| Under environmental stress, plants have many adaptation mechanisms by autogenously regulation such as photosynthetic response mechanism, mechanism of inorganic nutrition utilization, mechanism of carbon anhydrase function and others. While variation of the amount of root-exuded organic acids, derived from the photosynthetic fixed carbon, was an important adaptive mechanism of plant response to environmental stress. Through a series of complicated biochemical effect, organic acids of root exudations direct influenced many rhizosphere processes such as the nutrient acquisition in rhizosphere region, detoxification of heavy metal and microorganism, and then the plant growth and development were affected. Therefore, the root-exuded organic acids played a vital role in plant stress physiology.The study selected plant species adaptability to Karst----Broussonetia papyrifera (B. papyrifera) and Orychophragmus violaceus (O. violaceus) as the research objects, and the none Karst adapted plant species----Morus alba (M. alba) and Brassica napus (B. napus) as the control. By setting the artificial simulation of Karst environment (high pH, high bicarbonate concentration, low nutrition, drought), we explored the response mechanism of photosynthesis, and organic acids accumulation in plants and the root organic acids exudation, illuminated the source of organic acids in root exudations, discussed the characteristics of photosynthetic and root organic acids exudation under the long-term environmental stress, presented a method of expressing plant resistance to stress by root organic acid secretion characteristic. All of this results provided evidences for selecting plant adaptability to difference environment. This study had a great significance to understand adversity under the condition of root secretion, light respiration activity and plant water requirement and the relationship between the fertilizer requirements, provides a research basis for follow-up study of root secretion of adversity physiological response characteristics, potential of the excavation of the relationship between crop itself. In order to achieve the purpose of water-saving irrigation and precise fertilization, this research provides a preliminary research basis of precise fertilization and water-saving irrigation in the field of agricultural engineering.The experimental results are as following:(1) We studied the photosynthetic response mode of four plant species under interaction of Zn treatments (Zn-sufficient,0.02mM; Zn-deficient,0mM) and bicarbonate treatments (0mM and10mM bicarbonate). By measurement of the growth parameters, photosynthetic parameters, chlorophyll fluorescence parameters and chlorophyll content under Zn deficiency, or high bicarbonate concentration, or both. The results showed the photosynthetic response mode varied with species. Among the four plant species, B. papyrifera showed greatest adaptability to bicarbonate treatment, low Zn, or both, which resulted from the greatest of inorganic carbon assimilation and photosynthetic carbon metabolism in B. papyrifera; B. napus was sensitive to low Zn, bicarbonate treatment, or both, owing to the greatest damage in photosystem Ⅱ reactive center; the adaptability of O. violaceus and M. alba were between B. papyrifera and B. napus.(2) We studied the characteristics of allocation, transportation of organic acids in plant organs and root organic acids exudation. By artificial simulation of zinc treatments (Zn-sufficient,0.02mM; Zn-deficient,0mM) and bicarbonate stress (0mM and10mM bicarbonate), we investigated the amount of organic acids in plant organs (root, stem and leaf) and the amount of root-exuded organic acids, and reflected the characteristics of organic acids allocation, transportation and exudation. The results indicated that leave was the main region of organic acid production; the source of organic acids of root, stem, leave and root exudations was different, the source of organic acids of leave was main from Krebs cycle and glyoxylate cycle, the source of organic acids of root, stem and root exudations was main from Krebs cycle. Oxalic and citric acids are suffered the greatest influence in formation, allocation, translocation and exudation of organic acid.(3) We studied the dynamic variation characteristics of photosynthetic parameters, chlorophyll fluorescence parameters. Under the artificial simulation of interaction of zinc treatments (Zn-sufficient,0.02mM; Zn-deficient,0mM) and bicarbonate stress (0mM,1mM,5mM,10mM bicarbonate), the four plant species showed the difference variation of photosynthetic parameters, chlorophyll fluorescence parameters and chlorophyll content during the treatment time duration. The four plant species can adapt the environment of low bicarbonate concentration (1,5mM). Under the dual influence of Zn deficiency and excessive bicarbonate concentration (10mM), the adaptability of B. papyrifera and O. violaceus was greater than that of M. alba and B. napus under the long-term treatment.(4) We studied the dynamic variation characteristics of root-exuded organic acids. Under the artificial simulation of interaction of zinc treatments (Zn-sufficient,0.02mM; Zn-deficient,0mM) and bicarbonate stress (0mM,1mM,5mM,10mM bicarbonate), the amount of oxalic, citric, malic, succinic, and tartaric acids were depended on plant species and nutrition concentration during the treatment time duration. Under Zn-sufficient conditions, all three bicarbonate treatment stimulated root organic acids exudation in four plant species, the root-exuded organic acids was increased, particularly oxalic, citric, malic acids, under the5,10mM bicarbonate treatments at the same treatment time. Under Zn-deficient conditions, bicarbonate had no significant inhibition the exudation of root organic acids of four plant species during the treatment time duration.(5) We studied the characteristics of P-extraction cost by root-exuded organic acids in four plant species, and indicated a new method of expressing plant resistance to P deficiency stress by utilizing P-extraction cost. The amount of oxalic, citric and malic acids was measured in four plant species under P deficiency stress, and the models of the standard solution of oxalic, citric, and malic acids and the rhizosphere soil P extraction were established, and then the P-extraction of root-exuded organic acids was calculated. And we calculated the organic carbon loss of root-exuded oxalic, citric and malic acids. Therefore, we presented the concept of P-extraction cost by combining P extraction and organic carbon in root-exuded organic acids, and applied this expressed the plant resistance to P deficiency stress. The results showed the resistance to P deficiency stress of four plant species was the following series:O. violaceus≈B. papyrifera<M. alba<B. napus, which indicated that O. violaceus and B. papyrifera have the greater resistance to P deficiency stress.(6) We established the relationship between the chlorophyll fluorescence parameters and root-exuded organic acids. Under phosphorus stress, the linear relationship was between the chlorophyll fluorescence parameters (Fo, Fv/Fm) and the amount of root-exude malic acid, and founded a low-cost method was used for assessing plant adaptability to a low-phosphorus environment when no chlorophyll fluorescence instrument was available. And the method directly applied in the expression of drought resistance, the results showed that the drought resistance of B. papyrifera and O. violaceus was greater than that of M. alba and B. napus.In summary, we discussed the photosynthetic response characteristics and root organic acids exudations characteristics of four plant species in the long-term and short-term treatment under the simulation of Karst environment; and analyzed the source of organic acids in root exudations, and presented the method of expressing plant resistance to P deficiency and drought stress by the characteristics of root organic acids exudations, which provided strong evidence for plants resistance to stress. |
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