The effects of different levels of salinity stress on the yield and some physiological characteristics of different pinto bean cultivars

Introduction
Environmental stresses are among the major limiting factors of crop production at the global level. Environmental stresses affect the biochemical and physiological processes of plants, leading to reduced crop yield, and in severe cases, even plant death. Among the abiotic stresses, salinity stress ranks second after drought stress as a limiting factor for the yield of agricultural crops worldwide (Gupta & Huang, 2014). The range of saline ecosystems is very wide, extending from lowlands to mountain elevations. In total, about 1 billion hectares of the world's land (seven percent of the world's land) and about 1.5 million hectares of agricultural land in the world (five percent of the world's land) are affected by salinity (Balasubramaniam et al, 2023). Considering that different bean cultivars exhibit varying levels of susceptibility to salinity at different stages, it is essential to study the effects of salinity levels on the ecophysiological characteristics of different bean cultivars. Evaluating different plants under various salinity stress levels in greenhouse conditions allows for the possibility of multi-season cultivation and faster screening in the cultivars selection process for identifying and continuing research on salt-tolerant cultivars, which holds great importance. Under these conditions, by measuring certain parameters related to salinity at the growth stage and before harvesting, salt-tolerant cultivars can be identified with a high degree of probability, thus allowing for a more informed selection process. The lack of comprehensive information on the effects of salinity on the physiological aspects of beans, as well as the existence of a significant number of unknown cultivars in terms of salt tolerance, makes extensive research in this field unavoidable. Therefore, this research was conducted to investigate The effects of different levels of salinity on the yield and some physiological characteristics of different pinto bean cultivars, with the aim of gaining a deeper understanding of salt tolerance in these plants and potentially identifying cultivars with improved salt tolerance for further breeding efforts.

Methodology
Experimental model
To evaluate the yield and some physiological characteristics of four different pinto bean cultivars under salinity stress, an experiment was conducted in a greenhouse at the Research Greenhouse of Payame Noor University of Gandaman (Chaharmahal and Bakhtiari province). The experiment was arranged as a split-plot design within a completely randomized design with three replications. The first factor was Four levels of salinity including S0 (2), S1 (10), S2 (20), and S3 (30) (dS/m). where S0 was the Hoagland's solution as the control, and other salinity levels were the result of mixing sodium chloride (NaCl) and calcium chloride (CaCl2) at a ratio of 20 to 1 moles in the Hoagland's solution, respectively. The second factor comprised four pinto bean cultivars used were Kousha, Saleh, Ghaffar, and Talash. The experimental units included 40 × 40 cm pots, 35 cm in height, containing a completely homogenized washed sand. Due to lack of absorption and less the plants' need for a complete nutrient solution, the pots were irrigated every other day with a solution containing half the concentration of nutrients found in the Hoagland's solution (Dehdari et al, 2005). Two weeks after establishment, the plants were thinned from 15 to 8 plants per pot to achieve the desired plant density. From the 20th day after sowing (the four-leaf stage of the plants), salinity levels were gradually applied to acclimate the plants, such that all pots, except for the control level, received incremental salinity levels of 25% of each level in four irrigation shifts, thereby applying the total salinity treatment for each level.
 
Collecting and preparing plant samples
The salinity treatment continued at the specified ratios until the end of the growth stage. In this study, the traits of chlorophyll content, carotenoids, and proline levels were measured throughout the growth season; samples were collected at the flowering stage (60 days after sowing) by taking five fully grown young leaves from the upper parts of each experimental unit. To measure the content of chlorophyll, carotenoids, and proline, the methods of Arnon (1940), Gross (1991), and Pacquiao and Lecourier (1997) were used, respectively; the absorption of light from the extract was recorded using a spectrophotometer (SHIMADZO 54A). At the end of the growth stage (100 days after sowing), to measure the biological yield, grain yield, and harvest index, six plants in each pot were randomly selected, harvested, and the above parameters were measured and calculated. Seed protein was calculated using the Bradford method (Bradford, 1976) with a Kjeldahl apparatus, using bovine serum albumin (BSA) as the standard.
Statistical Analysis Statistical Analysis
Finally, data were analyzed using SAS software (version 9.1). Mean comparisons were conducted using Duncan's multiple range test at a 5% probability level.

 Results and discussion
In this study, all the examined traits were affected by salinity; increasing salinity levels decreased chlorophyll content, biological yield, harvest index, and grain yield in all studied cultivars. Cultivars under salinity stress maintained high levels of carotenoids, increased proline, and seed protein content, which mitigated the adverse effects of salinity stress. Salinity stress disrupts plant metabolism, affecting the grain filling process and reducing the translocation of assimilates due to secondary drought conditions. This disruption leads to the abortion of flowers and newly formed seeds in the pods, ultimately resulting in decreased grain yield in beans. (Gupta et al, 2014).

Conclusions
Environmental stresses such as drought and salinity have inhibitory effects on growth and plant development under stress, leading to reductions in yield and harvest index; this is associated with decreased levels of photosynthetic pigments and the remobilization of photosynthetic materials during grain filling. In the present study, cultivars under salinity stress maintained high carotenoid levels, increased proline, and seed protein content, mitigating the adverse effects of salinity stress; the Koosha cultivar showed higher yield compared to other cultivars in salinity stress and a lower yield reduction. Therefore, considering the different cultivars' responses to salinity stress and susceptibility of beans to salinity conditions and secondary drought stress, further research on different cultivars under diverse climatic conditions in Iran is necessary.