nitrate accumulation in the leaves of studied plants. The decrease in nitrate accumulation was accompanied by a decrease in nitrate reductase activity, an increase in glutamine synthetase and high production of amino acids and chlorophyll content. The amino acids present in the blood meal were more effective than the L-glycine treatment to reduce the nitrate concentration in spinach (1.72%) and lettuce (17.5%). Compared to the full nitrate treatment, partial replacement of blood meal amino acids increased the soluble protein content in the leaves of spinach (67.36%) and lettuce (83.82%). Supplying with amino acids significantly enhanced total nitrogen and dry matter in the studied plants, although effects of blood meal amino acids treatment were higher than L-glycine. Based on the results, partial replacement of nitrate with amino acids could cause decrease in nitrogen accumulation in spinach and lettuce plants.