Effect of Hexaconazole and Penconazole on Kochia (Kochia scoparia) Freezing Tolerance

Cold and freezing are the most important limiting factors on development of sensitive plants. When the ambient temperature deviates from optimal, physiological, and biochemical, metabolic and molecular changes will occur within plants. This is an effort of plants to maximize growth and developmental processes and to maintain cellular homeostasis during such adverse conditions. At the extremes of the natural temperature range of plant, the degree of physiological, cellular, metabolic and molecular dysfunction becomes so severe that it leads to death. Triazoles are the most potent groups of growth retardants with multiple effects. Plant growing with paclobutrazol generally has little effect on rates of net photosynthetic rate. However, because the compound reduces leaf area, net photosynthesis on a per plant basis is probably reduced. They have exhibited growth regulating, fungicidal, herbicidal, apicidal and antibacterial activities. More recently, it was found that triazole compounds are able to increase tolerance of plants to cold and freezing stress. Kochia (Kochia scoparia (L.) Schrad.), an out crossing species whose pollen move between plants in windy areas, has recently been considered as a forage or fodder crop in marginal lands. Steppuhn and Wall (1993) claimed that Kochia offers great potential as a crop that can be grown on saline soils, yielding fodder in quantities approaching that produced by alfalfa (Medicago sativa L.). Shamsutdinov et al. (1996) also reported more than 15 Mg ha-1 dry-matter production for Kochia under saline conditions and concluded that it is a good candidate for forage hay. Thus, the aim of the present study was to determine whether the triazole compounds could increase tolerance of kochia against freezing stress.
An experiment was carried out with hexaconazole and penconazole in 0, 10 and 20 mg.L-1 and freezing temperatures 0, -2, -4, -6 and -8 degree centigrade in factorial based on CRBD with three replications at research greenhouse of college of agriculture, Ferdowsi University of Mashhad in 2009. Plants were kept in natural condition until three weeks after planting, which in this stage they had three leaves, and then treated with fungicide. After 24 hours plants were frozen in thermo gradient freezer, in the dark. At first, the temperature of thermo gradient freezer was five degrees centigrade, and then temperature decreased two degree centigrade per hour. In order to make ice nucleation in plant at -3-degree centigrade ice nucleation active bacteria was sprayed. Plants were keeping an hour in each temperature treatment. In order to balance the ambient temperature, the plants are kept at the desired temperature for one hour and then the pots transport in a cold chamber at a temperature of 5 ± 2°C for 24 hours. The cell membrane stability was measured through electrolyte leakage (EL) and the lethal temperature 50 (LT50el) (according to El) also were determined in the youngest developed leaf from each plant. The electrolyte leakage in the solution was measured after 24 h of floating at room temperature using a conductivity meter. Total conductivity was obtained after keeping the flasks in an oven (75°C) for 90 minutes. Results were expressed as percentage of total conductivity. Survival percentage, plant height, dry weight and (LT50su) (according to survival) were determined after three weeks recovery in the natural condition. The data were analyzed statistically using a two-way ANOVA, applied to the various measured and calculated parameters, followed by a Duncan test for mean comparison between treatments at a 95 % confidence level by MSTAT-C program.
The results showed that increase of fungicide concentration by 10 and 20 mg.L-1 higher than control decreased electrolyte leakage but, electrolyte leakage build-up with decrease freezing temperature. Fungicide application increased -2 °C freezing tolerance in kochia. Increasing fungicide concentration in 10 and 20 mg.L-1 than control causes decreased LT50el from -5.9 to -7.2. Application of triazoles responsible of reduced electrolyte leakage but they did not effect on survival. The use of different concentrations of fungicides hexaconazole and penconazole of photosynthetic pigments in Kochia showed no significant effect. However, the concentration of chlorophyll a and b in the use of fungicides hexaconazole was more than penconazole.
Triazole application initially had a positive effect on the reduction of electrolyte leakage, but survival and re-growth did not affect. Generally, the use of Triazoles compounds prior to stresses such as cold, could expect the plants can tolerate oxidative stress, which usually occurs after cold stress.