Effect of Drought Stress on Seed Germination Variables and Physiological Traits of Ornamental Hollyhock (Alcea rosea L.)

The Hollyhock (Alcea rosea) is a summer flowering biennial plant that is native to China and belongs to the Malvaceae family. It is one of the most valuable ornamental plants, whose 36 species are cultivated in Iran. It is an increasing garden escape, especially in urban areas, and is usually found at foot of walls, in ruderal areas, and in cracks of pavements and old walls. Also, it is sometimes seen on riverbanks (for instance on dikes of the river Maas), dumps or road- and railway banks. Alcea rosea has been used as an herbal plant in folk medicine for treatment of different diseases such as common cold and cough. This plant is antiphlogistic, astringent, demulcent, diuretic and expectorant. Drought is the most significant environmental stress in agriculture worldwide, and improving yield under drought is a major goal of plant breeding. Seed germination and early seedling growth are potentially the most critical stages for water stress. When subjected to drought stress, plant metabolism is interrupted or inhibited by increasing reactive oxygen species (ROS) and lipid peroxidation, resulting in reduced germination, weaker root and shoot growth and even mortality. Plants have evolved oxygen-scavenging systems consisting of non-enzyme antioxidant metabolites, such as proline and various antioxidant enzymes including superoxide dismutase, peroxidase and catalase.

In order to examine the effect of drought stress on germination indexes, various antioxidant enzyme changes and non-enzyme antioxidant metabolites in Alcea rosea, an experiment was conducted in complete randomized design with three replications. The drought stress treatment was conducted in five levels with osmotic pressures 0, -2, -4, -6, and -8 bar and with using PEG (Poly Ethylene Glycol 6000) on two ecotypes of Alcea rosea (ecotype 1= Mashhad and ecotype 2= Tehran). The seeds are at first sterilized with hypo chloride sodium for two minutes and then washed superficially three times with distilled water. 25 seeds were transferred to a glass petri dish with 10 cm diameter, and for the duration of the experiment, 5 ml solution with different levels was added to each petri dish. After 14 days at 25±1 ºC temperature, the number of geminated seeds in each day was counted and recorded. In the first part of the experiment, after the end of the germination period, the following growth parameters were measured: germination percentage, germination rate, root and shoot length, and the fresh weight of seedling. In the second part, the activity of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), malondialdehyde content, lipid peroxidation in terms of malondialdehyde (MDA) content, and proline index were measured.

The ecotype, drought treatments and their interaction had significant effects on growth parameters (germination percentage, germination rate, radicle and plumule length, and seedling fresh weight), as well as physiological and biochemical parameters (SOD, POD, CAT, MDA and proline). The highest germination parameters were recorded at Mashhad ecotype. Germination percentage and germination rate were severely affected by drought. Maximum percentage of germination (36) was recorded at control group followed by 32.1 at -2 bar and -4 bar PEG treatments in ecotype 1. Germination percentage in two ecotypes further declined to 33% at -8 bar compared to the control treatment. In both ecotypes, plants had the highest germination rate in control media (zero osmotic potential) but germination rate decreased significantly by decreasing water potential. Germination rate of control seeds of ecotype 1 was 11.7, while that of ecotype 2 was 8. 3. Germination rate in two ecotypes further declined to 56% and 38 %, at -8 bar compared to the control treatment, respectively. Ecotype 2 displayed a significant reduction in radicle and plumule length compared to the ecotype 1. Generally, the radicle and plumule length decreased significantly in relation to the drought stress caused by PEG. Ecotype 2 in -8 bar PEG treatment had the highest (41 and 32%) decrease in radicle and plumule length compared to the control temperature. In two ecotypes (Mashhad and Tehran) of Alcea rosea, increasing PEG concentrations resulted in a decrease in fresh and dry weights. Water absorption is the first germination stage. Due to the probable resistance of the ecotype one, water absorption rate is higher, and as a result, the percentage and rate of germination have increased. Ecotype 1 exhibited higher leaf SOD activities in response to -2 bar compared to -8 bar, but the SOD activities in ecotype 2 showed a general increase trend with increases in the PEG concentration. Drought stress resulted in lower antioxidant enzyme activities (POD) in leaves of both ecotypes compared to that observed at the control treatment, but the activity of CAT increased with the increase of drought stress. In both ecotypes, exposure to -8 bar resulted in significantly higher leaf MDA activities. Plants exhibited higher proline in response to -8 bar treatment compared to the control. By increasing the PEG concentration from control to -8 bar, proline content increased about 90 percent. In this experiment, drought stress reduced the rate and germination percentage and delayed subsequent plant deployment. When plants are subjected to drought stress, their metabolism is interrupted or inhibited by increasing reactive oxygen species (ROS) and lipid peroxidation, resulting in reduced germination, weaker root and shoot growth and even mortality. The recent experiment showed that the activity of two superoxide dismutase and peroxidase enzymes decreased with increasing drought stress, and the activity of the catalase enzyme increased; this is in agreement with the results reported by other studies. On one hand, the increase in the activity of the catalase enzyme indicates that it is perhaps the most important enzyme involved under drought condition, which increased over the course of 14 days. On the other hand, limiting the activity of enzymes and increasing the amount of proline showed that increasing the resistance to drought stress in the plant depends on the accumulation of contaminating substances such as proline.

Due to the higher resistance of the ecotype 1 and increase in water absorption, the percentage and rate of germination were increased. If water absorption is disturbed by the seed, the germination activity is slowly applied and the later growth of the roots will decrease the germination rate. Therefore, it seems that Mashhad ecotype with increase of catalase and proline has the highest drought tolerance compared to the other ecotype at germination stage. There was a significant correlation between germination percentage with SOD and POD.