Document Type : Full Paper
Extended Abstract
Introduction
Citrus is one of the most economically important fruit crops in the world, mostly belonging to the genus Citrus and the family Rutaceae. They are mainly trees or shrubs adapted to tropical and subtropical regions. The trend of citrus production is continuously increasing, so citrus is among the most widespread tree fruit crops in the world, although its production can be limited by abiotic stresses. Salinity is one of the major abiotic stresses that significantly affects plant growth and yield. The continuous increase in salinity in agricultural lands due to poor cultivation practices and climate change has devastating global effects, and approximately 50% of arable land will be lost by the middle of the 21st century. Another limiting factor for plant growth and development is alkaline stress, which directly affects the absorption of mineral elements due to high pH and disrupts ionic homeostasis. Alkaline stress is one of the most important abiotic stresses that has been increasing in recent years, especially in areas with calcareous waters. High pH can lead to reduced seed germination, destruction of root cell structure, changes in nutrient availability, especially iron, and consequently a significant reduction in plant growth and yield.
Simultaneous salinity and alkalinity stress affect root acidification and iron absorption due to their negative effects on the activity of enzymes involved in iron absorption or siderophore production in plants. The response of citrus trees to stress conditions depends on several factors such as irrigation system, soil, climate, and rootstock type. Rootstock selection is a very important decision that is made based on the climatic and soil conditions of the region.
Materials and Methods
This experiment investigated the tolerance of two citrus rootstocks, including sour orange and Volkameriana, to salinity and alkaline stress. For this purpose, one-year-old seedlings of these rootstocks were obtained from a commercial nursery in Dezful in 2019 and transferred to the greenhouse of the Faculty of Agriculture, Shahid Chamran University of Ahvaz, and then cultivated in 7-liter pots (containing a sand substrate). Hoagland nutrient solution was used to nourish and facilitate the establishment of the seedlings. After establishing the seedlings in the new conditions in June, they were subjected to salinity (sodium chloride) and alkaline stress conditions. Salinity stress included concentrations of 0, 30, 60, and 90 mM sodium chloride. (To prevent osmotic shock to the plant, the salinity treatment was gradually applied in three stages). Alkaline stress was applied by increasing the pH of the nutrient solution using sodium carbonate from 6.5 to 8.2. Stress was continued until the plants showed signs of stress. This experiment was a factorial experiment in a completely randomized design with four replications. During and at the end of the experiment, traits such as fresh and dry weight of shoots, fresh and dry weight of roots, photosynthesis, transpiration, ion leakage, relative water content, chlorophyll, carotenoids, proline, and carbohydrates were measured.
Results and Discussion
According to the results obtained in sour orange and Volkameriana rootstocks, increasing salinity concentration caused a decrease in morphological characteristics, including fresh and dry weight of shoots and fresh and dry weight of roots at both pH levels, with a greater reduction at pH 8.2 than at pH 6.5. The lowest fresh weight of shoots (89 g) was obtained in Volkameriana with pH 8.2 and 90 mM NaCl, and the highest fresh weight of shoots (180.50 g) was observed in sour orange with pH 6.5 and 0 mM NaCl. Volkameriana rootstock with 90 mM NaCl and pH 8.2 had the lowest dry weight of roots (15.88 g), which was a decrease of 48.88% compared to the control of this rootstock. In sour orange and Volkameriana, salinity and alkalinity stress significantly reduced the rate of photosynthesis, which was directly related to the increase in stress intensity and reached its lowest level at pH 8.2 with 90 mM NaCl, which caused a 51.83% reduction in sour orange and a 57.5% reduction in Volkameriana compared to the control. Salt-alkali stress increased the amount of ion leakage, proline, and carbohydrate in both sour orange and Volkameriana rootstocks. The highest amount of ion leakage (42.57%) was obtained in Volkameriana rootstock at 90 mM NaCl with pH 8.2, which was an increase of 74.03% compared to sour orange with pH 6.5 and 0 mM salinity. The highest amount of proline (23.02 μmol/g f.w.) was observed in sour orange with pH 8.2 and 60 mM NaCl, and Volkameriana at pH 8.2 and 30 mM NaCl had the highest amount of carbohydrate (29.89 mg/g f.w.). Stress conditions caused a decrease in photosynthetic pigments and gas exchange due to reduced absorption of nutrients, including magnesium, and increased chlorophyllase activity, which ultimately reduced the fresh and dry weight of the plant. Also, to increase tolerance to stress conditions, the amount of osmolytes increases. The accumulation of compatible nitrogenous osmolytes, including proline, plays a role in osmotic regulation, protecting cellular macromolecules against salinity damage, nitrogen storage, and free radical inhibition. It increases the activity of sucrose phosphate synthetase, which increases the amount of soluble sugars.
Author Contributions
A. D. designed the study, conducted the experiments, and wrote the draft. E. K. contributed to data analysis and methodology. Sh. Z. and Kh. M. reviewed and provided critical revisions to the manuscript. All authors read and approved the final manuscript.”
Data Availability Statement
The datasets generated and analyzed during the current study are available from the corresponding author on reasonable request.
Acknowledgment: This project was supported by Shahid Chamran University of Ahvaz. The authors
would like to thank the reviewers for their constructive comments and suggestions.
Ethical considerations
Not applicable: the study was conducted on plant samples only, and no human or animal subjects were involved.
The authors declare no conflict of interest.
