the effect of drought stress and plant density on biochemical and physiological characteristics of two garlic (allium sativum l.) ecotypes

introduction drought stress is the most important growth limiting factor for crop production. sugar accumulation under drought stress strengthens and stabilizes cell membranes and maintains the water absorption and turgid property. under stress conditions, proline will also maintain the turgor pressure and decreased the damages caused to plant membrane. although proline concentrations may have undesirable effects on plant growth, because of deflecting photosynthetic resources to the processes that are not involved in plant growth. chloroplasts and its pigments are also affected by drought stress. density is one of the factors that has a significant impact on plant growth. garlic is one of the edible plants which has generated considerable interest throughout human history because of its pharmaceutical properties. this study aimed to determine the effects of drought stress and plant density on some biochemical and physiological treats of two garlic ecotypes and determining the more resistant ecotype. materials and methods the study was carried out in 2011-2012 in a farm land at the south east of semnan city. the experimental layout was a split-plot factorial with a randomized complete block design in three replications. the treatments were comprised of three factors: irrigation regimes (60%, 80% and 100% of estimated crop evapotranspiration (etc)) that were assigned as the main plot and the factorial combination of 3 levels of planting density (30, 40 and 50 plants. m-2) and two ecotypes (tabas and toroud) made up the sub-plots. the water requirement was calculated based on fao-56 crop water requirements instruction. fao-56 penman-monteith equation was used to calculate evapotranspiration. to calculate the content of soluble sugar, proline and leaves pigment, the samples were collected in a random way from the youngest fully expended leaves one month before the final harvest. relative water content was estimated by measuring dry weight, fresh weight and turgor weight. all calculations were performed with the help of the sas and mstatc soft wares. the statistical significances of mean values were assessed by analysing of variance and lsd tests at p results and discussion drought stress increased the content of soluble sugars significantly. as an osmotic agent, the increased sugars, induced by water stress, was significantly correlated to osmotic adjustment and turgor maintenance. no significant differences were observed between different ecotypes under the interaction of ecotype–irrigation regime and this indicates that none of the ecotypes were superior in resisting drought stress by increasing soluble sugar. the drought stress, ecotype and plant density did not have significant effect on proline content. regarding ecotype effects, a significant increase was observed in the content of chlorophyll a, chlorophyll b and total chlorophyll in tabas ecotype in comparison with toroud ecotype. the carotenoids amount in the treatment of 100% etc was significantly higher than 60% etc treatment. carotenoid content can be reduced due to oxidation by active oxygen and destruction of their structure. tabas ecotype had significantly more carotenoid content in comparison with toroud ecotype. drought stress significantly reduced the rwc. the effect of water decrease in the plant can vary from a simple turgor pressure decreasing, to severe damages which can lead to protein crystallization. rwc was significantly higher in tabas ecotype in comparison with toroud ecotype. difference in rwc of cultivars that are under drought stress may be due to the ability of more absorption of water from soil or ability of stomata to reduce the loss of water. conclusions the soluble sugar content was increased by drought stress but proline content was not significantly changed under drought stress. therefore, it can be tell that in this study, the soluble sugars had more important effect on osmoregulation and plant tolerance against drought stress rather than proline. under drought stress, the rwc and carotenoid content were decreased significantly. tabas ecotype had significantly more rwc, chlorophyll a, chlorophyll b, total chlorophyll and carotenoid content in comparison with toroud ecotype. a significant difference in the amount of soluble sugars, proline, chlorophyll a, chlorophyll b, total chlorophyll, carotenoids and rwc was not observed in two ecotypes under different irrigation levels. this study showed that both ecotypes in order to withstand drought conditions were similar to each other and did not have advantage to other one.