Drought Resistant Physiological Basis Of Six Xerophilous Shroubs

In this paper, by using potted-seedlings and weighing method to control soil moisture the water parameters from pressure-volume curves, important osmotic adjustment substances, antioxidation protection system and light response characteristics of six xerophilous shrubs with high utilization value in arid and semiarid areas, Caragana korshinskii, Atraphaxis bracteata, Hedysarum mongolicum, Hedysarum Scoparium, Buddleja alternifolia and Atriplex canescens were studied under suitable soil moisture, medium drought and severe drought. Some important physiological responses to drought stress were explored in order to get some important physiological adaptive mechanism of six xerophilous shrubs to drought. And also selection methods of representative indexes about leaf anatomical structures of six xerophilous shrubs were explored; the drought resistances of six xerophilous shrubs based on leaf anatomical structure were evaluated comprehensively. The main results were as follows:1. Compared to control of suitable soil moisture, as the drought stress degree was enhanced from medium to severe level, the water parameters from pressure-volume curvesψtlp,ψssat,RWCtlp and ROWCtlp of six shrubs showed a trend of decrease, while the value of Va showed a trend of increase. It indicated the abilities of maintaining turgor and capturing and keeping water of six shrubs were improved obviously.2. Based on seven water parameters from pressure-volume, the comprehensive estimating results on abilities of turgor maintaining showed that they were Hedysarum Scoparium> Caragana korshinskii> Hedysarum mongolicum> Atraphaxis bracteata> Atriplex canescens> Buddleja alternifolia under medium drought stress and Caragana korshinskii> Atraphaxis bracteata> Hedysarum Scoparium> Hedysarum mongolicum> Atriplex canescens> Buddleja alternifolia under severe drought stress.3. Under drought stress, K+ is a kind of substance that didn't have the common osmotic adjustment action in the six xerophilous shrubs, while soluble sugar,proline,betaine and soluble protein had a common osmotic adjustment action. Under the different drought stress degrees, the kinds of substance taking part in the osmotic adjustment were different for the six xerophilous shrubs.4. For the six xerophilous shrubs, the cumulants of osmotic adjustment substances were affected by both drought stress degree and drought stress period. The changes of the cumulants of osmotic adjustment substances decided the osmotic adjustment abilities depending on them. Therefor, the osmotic adjustment abilities of six shrubs were in a dynamic changing condition.5. Under drought stress, the response models of antioxidation protection system of six xerophilous shrubs were closely related to drought stress degrees, drought stress periods and tree characteristics, and therefore showed some speciality, diversity, dynamic and complexity.6. For the six xerophilous shrubs, the cooperation between SOD and CAT and the cooperation among ASA,GSH,GR and APX in antioxidation protection system could be kept well or enhanced in the prophase and metaphase of medium and severe drought stress, but be weakened in the anaphase of drought stress, at this time the main method to Scavenging H2O2 in the chloroplast was usually direct Scavenging of ASA or GSH. In the long period of mederate and severy drought stresses, all the six xerophilous shrubs were poisoned by active oxygen, oxidative damage was inevitable and happened earlier with the enhance of drought stress degree.7. The changes of light response curve showed same trends for the six xerophilous shrubs. The net photosynthetic rate (Pn) is sharply increased at low photosynthetically active radiation (PAR), but after PAR reaches certain degree the Pn is increased slowly and the curve becomes flat. Curve fittings by nonrectangular hyperbola model were good. For different xerophilous shrubs the high-low changes of light response curves showed a similar regulation with soil moisture reducing, it was Pn at same PAR that showed suitable soil moisture> medium drought stress> severe drought stress.8. For the six xerophilous shrubs, the apparent quantum yield (AQY) and maximum net photosynthetic rate (Pnmax) of light response characteristic parameters showed no certain corelation. The drought-resistant adaptations of the six xerophilous shrubs were respectively compared by the average changed degree of Pnmax and AQY under medium and severe drought stress. According to Pnmax,Caragana korshinskii was the strongest, the next was Hedysarum Scoparium, and Atriplex canescens, Atraphaxis bracteata and Hedysarum mongolicum were almost equal, but Buddleja alternifolia was the weakest. According to AQY, Caragana korshinskii and Hedysarum Scoparium were still ranked first and second in drought-resistant adaptations, Atriplex canescens was nearly close to Hedysarum mongolicum, and Buddleja alternifolia was at the penultimate place, then Atraphaxis bracteata was the weakest.9. Under drought stress, the change of light saturation point was the main reason to contribute to the change of light condition required by six xerophilous shrubs. According to the changes of valid intensity range resulted from variations of light compensation point and light saturation point, Caragana korshinskii and Hedysarum Scoparium showed considerably strong adaptability to light environment under medium and severe drought stress. The adaptabilities of Hedysarum mongolicum, Atraphaxis bracteata and Atriplex canescens to light environment were relatively strong under medium drought stress, but very weak under severe drought stress. The adaptability of Buddleja alternifolia to light environment was seriously reduced under medium and severe drought stress.10. According to the principles of comparability, mensurability, variability and correlation, three indexes from leaf drought-resistant anatomical structure involving upper epidermis cell thickness, leaf thickness and stoma density were considered as representative to contributing drought-resistant ability to six xerophilous shrubs basing on leaf anatomical structure. By using subordinate function values with the three indexes the drought resistances of six xerophilous shrubs on the basis of leaf anatomical structure were arranged. The result was Hedysarum Scoparium> Atraphaxis bracteata> Hedysarum mongolicum> Atriplex canescens> Caragana korshinskii> Buddleja alternifolia.