Evaluation of the effect of putrescine and brassinosteroid on induction of drought tolerance and physiological changes in basil plant (Ocimum basilicum L.)

Introduction Polyamines include spermidine, spermine and putrescine, are the polyacetyl compounds with low molecular weight, which play a role as osmotic regulators. They also contribute to the regulation of some of the essential cellular processes such as DNA replication, transcription, translocation, cell division, regulation of enzymes activity, maintaining membrane stability, sweeping active radicals, and tolerance to various stresses. Brassinosteroids have increased the tolerance in a wide range of environmental stresses such as drought, salinity, cold and heat. This increase is generally dependent on production and increase of transcription of genes responsible for stress response and in order to increase the tolerance of plants treated with brassinosteroids. In this regard, a trial was conducted to clarify the role of growth regulators (putrescine and brassinosteroid) on controlling oxidative stress and physiological changes during tolerance to water deficit in basil plant. Materials and methods This experiment was conducted as split plot factorial in a randomized complete block design with three replications at Research Farm, Faculty of Agriculture, Ilam University. Experimental treatments consisted of drought stress at three levels (I1: 40, I2: 80 and I3: 120 mm evaporation from the evaporation pan) as the main factor, putrescine spray at three levels (Put0:0, Put1: 0.5 and Put2: 2 milli molar) and brassinosteroid spray in three levels (Br0: 0, Br1: 0.5 and Br2: 2MM) as sub plots. Seeds of the Afghani cultivar were planted on May 20th. To prevent the lateral movement of the water, stacks 1 meter width were placed around each plot. To ensure greater on the lateral water movement, the spacing between the plots for different irrigation treatments was considered as 2 m. No herbicide was used to control weeds in this experiment. Instead, weeding was done to control them. Due to the absence of pests and diseases in experimental plots, pesticides were not used. In this research, changes in the content of photosynthetic pigments of leaves, flavonoids, leaf relative water content, photosynthesis rate, co2 concentration, proline and ion leakage were investigated on basil. Variance analysis of data was performed using SAS 9.1 software and comparison of meanings was made with a minimum difference of 5% probability level and drawing the shapes was carried out in Excel software environment. Results and discussion According to the results of analysis of variance, the traits of leaf relative water content, ion leakage, flavonoids, photosynthetic pigments, photosynthesis velocity, sub-stomatal CO2 concentration and basal significantly affected by the triple interaction (Wetting treatment × putrescine × brassinosteroid). According to the results of this study, the amount of chlorophyll a and b, flavonoids and the concentration of Co2 under the stomata in Br2× Put2 treatment under non-stress conditions (I1) were increased as 74.77, 64, 68/45 and 19.5%, in medium stress (I2) were increased as 61.4, 33.9, 6.3 and 10.3%, and in severe drought stress (I3) were increased as 62.1, 68.6, 33.1 and 15.7, compared to control treatment (Br0 Put0 ×), respectively. The rates of photosynthesis was reduced under drought stress. However, under Put2 × Br1 in moderate (I2) and severe (I3) drought stress (I3), they were increased as 76.59% and 83.33% as well as 10% and 16% in compare with Br0 Put0 treatment, respectively. Put2 × Br1 treatment reduced the level of ion leakage under all drought stress levels. Conclusions Since polyamines are considered as effective molecules in the signaling pathway for drought stress, the spraying of these substances will lead to the induction of plant defense responses. Most likely, these responses led to increased tolerance of basil plant to drought stress and increased photosynthetic pigments as well as physiological characteristics. In general, it seems that high consumption of putrescine and brassinosteroid (Put2 and Br2) in drought stress can improve the physiological parameters in basil plant.