Evaluation of physiological responses of cannabis (Cannabis sativa L.) ecotypes under different levels of irrigation

Cannabis (Cannabis sativa L.) is an herbaceous annual plant belongs to Cannabacea Family. (Ahmad et al., 2008). The resistance to water shortage, the ability to grow in different climatic conditions, and great genetic diversity are features of this plant (Amaducci et al., 2008). Drought is one of the most important environmental stresses limiting crop production worldwide and has adverse effects on plant growth, development, which may result in decreased chlorophyll a and b and increased proline content of leaf (Lum et al., 2014; Karimi et al., 2016). Plants generally adapt to drought stress by inducing a variety of physiological, biochemical, and morphological responses, and each of these factors can be effective in introducing drought tolerant cultivars. Among the physiological properties, leaf water condition, membrane stability, photosynthetic changes and related factors are of great importance (Farooq et al., 2009). Considering the pharmaceutical and industrial importance of cannabis, this study was conducted to identify the drought tolerant and sensitive ecotypes of cannabis based on physiological responses.

This study was done in research greenhouse of University of Tehran, Iran, from February to July 2017 on the base of factorial experiment in as a completely randomized design (CRD) with three replications. The first factor consisted of three soil moisture levels [100% (normal irrigation), 75% (mild drought stress), and 50% (serve drought stress)] of field capacity (FC). Also, the 12 Iranian ecotypes of cannabis were the second factor where collected from different geographical regions of Iran including Urmia, Tabriz, Sanandaj, Dasht-e-Moghan, Rasht, Khomein, Daran, Qom, Shahrood, Kerman, Tabas, and Saravan. The seedlings thinning was done at 3-4 leaf pairs stage and four plants were maintained in each pot. At the time point of sex determination of plants, one female plant was kept for future study. The irrigation was done uniformly to all pots until the emergence of fifth pair of leaves and afterwards, irrigation treatments were applied. During applying irrigation treatments, the soil humidity of the pots was measured before each irrigation cycle. Relative water content, electrolyte leakage, chlorophyll a, chlorophyll b, total chlorophyll content and carotenoid pigments, proline content, catalase and guaiacol peroxidase enzymes were measured at full flowering period - early fruiting. The analyses of variance of obtained data were down using SAS software (v.9.2) and Duncan's multiple ranges test was used for comparing the averages at the significance level of α = 0.05.

The results showed that the highest value of relative water content was obtained from the normal irrigation, which was 77.21% and was reduced to 16.70 and 31.13% under mild and serve drought stress, respectively. Interaction effect of Irrigation levels and ecotypes showed that Urmia ecotype had the highest value of relative water content in normal irrigation treatment, and Tabriz ecotype had lowest value of this parameter in severe drought stress. The electrolyte leakage Index was decreased by 10.54 and 24.11% at mild and severe drought stress, compared to normal irrigation, respectively. The highest value of electrolyte leakage was obtained from Tabriz ecotype in severe drought stress, and the lowest value of this parameter was obtained from Tabas and Saravan ecotypes in normal irrigation treatment. The highest values of chlorophyll b and total chlorophyll content were obtained for Tabas and Urmia ecotypes with 0.61 and 2.25 (mg.g-1 fw), respectively, at normal irrigation treatment. The lowest values of this parameters were obtained for Tabriz and Dasht-e-Moghan ecotypes with 0.17 and 0.84 (mg.g-1 fw), respectively, in severe drought stress. Water deficit decreased 28.12% of carotenoid pigments at severe drought stress compared to normal irrigation, and it increased values of proline, catalase and guaiacol peroxidase enzymes with 47.06, 29.18 and 22.78 (%) respectively, at severe drought stress compared to normal condition. The highest values of carotenoid pigments, proline, catalase and guaiacol peroxidase enzymes were observed in the ecotypes of Tabas, Urmia, Qom and Urmia [0.79 (mg.g-1 fw), 1.27 (mg.g-1fw), 0.0820 and 0.5800(Mc.min-1 mg-1 pro), respectively], and the lowest values of them were obtained for Tabriz, Dasht-e-Moghan, Khomein and Rasht ecotypes [0.34 (mg.g-1fw),0.48 (mg.g-1 fw), 0.0396 and 0.2744 (Mc.min-1 mg-1 pro), respectively].

The results of this study showed that Tabas ecotype had a significant advantage in maintaining relative water content, maintaining chlorophyll content and maintaining membrane stability. The Tabriz Ecotype is the most sensitive ecotype for drought conditions. Because it lost the most values of the relative water content, chlorophyll content and membrane stability in stress condition compared to other ecotypes. Therefore, it can be concluded that the physiological parameters measured under drought stress conditions can be used as a criterion for the identification of tolerant and sensitive ecotypes.