Morphological, physiological and biochemical effects of some abiotic stresses on proline-supported citrus rootstocks

The Aurantioideae sub-family, in which citrus species are classified, consists of a dynamic plant group with approximately 3600 species. As in the world, among the all fruit groups, citrus is the highest in both amount produced and export value in our country. The highest commercial production values belong to the types of orange, mandarin, lemon and grapefruits, respectively. Like all plants, the physiological and metabolic activities of citrus fruits can be affected from different ecological conditions encountered during thei life cycle. The situations in which plant's biological system or function damaged, plant growth and development adversely affected are defined as stress. Stress factors can be classified in different ways according to their origins. These are called biotic (parasitic plant, animal, pathogen and various anthropogenic activities) and abiotic stress factors (oxidative stress, heavy metal, salt, drought, low-high temperature, water flooding, radiation, agricultural drugs). Abiotic stresses, such as salinity and drought, negatively affect all living and non-living elements that form the biosphere and its components, and can lead big losses in agricultural systems. The salt accumulation within the plant affects the physiological activity of plants, decreasing their productivity, resulting in product loss and decrease in quality. The performance of the plants against the harmful effects of stress varies depending on the metabolic activity of the plant, type of stress, duration of exposure and dose of stress. Osmotic stresses such as drought and salinity affect plant morphology and cause an increase in fibrous roots as a common defence mechanism. For this reason, stress type, amount, severity and benefit-harm threshold in plants are of great importance in terms of plant physiology and development. Drought stress affecting osmoregulation is also one of the most important abiotic stress that causes decrease in plant growth, development and yield in semi-dry climates. The drought stress is a result of global warming, especially due to decreasing water resources, leads to drought and desertification and this disrupts the biosphere and consequently decreases agricultural productivity. Understanding the responses and defence mechanisms of plants in environments affected by drought stress is an important part of making plants more tolerant to stress. In this study, the preference of drought and salinity stress on Carrizo rootstocks is to determine its potential defence mechanisms against these stresses. Stress effects to plants can be reduced, increased, or repaired with external applications. Proline is an amino acid with the highest accumulation in plants grown under stress conditions and having the lowest molecular weight with a simple chemical structure. Increased amount of proline concentration in vacuoles and within the cell is the first physiological reaction of plants in osmotic stress conditions including; water deficiency, cold, heavy metals, temperature and salinity and it is an indicator of how much and the extent of the plant can be resistant to a given stress. The increase in the concentration of the proline in the cell is an indicator of stress, and it is also the first step in a series of metabolic reactions that trigger the plant's defence mechanism against stress. Knowing the mechanisms of behaviour of citrus against the loss of production caused by abiotic stresses and taking measures to protect against stress may prevent the reduction of production amounts. In this study, individual and combined effects of drought and salinity stress were investigated on Carrizo rootstocks grown in soilless conditions in terms of physiological, morphological and biochemical parameters. For this purpose, 10 ten different treatments including; control, PEG 6000, peg 6000 + prolin, PEG 6000 + Mikoriza, NaCl, NaCl + PROLIN, NaCl + Mikoriza, NaCl + peg 6000, NaCl + mikoriza, NaCl + peg 6000 + mikoriza were studied on all organs of Carrizo seedlings grown in the soilless media. In the experiment, split parcel design with three replication each of which contains five seedlings were used. The experiment were carried on 2-3 months old real leaf formed seedlings that were subjected to 15 weeks of stress. While planned morpho-physiological analyses were carried during the assaying period, chemical and biochemical analyses (MDA, LOX, APX, GR, SOD, POD, CAT ) were performed at the end of the trial. In summary; as a result of the parameters examined on plant growth and development, inoculation with arbuscular mycorrhizal fungi increased the growth of citrus seedlings grown in saline environment. According to the results of physiological leaf parameters, the best results were found in mycorrhizal colonization on Carrizo seedlings. Mycorrhizal colonization was found to limit the intake of Cu. Each of the antioxidant enzymes (MDA, LOX, APX, GR, SOD, POD, CAT) shows the highest value in seedlings with salt application. In the study, it was concluded that there was no effect of exogenous proline application on the plant due to the fact that there was no difference leaf prolin levels among the treatments. In the roots, on the other hand, the lowest proline level found in the roots of M treatment suggested that mycorrhizal colonization was probably reduced the effect of stress that stimulates the proline production in the roots.