Effect of freezing stress on faba bean (Vicia faba L.) under controlled conditions

Faba bean as one of the most important legumes is an annual and cool-season plant which plays an important role in feeding human and livestock due to its protein content. Since unfavorable environmental conditions negatively affects plant growth and development, survival, productivity and natural distribution, the ability of faba bean to tolerate this conditions is of high paramount important. Although autumn sowing of faba bean results in effective use of precipitation, no need for irrigation and grain yield improvement in comparison with spring sowing, but cold stress is one of the main growth limiting factors in this season. Cold acclimation in plants is an effective process for improving cold tolerance and determining winter survival capacity. Evaluation of survival percentage and regrowth traits of plants after freezing stress is a very important criterion in cold related studies. Therefore, the present experiment was conducted to determine damage threshold of freezing stress in two faba bean landraces through survival percentage and other regrowth traits. This study was carried out as a factorial experiment based on completely randomized design with four replications under controlled conditions in research greenhouse of the research center for plant sciences, Ferdowsi University of Mashhad. Experimental factors included two faba bean landraces (Borujerd and Neyshabur) and 7 freezing temperatures (0, -4, -8, -12, -16, -20 and -24°C). Plants were cold acclimated in natural environment till to 4-6 leaf stage, before freezing stress was imposed in a thermogradient freezer. Then plants were transferred to greenhouse to be recovered. Treated plants were evaluated for survival percentage and characteristics related to regrowth such as stem height, leaf number, leaf area and leaf, stem, shoot, and root dry weight 21 days after freezing stress. Analysis of variance was performed by MINITAB ver 17 and means were compared by LSD test at 5% probability level.Graphs were drawn by Excel. Results indicated that freezing temperatures significantly affected survival percentage and other regrowth traits in a way that all the traits were decreased as freezing temperature decreased. This was more severe in temperatures lower than -12oC. All the plants were killed in -16oC. It seems that the effectiveness of cold acclimation for freezing tolerance was up to -12°C. More reduction was found in Neyshabur landrace in almost all regrowth traits in the range of Zero to -12oC compared to Borujerd and reduction of 24cm2 in leaf area and 106, 77, 184 and 101 mg.plant-1, respectively in leaf, stem, shoot and root dry weight was observed, indicating higher sensitivity of this landrace to freezing stress. A significant positive correlation was found between stem height and leaf area with shoot dry weight (r=0.98**, r=0.96**) and root dry weight (r=0.96**, r=0.91**), respectively, indicating the consistent and coordinated changes between the two mentioned traits with plant dry weight. So that, plant dry weight was decreased as stem height and leaf area were decreased at lower temperatures and these traits in -16oC were not evaluated due to the death of all the plants which indicates that the plants were not the suitable recovery at low temperatures. Despite survival percentage and other regrowth traits of faba bean were declined under freezing stress, but the reduction rate was different depending on landrace and temperature. Generally, in the present study, faba bean tolerated freezing stress up to -12oC with slight damage. So it could be concluded that in areas susceptible to freezing stress, if the planting date of faba bean is set properly, in a way that, low temperatures up to -12°C occur when plant is infour to six leaf stages minimal damage will be observed. Also, if high correlation would be found between controlled and field experiments, Borujerd landrace could be introduced as a more appropriate landrace compared to Neyshabur due to its lower reduction of regrowth traits in temperature range from Zero to -12oC.