Number and Length of Shoots
The results of variance analysis indicated a significant effect (
p≥0.05) of the drought stress on the number of shoots, shoot length and chlorophyll a, MDA and proline contents. In contrast, the effect of drought on chlorophyll b was not significant (Table 1). The highest mean number of shoots in the control treatment was obtained for MK rootstock as 12.81 shoots plant
-1 and the lowest mean number of shoots was recorded as 5.47 shoots explant-1 for SL-64 rootstock. The mean number of shoots sharply decreased in 4.0% PEG treatment, it was 1.30 shoots explant
-1 for MK and 1.54 shoots explant
-1 for SL-64 rootstocks. The highest number of shoots in the highest PEG dose (4%) was recorded in Pixy (2.81 shoots explant
-1) rootstock (Table 2). The adverse effects of PEG applications on the number of shoots and shoot length in different rootstocks have also been reported by Deblonde and Ledent (2001). The number of shoots recorded in 2% PEG treatment for SL-64 and Pixy rootstocks indicated that these two rootstocks are more active under slight drought stress than the control conditions. In contrast, the highest number of shoots for MK (12.81 shoots explant
-1) and Pixy (10.28 shoots explant
-1) rootstocks were obtained in the highest PEG application dose (Table 2).
Shoot length of rootstocks significantly decreased with the increased PEG doses (Table 3). The lowest shoot length in 4% PEG treatment was recorded in Garnem rootstock (0.50 cm). The shoot length of MK rootstock in PEG 2% dose was lower than the control. Similar to the number of shoots, the mean shoot length of all rootstocks significantly decreased with the increasing PEG doses. The mean shoot length in general was 1.38, 1.21, 0.92 and 0.66 cm for 0, 2, 3 and 4% PEG treatments, respectively. The lowest shoot length in control treatment was recorded in Pixy (1.00 cm) and the highest shoot length was in SL-64 rootstock (1.78 cm). The mean shoot length of Pixy rootstock in 2% dose PEG and control treatments (1.00 cm) was the same. Higher mean shoot length of Gis-6 and MK rootstocks in 2% PEG dose compared to the control indicated higher drought resistance of these two rootstocks compared to other three rootstocks. Therefore, prolongation of side shoots in arid regions can be attributed to the disruption of shoot apical meristem and growth inhibition of necrosis. Similar findings for the effects of drought stress on plant growth
in vitro conditions have been reported by Dami and Harrison, (1995) and Al-Khayri and Al-Bahrany (2004).
Effects of drought stress on chlorophyll, proline, lipid peroxidation content
The chlorophyll a and chlorophyll b contents of all rootstocks investigated significantly decreased with the increasing PEG doses (Table 4 and 5). The lowest chlorophyll a (1.27 mg/f.w) and chlorophyll b (0.51mg/f.w) contents were recorded in 4% PEG treatment for Gis-6 rootstock. The chlorophyll a content in 0, 2, 3 and 4% PEG treatments were 10.66, 8.01, 3.68 and 2.03. mg/f.w, respectively. Similarly, the chlorophyll b content of rootstocks in 0, 2, 3 and 4% PEG doses were 3.63, 3.13, 1.55 and 1.15 mg/f.w, respectively. The chlorophyll contents gradually decreased compared to the control treatments in different almond rootstocks under drought stress (Ýpek, 2015). In addition, Cerci (2012) stated that the decrease in chlorophyll contents caused leaf color bleaching of six different rootstocks under different drought stress conditions.
Yarsi et al., (2017) reported that abiotic stress reduced the chlorophyll a and b contents of leaves, which significantly reduced the plant growth and altered the morphology.
Malondialdehyde (MDA) contents of all rootstocks increased under drought stress. The highest MDA content in PEG treatment was obtained in Garnem rootstock (1.31µmolg
-1), while the lowest value was noted in MK (0.25 µmolg
-1) rootstock (Table 6). The mean value of MDA content for all rootstocks increased with the increasing PEG doses up to 3.0% then significantly decreased in 4.0% PEG dose (Table 6). The increasing MDA contents of rootstocks in 0, 2 and 3% PEG doses were ranked as 0.57, 0.80 and 1.02, respectively. The difference in MDA content of rootstocks was attributed to the relationship between MDA content and membrane coalescence; confirming that, the higher the MDA content, more damage occurs in the cell membranes (Liu and Zhao, 2005). Lipid peroxidation indicated by MDA content increased markedly in all rootstocks up to 3.0% PEG dose. The results obtained in this study are consistent with the findings of
Asghar et al., (2016). The highest impact of drought stress on MDA content of rootstocks was recorded in 3% PEG treatment. The results indicated that all rootstocks have higher adaptability to 4% PEG dose compared to 3% PEG dose (Table 6).6
The proline content of rootstocks under drought stress significantly differed from the control treatment. Similar to the MDA values, the mean proline contents of all rootstocks in PEG treatments increased up to 3% PEG and significantly decreased by the application of 4.0% PEG dose. The mean proline content of all rootstocks in 0, 2, 3 and 4% PEG treatments were 0.26, 0.65, 1.16 and 0.85 µmolg
-1, respectively (Table 7). The highest proline content in 4.0% PEG treatment was recorded in SL-64 (1.33 µmolg
-1) and MK (1.09 µmolg
-1) rootstocks, which can be considered more tolerant under high drought stress conditions compared to the other rootstocks (Table 7). Proline controls the production of free radicals like different antioxidants and proline is the only component that protects plant cells from oxidative damage
(Baxter et al., 2014). The differences in proline accumulation in various rootstocks under abiotic stress have also been mentioned by
Sorkheh et al., (2012).