**1. Introduction**

When plants are exposed to adverse environmental conditions, such as nutrient deficiency, lack of water, low or high temperature, ultraviolet radiation, salinity, insufficient oxygen, heavy metal toxicity, diseases, and pests, their growth is adversely affected. This condition is called stress. Stress can last for a long time or be temporary for a short time. Agricultural productivity is decreasing due to the detrimental impacts of climate change. Therefore, in order to extend sustainable agriculture and to increase crop products for food in the world, it seems necessary to use the appropriate solutions

to decline the negative effects of stresses on agricultural plants [1]. Salinity is one of the most important abiotic stress factors that adversely affects growth and development in plants, limiting yield and quality. The salinity-affected area is expected to reach about 50% of total agricultural land by 2050. Salinity stress generates various detrimental effects on plants' morphological, physiological, biochemical, molecular, and agronomic characteristics and decreases productivity. Reduced plant growth under salinity stress is due to decreased nutrients, hormonal imbalance, generation of reactive oxygen species (ROS), ionic toxicity, and osmotic stress [2].

In recent years, improvements in beneficial microorganisms have raised the tendency to use biofertilizers as valuable tools in sustainable agriculture. Biofertilizers have various benefits for plant growth. They regulate the soil texture and activate the soil biologically. It has been reported that many biofertilizers suppress plant pathogens and protect the plant against soil-borne diseases, so they are known as environmentally friendly. In terms of agricultural sustainability, biofertilizers do not harm the ecological system and do not contain harmful substances, they are proportionally cheaper when compared to commercial chemical fertilizers. Biofertilizers stimulate plant growth and produce phytohormones, thus increasing the yield and quality of the plant. In the fight against salinity, biofertilizer applications are widely preferred all over the world because they significantly increase salt tolerance [3].

One of the most effective alternatives among biofertilizer applications is mycorrhiza. Mycorrhizal fungi, which have the ability to establish a symbiotic relationship with plant roots, take carbohydrates that they cannot synthesize from the plant itself and contribute to the ability of plants to take in more water and nutrients by expanding their root domain thanks to their hyphae [4, 5]. It has been reported that the positive effect of mycorrhiza is not only to increase the intake of water and nutrients but also to increase the tolerance of plants to abiotic and biotic stress conditions [4, 6, 7]; mycorrhiza and beneficial bacteria have taken their place in the biofertilizer industry in recent years. The effectiveness of these fertilizers has positive effects on the nutrition of the plants by increasing the solubility of nutrients in the root area, with benefits, such as lowering the pH in the root zone, secretion of chelators, production of special ion carrier proteins [8–12]. While the solubility and availability of nutrients, such as phosphate, Fe, Zn, and Mn, increase by decreasing the pH in the root zone, some bacteria also fix the nitrogen to the soil from the air. It is reported that PGPR (plant growh promoting rhizobacter) bacteria that promote plant growth produce hormones, fix nitrogen in the air, and dissolve phosphate [13].

*Chlorella vulgaris*, one of the microalgae species with the highest biotechnological applicability, has been widely commercialized and is used as a food supplement for humans and as a feed additive for animals. These algae, a member of *Chlorophyta*, is seen as an alternative protein source due to their high protein content of 42–58% and has been cultivated for various purposes by many countries [14]. Instead of chemical fertilizers, which are generally used as a nitrogen source in agriculture, the use of *C. vulgaris* with high protein content will be a cheaper and environmentally friendly application. However, studies on the use of microalgae as biofertilizers, both in the world and in our country, are limited.

The origin of lettuce is accepted as Anatolia, Caucasus, and Turkestan regions. Some researchers stated that different forms of salad and lettuce are found in central Europe and southern Europe and the Canary Islands, some African countries, Mesopotamia, Kashmir, Nepal, and even Siberia [12]. Its Latin name "*Lactuca sativa* L. var*. longifolia*" was used in this study. It is also called Romain lettuce or Cos lettuce. *Impact of Biofertilizers on Plant Growth, Physiological and Quality Traits of Lettuce… DOI: http://dx.doi.org/10.5772/intechopen.108710*

This lettuce is a species whose leaves are longer than wide, the leaves overlapping each other and often forming a loose and oval core.

The aim of this study is to determine the effect of microalgae, bacteria, and mycorrhiza biofertilizers on plant growth, yield, and plant nutrient content of lettuce grown under salt stress. It also revealed the effects of using less chemical fertilizers in lettuce cultivation, thus saving fertilizer and protecting the environment, as well as the yield and quality of the plant.
