The Effect of Irrigation Levels on Improvement of Cold Tolerance in Viola Plants under Controlled Conditions

Climate change is expected to have impacts on ecosystems worldwide. During the last 50 years, the greatest warming trends have been observed in winter months and significant increases in both the occurrence and duration of winter warming have already been reported. In general, predicted future climate change scenarios will result in less than optimal cold acclimation conditions, leading to decreases in freezing tolerance and predisposition of plants to winter injury. Nonetheless, it is not clear whether water stress induced during cold hardening is of high importance in inducing freezing tolerance in plants or it is an integral part of typical cold hardening process. Since rapid and effective assessment of plant cold tolerance is important for researchers and also field trials have no regular process and have high error, different kinds of artificial freeze tests such as survival percentage test and regrowth after imposing stress have been developed.

In order to evaluate the effect of drought stress on plant freezing tolerance of viola, a factorial experiment was conducted based on completely randomized design with three replications in faculty of Agriculture, Ferdowsi University of Mashhad. Experimental factors include three water treatments (80% FC, 60 % FC and 40% FC) and 10 temperature levels (Control, from zero to -24 with 3 °C intervals). Pansy seeds sown in a nursery in the summer of 2015 and after reaching the five-leaf stage in the fall plants were transferred to the pots. After the potted plants spend cold acclimation in nature conditions, plants were subjected to water stress including control (80% FC), 60% and 40% FC for two weeks. After drought stress, whole plants were sampled for freezing tolerance assessment and they were transferred to the freezer thermos-gradient. After applying the stress, electrolyte leakage, lethal temperature 50 according to the electrolyte leakage percentage (LT50el) were measured. One months later, survival percentage, lethal temperature 50% of plant according to the survival percentage (LT50su), leaf area, number of flower and bud, dry weight (dry weight of vegetative, reproductive, root and total) and reduced dry matter temperature 50 (RDMT50) were evaluated.

Electrolyte leakage percentage (EL %) and survival (%) were significantly (p ≤ 0.01) affected by irrigation treatments in the freezing conditions. By lowering the temperature from 20 to -24 °C, the EL% significantly increased in three irrigation treatments and it increased in 80% FC compared to 60% (by 16%) at -24°C. plants under 60% FC treatment exhibited higher baseline freezing tolerance (LT50 of −18.4 °C) compared to 80% FC (LT50 of −11.8 °C).Treated plants (except 80% FC) were able to tolerate lowering the temperature to -21°C. Lowering the temperature to -24°C caused the total mortality. According to the LT50su index, 60% FC treatment was less than compared to other treatments. Leaf area significantly increased by 16%, respectively, when plants were under water deficit (60% FC) compared to 80% FC at 0 °C. The maximum number of flower were seen in 60% FC at – 3 °C and the maximum number of bud were observed at 0 °C. The results showed that dry weight was significantly (p ≤ 0.01) increased by drought stress in the freezing conditions. Plants under 60% FC at 0 °C had the highest increase (55, 62 and 64%, respectively) dry weight of vegetative, reproductive and total growth, respectively compared to control. By lowering the temperature to -18 °C in 80% FC vegetative, reproductive and root growth decreased (36, 38 and 42%, respectively) compared to control plants. RDMT50 significantly affected by drought stress. There were significantly correlation between EL with LT50el and RDMT50 (r =0.25* and r = 0.72**, respectively). In total, plants under 60% FC showed highest freezing tolerance compared to the other treatments.

In the current study, we found that the greatest gain in freezing tolerance was associated with cold and that the effect of drought stress on freezing tolerance varied with temperature. Drought stress resulted in an improvement in freezing tolerance of viola (lower LT50). Among the different parameters evaluated, 60% FC treatment at 0 °C most consistently induced increases in survival percentage, reproductive and vegetative growth which suggested a synergistic effect between drought exposure and low temperature. Higher dry weight of viola plants may contribute to better plant overwintering capacity. In addition, future research should explore the effect of repeated mild drought events on freezing tolerance of acclimated plants, by using strategies such as wilt-based irrigation scheduling, partial root zone drying, and deficit irrigation.