Effect of drought stress and plant density on some quantitative and qualitative characteristics of two local stands of garlic in Semnan

Dissertation for Master's Degree in Agricultural Engineering

Agriculture Orientation

Abstract

Environmental stress is one of the most important factors that determine the pattern of plant distribution in the world, and drought stress also determines part of this distribution. Garlic is a plant that has been of special importance in terms of food and medicine in human life for centuries. Knowing the physiological parameters of different ecotypes of garlic under drought stress in different planting densities can help researchers in choosing the best and most tolerant ecotype and optimal density and the possibility of measuring the response of this plant to stresses. In this regard, an experiment was conducted on two ecotypes of garlic in the form of split-factorial in the form of randomized complete block design with 3 replications in the climatic conditions of Semnan. In this experiment, 3 levels of drought stress were considered based on 60 and 80% water requirement of garlic, along with the control treatment (100% water requirement), as the main factor and the factorial combination of density in 3 levels (30-40 and 50 plants per square meter) and ecotype in 2 levels, Turud and Tabas, were considered as secondary factors. To calculate the water requirement of garlic, the daily meteorological parameters recorded at the Semnan synoptic station were used and the water requirement was determined based on the instructions for calculating the water requirement of plants FAO-56. The results showed that with the application and intensification of drought stress, the maximum leaf area index, maximum dry matter accumulation, maximum crop growth rate, maximum net assimilation rate, relative leaf water content, yield, percentage of dry matter, fresh weight and dry weight of shoots, diameter of shoots and length of shoots, dry weight of shoots, diameter of shoots and length of shoots were significantly reduced compared to the condition of 100% irrigation. The carotenoid content and the number of garlic in Sukh did not show a significant decrease with the application of drought stress at the level of 80% water requirement compared to the irrigation condition of 100% water requirement, but the intensification of the stress at the level of 60% water requirement caused a significant decrease compared to the other two levels of irrigation. By applying stress, a significant increase in the amount of soluble sugars and non-enzymatic antioxidants was observed. Turood ecotype had significantly higher values ??than Tabas ecotype in terms of traits such as plant height, multiple crop growth rate, relative maximum growth rate, leaf non-enzymatic antioxidant, fresh and dry weight of garlic and garlic length. On the other hand, traits such as number of leaves, maximum accumulation of dry matter, chlorophyll a, chlorophyll b, total chlorophyll, carotenoid, relative water content, and the number of shoots per stem were significantly higher in Tabas ecotype than in Troud ecotype. But in general, the ecotype did not have a significant effect on the yield and percentage of dry matter, and it can be concluded that the two ecotypes were not superior to each other in terms of obtaining a higher yield. Due to the increase in the amount of non-enzymatic antioxidants in the Troud ecotype when faced with drought stress, this ecotype has been more successful than the Tabas ecotype in terms of creating a tolerance mechanism against stress. On the other hand, due to the non-significance of the interaction effect of drought stress and ecotype on the final yield of Sukh, it can be concluded that both ecotypes were not superior to each other in relation to drought stress and had relatively the same trend. Planting density of 30 plants per square meter created the highest maximum rate of net assimilation, and then with increasing planting density, a significant decrease was observed in relation to this trait, while planting density of 50 plants per square meter had the highest value of leaf surface index and maximum dry matter accumulation, and with the decrease of density, these traits also decreased significantly. Planting density was not significant on all traits related to yield components, but it had a significant effect on yield, and by increasing the planting density to 50 plants per square meter, the yield of Sukh showed a significant increase compared to the other two planting densities. Considering the significant decrease in yield and many parameters measured under the influence of drought stress, it can be seen that in order to obtain a suitable yield in relation to the garlic plant, it is necessary to avoid facing this plant with drought stress, and the density of 50 plants per square meter was recognized as the optimal density due to the appropriate response of garlic and obtaining a higher yield than the other two planting densities.Keywords: antioxidant, density, drought stress, garlic, soluble sugar, relative water content. rtl;"> 

Chapter 1

1- Generalities of the research

1-1- The importance of studying drought stress

Environmental stresses are one of the most important factors that determine the pattern of plant distribution in the world, and drought stress also determines a part of this distribution (Kafi et al., 1388A). The world's population is increasing at an alarming rate, on the other hand, it is decreasing due to the existence of numerous non-living tensions of food production and provision; In this case, any decrease in product production and food provision due to the increase in the world's population is worrying due to the lack of response to food needs. The growth and fertility of plants is negatively affected by natural disasters in the form of living and non-living stressors. Non-living stresses every year cause the loss of huge costs due to the reduction of production and the loss of the product. In fact, these tensions threaten the stability and sustainability of agriculture (Mahajan and Tutja [1], 2005). Drought stress is the most important environmental factor limiting the growth and development of plants all over the world, so that the reduction in growth due to drought stress is much more than other environmental stresses (Vasipour et al., 2013).

Rainfall in Iran with an average of 240 mm is less than a third of the global annual precipitation (700 mm) and Iran has a dry and semi-arid climate. (Bagheri et al., 2018). In dry environments, the atmospheric demand for evaporation and transpiration is higher and to produce a unit of dry matter, the plant is bound to lose more water (Wafabakhsh et al., 2018) and the yield of the crop is significantly reduced due to drought stress (Ashraf [2], 2010). Severe drought stress reduces plant growth by affecting physiological and biochemical processes, such as photosynthesis, respiration, transport, ion absorption, carbohydrates, and metabolism (Jalal et al. [3], 2012). The response to water shortage depends on the plant species and variety, the duration and duration of drought stress, the age and developmental stage of the plant, the type of plant cells and organs, and subcellular components and its structure (Berry[4], 1997 and Levitt[5], a1980).

Since most of the reports related to climate change indicate an increase in dryness in many regions of the world (Houghton et al.[6], 2001), examining the behavioral and physiological responses of plants to drought stress and the mechanisms of creating resistance and studying the effects of stress on yield and its components in plants are of particular importance. Air, light and soil are provided to the plant in a more appropriate way, and at the same time the competition between plants is reduced to the lowest level (Khajepour, 2018). Structural characteristics and vegetation cover are related to radiation absorption and have a decisive role in plant performance (Kochaki and Sarmadanya, 2019).  In one-year plants, the main reason for yield reduction is poor vegetative growth and as a result, low leaf area at the beginning of the growing season, and in this case, most of the solar radiation is absorbed by the ground and remains unusable, so in such conditions, increasing the density of plants may help to absorb more solar radiation, especially in the early stages of growth (Amir Moradi and Rezvani Moghadam, 2019). The efficiency of absorption of radiant energy that shines on the surface of a product requires sufficient leaf surface and its uniform distribution so that it completely covers the surface of the earth. This goal can be achieved by changing the density of plants and the proper arrangement of plants on the surface of the soil. Also, with the increase in plant population, the light intensity in the cover is reduced and this action causes the reduction of plant biomass (Razmi, 2010). Considering the competition that occurs between plants in different planting densities to absorb light and nutrients, planting intervals can have a significant effect on crop yield (Mirshekari and Mobasher, 2015).