عنوان مقاله [English]
Introduction: Drought is a major environmental stress that affects agricultural systems and induces several physiological, biochemical and molecular responses in plants. Drought inhibits the plant photosynthesis causing changes of chlorophyll contents, damage the photosynthetic apparatus and decreases plant growth and development. Generally, the environmental stresses, especially drought stress, give rise to accumulation of soluble carbohydrates, proline and free amino acids as well as antioxidant compounds. Triazoles are the active ingredient of fungicides (propoconazole, penconazole, epixiconazole) and some growth regulators. The fungicidal properties of triazoles depend on inhibition of the C4-demethylase reactions in sterol biosynthesis of fungi. However, triazole-based fungicides induce a suite of morphological and physiological adaptations and allow plants to tolerate a broad range of environmental stresses including drought, herbicide treatment and elevated temperatures. The growth inhibitor paclobutrazol (PBZ) is a triazole and has been reported to protect plants against several environmental stresses, i.e. drought, low and high temperature. The purpose of this study was to evaluate the effect of palobutrazol on vegetative, physiological and gas exchange characteristics of pear (Pyrus communis cv. ShahMive) under different irrigation regimes.
Materials and Methods: In March, 2011, 1-year-old pear (Pyrus communis cv. ShahMive) saplings 80±2 cm high were planted in 20-l plastic pots filled with loamy sand soil (8% clay, 15% silt, 77% Sand) in experimental greenhouse. Paclobutrazol was added to soil at the same time with sapling cultivation at rates of 0, 0.15 and 0.3 g active ingredient per pot. PBZ was diluted in 500 ml distilled water and solution applied to the soil at the base of the saplings on pots. The control saplings were treated with distilled water of equal volume. Vegetative (stem growth, stem diameter, leaf number, shoot dry weight, root dry weight and plant dry weight), physiological and biochemical (leaf relative water content (RWC), total soluble sugar(TSS), proline and membrane stability index (MSI)) and gas exchange (Photosynthetic rate, sub-stomatal CO2, stomata conductance (gs) and transpiration) characteristics were measured.
Results and Discussion: The results showed that paclobutrazol treatments had significant effect on growth parameters, except root dry weight. Paclobutrazol significantly reduced stem height and stem diameter increment, mean leaf area, shoot dry weight and whole plant dry weight. Root: shoot ratio was increased in paclobutrazol-treated saplings. No significant differences in any characteristic were found between 0.15 and 0.3 g active ingredient PBZ per pot for growth parameter. Waters stress decreased leaf relative water content (RWC), photosynthetic rate, stomatal conductance and transpiration rate, membrane stability index and chlorophyll content and increased leaf proline content, total soluble sugar and sub-stomatal CO2. Significant interaction between PBZ and irrigation regimes was seen for RWC, proline and sub-stomatal CO2. PBZ-treated saplings had higher RWC than untreated ones. The effects of treatments on physiological and gas exchange traits were significant. RWC was high in all non-water-stressed (with or without paclobutrazol) treatments and decreased in water stressed treatments. It was higher in PBZ-treated than PBZ-untreated treatments in similar water stress condition. But there was not significant differences between 0.15 and 0.3 g PBZ in a given water stress condition. For example, RWC was 89.76 and 85.56 percent in -0.4 MPa water stress plus 0.15 and 0.3 gr PBZ treatments, respectively. The results showed that leaf proline content was increased under water stress condition. Leaf proline content of the PBZ-untreated sapling, subjected to water stress increased to 32.13 and 61.82 µmol.gr-1DW in -0.4 and -0.8 MPa water stress conditions, respectively. The PBZ-treated saplings accumulated less proline content than the PBZ-untreated ones. The highest proline concentration was founded in PBZ-untreated and -0.8 MPa water stress treatment. TSS was decreased in water stress treatments. TSS concentration was increased in water stress treatments. The highest TSS concentration was founded in PBZ-treated and untreated -0.8 MPa water stress treatments. PBZ- treated saplings had more TSS than untreated ones in -0.4 MPa treatments. Water stress was decreased leaf chlorophyll (a, b and total) content of saplings. PBZ-treated saplings had higher leaf chlorophyll content than PBZ-untreated ones in non-water stress treatments. The interaction of PBZ treatment and water stress moderated the negative effect of water stress on the chlorophyll b and total chlorophyll.
Conclusions: Generally, the results showed that PBZ allowed plants to tolerate water stress by morphological and physiological traits modification. On the other hand, paclobutrazol stimulated a more efficient stomatal regulation, which affected photosynthesis, but permitted significantly better levels of water status in treated plants.