Introduction: One of the most important strategies to reduce the damage caused by environmental stresses, especially in arid and semi-arid regions, is use of beneficial microorganisms including mycorrhiza fungi. Plant-mycorrhiza fungi symbiosis by improving physiological and growth characteristics alleviates the negative effects of water stress on plants. Symbiosis of mycorrhizal fungi with plants under water stress conditions improves crop production by increase uptake of more immobilized nutrients such as phosphorus, zinc and copper via increasing the root development, chlorophyll content and Fv/Fm. Lettuce (Lactuca sativa L.) is used as a fresh vegetable because of its high nutritional value. Since there is no research about the ameliorating effects of mycorrhiza symbiosis on the growth characteristics of lettuce under water stress conditions in Iran, therefore the purpose of this study was to investigate the effect of different levels of water stress and mycorrhiza inoculation on some physiological traits, nutrient uptake and economical yield of lettuce.
Materials and Methods: In order to investigate the effect of mycorrhiza biofertilizer on physiological traits, nutrient uptake and economic yield of lettuce under drought stress conditions, a split plot layout based on a randomized complete block design with three replications was conducted at University of Hormozgan in 2017-18. Experimental factors were included water stress levels as the main factor in three levels including irrigation of lettuce plants based on providing 50, 75 and 100% of readily available water, equal to mild and moderate deficit irrigation and control respectively, and mycorrhiza fertilizer, it consists of three species of Glumos intraradices, G. mosseae and G. fasciculatum, at 0, 5, 10 and 20 g.m^-2 as sub factor. The studied traits included physiological traits (chlorophyll a, b and total, maximum photosystem II efficiency (Fv/Fm) and catalase activity, relative leaf water content) leaf P and K content and economical yield of lettuce.
Results and Discussion: The results of this study showed that water stress and mycorrhiza fertilizer significantly affected physiological traits of lettuce. With increasing in water stress level, root colonization percent, leaf chlorophyll a, b and total content, maximum photosystem II efficiency (Fv/Fm), leaf relative water content, P and K content and overall economic yield of lettuce were decreased, with the most reduction being in moderate water stress condition. While catalase activity and proline content was significantly increased under this condition. Application of mycorrhiza fertilizer under water stress conditions could improve physiological traits and nutrient content, so that in moderate water stress condition, application of 20 gm^-2 of mycorrhiza fertilizer significantly increases the activity of catalase, Fv/Fm, chlorophyll a and total content, relative leaf water content, K and P of leaves by 53.1, 37.9, 30.1, 7.0, 105.4 and 52.7% respectively, compared with no fertilizer application. At each level of drought stress, mycorrhiza fertilizer often causes a significant increase in lettuce economic yield compared to no fertilizer application. So that, application of 20 gm^-2 of mycorrhiza fertilizer in control irrigation, mild and moderate drought stress, increased the economic yield of lettuce by 43.9%, 80.8% and 59.5%, respectively, compared to the control (no application of mycorrhiza). These results showed that mycorrhiza fertilizer likely by improving the absorption of potassium and phosphorus of the leaf and increasing the synthesis of plant pigments and increasing the leaf relative water content, could increases Fv/Fm and the photosynthesis efficiency and thereafter economical yield of the lettuce plant under water stress condition. Similar to these results, some researchers have reported that mycorrhiza fertilizers could increase plant tolerance to water stress, via improving physiological and vegetative traits such as catalase activity and chlorophyll content and vegetative characteristics including plant relative water content, fresh and dry weight and economical yield. Mycorrhizal fungi also increase drought resistance by increasing plant antioxidant capacity and thus improve vegetative characteristics and plant yield (Wang et al., 2019; Rapparini & Penuelas, 2014).
Conclusion: In the present study, likely mycorrhiza fertilizer increased photosynthesis capacity by increasing leaf chlorophyll, relative leaf water content, photosystem II efficiency and catalase activity, which resulted in improvement of vegetative characteristics and, consequently, fresh weight, as an economical yield of lettuce under water stress conditions. In the present study, application of mycorrhizal biofertilizer through improved biochemical and vegetative characteristics increased tolerance of lettuce under water stress conditions. Therefore, it seems that the application of mycorrhiza can be considered as one of the feasible solutions to deal with environmental stresses in sustainable agriculture.