Drought is the most important factor limiting plant establishment, growth and distribution in many regions of the world. The Loess hilly region is a typical semi-arid and arid region in northwest China. The harsh environments and long-term anthropogenic disturbance had resulted in severe soil erosion and water loss, vegetation degradation and fragile ecosystems. These conditions seriously impede the fast and coordinated social, economic development and the safety of the lower Yellow river. Therefore, it is the key to solve these problems that carrying out ecological restoration and reconstruction in the Loess hilly region. Water deficit is an important limiting factor for the ecological restoration in this region. Moreover, due to the global climate change, an increased frequency and severity of drought stress might occur at a regional scale in the Loess hilly region, which will worsen the ecological environment. The selection of appropriate plant species for vegetation restoration is an important issue at present and in the future. The research on responses of native species to water deficit conditions and their eco-physiological adaptation strategies to drought environment in the Loess hilly regions could provide insights into the improvement of the vegetation restoration, and therefore, are of great theoretical and practical significance.
In this paper, Periploca sepium Bunge which is a native species and widespread in the Loess hilly region was chosen as experimental material to study the eco-physiological adaptation strategies of native species to local drought environment. Using the method of field investigation and manual control experiments, we systematically investigated the responses of Periploca sepium plants to different water conditions or drought patterns. The drought-resistance of Periploca sepium was analyzed and discussed. The adaptive strategies including reproductive strategy, seed germination strategy, morphology and anatomy strategy, the whole-plant strategy by coordination of all organs and the survival strategy under extreme drought condition were revealed. The main results were as follows:
(1) As a heliophilous plant, the vegetative growth and reproductive growth of Periploca sepium plants in sunny slope were significantly better than that in shady slope, although the soil moisture is a major limiting factor of plant growth in the Loess hilly region. The average seed production of single Periploca sepium plant in sunny slope was 19 pairs of follicles, which was 8 times higher than that in shady slope. Moreover, the follicles produced by Periploca sepium plant in sunny slope were significantly larger than that in shady slope. The single former follicle contained about 66±7.25 seeds, while the later one contained 45±8.09 seeds. The single sunny slope plant could produce seeds as many as 9900, and the average production (seed number) was 2376, while the shady slope plants produced only 180 seeds on average.
(2) Seed germination of Periploca sepium was not influenced by light. Seeds germinated at a wide range of temperatures (constant temperature: from 4 to 40 ; ℃alternating temperature with 5 difference: from 10/4 to 40/35 ; alternating ℃℃temperature with 10 difference: from 15/4 to 40/30 ). Seed germination of ℃℃Periploca sepium was thermophilic, and the germination rate at 30-40 was much higher than that ℃at lower temperatures. The alternating temperature regimes accelerated the germination rate. Seed germination of Periploca sepium could tolerate moderate PEG osmotic stress (-0.6 MPa). The ungerminated seeds in osmotic stressed condition remained viable and germinated more rapidly when the stress was relieved. Hydration-dehydration pretreatment did not affect seed viability, and the pre-treated seeds germinated much faster when re-watered. The germination process ceased when hydrated seeds re-dried, however, the physiological advancement during hydration was maintained. Osmotic stress prolonged the second phase of seed germination to make sure slow but sufficient hydration and to accumulate enough energy. The low or none germination under water deficit conditions and the larger and more rapid germination when seeds are provided with enough water is two strategies for seedling establishment of Periploca sepium in drought-prone regions. Rapid germination occurs only after a major rainfall event or prolonged rainy weather. In this case the emerged seedling has access to sufficient water to enhance its chances to survive and to complete its life cycle.
(3) Stomata closure was the main factor limiting Periploca sepium photosynthesis when plants were with-held water for 6 d during summer, and the photosynthetic apparatus did not show any irreversible damage. The photosynthesis rate of re-watered plants was significantly higher than that of control plants, showing a significant compensation effect. The production rates of superoxide anion radical (O2·-) and malondialdehyde (MDA) contents in all organs were expressed as: old leaves> mature leaves> young leaves > new stems> fine roots. The MDA contents in young leaves, new stems and fine roots declined, although their production rates of O2·- increased under drought conditions. SOD、CAT and POD contributed differently to the antioxidant capacity. These three enzyme activities in fine roots were all sensitive to soil moisture, increasing under water deficit conditions and declining when re-watered. The solute accumulation of Periploca sepium plants showed an organ-specific characteristic, which were reflected both in magnitude and in the type of solutes involved. The source organs (mature and old leaves) were stable under repeated stress conditions, and the accumulation of compatible solutes was important mainly for sink organs (young leaves, new stems and fine roots) with increasing drought cycles. The preferential protection of young tissues from the oxidative stress induced by water deficit conditions, the strong repair and compensation capacity during rehydration, the storage role of stem and root, and the cooperation of all organs are the important adaptive mechanisms and strategies of Periploca sepium to the drying and wetting conditions in drought-prone regions.
(4) The leaves of Periploca sepium plants in the Loess hilly regions were elongated, while plants in Yangling where is of warm sub-humid monsoon climate had ovaideleaves. Moreover, the former leaf area was significantly smaller than the later, but the leaf number was exactly reversed. Compared with plants in Yangling region, the Periploca sepium plants in the Loess hilly region showed greater leaf specific gravity, smaller leaf tissue structure loose ratio, thicker upper and lower cutin layers, smaller but more stomata, and these differences were statistically significant. Compared with plants in the Loess hilly region, the Periploca sepium plants in Yangling region showed significantly smaller seed production, number of follicles per plant, average length and diameter of follicles and the ratio of fruit biomass to total biomass, and these index of Yangling plants were more sensitive to soil moisture, for example, its seed production was low under moderate drought condition and no seeds were produced under severe drought condition.
(5) The growth rate and water consumption of Periploca sepium plants decreased abruptly when water was with-held. However, after a period of constant stress, the predawn leaf water potential, relative water content, MDA content and activities of 3 antioxidant enzymes showed none significant changes with the stress time under all 3 constant drought conditions. The MDA content in plants under 35%FC soil water condition was significantly lower than that under 80%FC soil water condition all the time. These results indicate that Periploca sepium is able to adjust its growth and physiological state to the new soil water condition under constant drought stress, and its adaptive strategy is achieving new stable state and turning the stress to “non-stress”.
(6) The survival rates of Periploca sepium were 100%, 90% and 30% when soil water content dropped to 7.07% (water with-held for 15 d), 4.65% (water with-held for 27 d) and 3.09% (water with-held for 42 d), respectively. The leaf MDA content in plant increased sharply when naturally drought for 18 d (with soil moisture content of 6.35%), and until then, all leaves were senescent and dead, except leaves of individual young shoots sprouting directly from roots. Under extreme drought conditions, Periploca sepium accelerated its leaf senescence in order to slow down the metabolism and the use of limited water and nutrient resources. At the same time, lots of dormant buds generated and the whole plant became relatively dormant. In this way, Periploca sepium could live through the extreme drought condition. The dormant buds sprouted rapidly when plants were re-watered and showed obvious compensatory growth. These results suggest the survival strategy of Periploca sepium by defoliating leaves and entering relative dormant state under drought and recovering quickly when re-watered.
(7) The root bark of Periploca sepium, also known as cortex periolocae, is a traditional Chinese medicinal material. In our experiment, the content of 4-metho- xysalicylaldehyde in root bark of Periploca sepium plants in sunny slope were significantly higher than that in shady slope where the soil moisture condition is much better. The 4-methoxysalicylaldehyde content in Periploca sepium plants growing in the Loess hilly region were higher than 0.35% during the whole growth period, while the standard content