Studies On The Role Of Osmotic Adjustment Under Drought And Salt Stress In Desert Plant Reaumuria Soongorica

The term osmotic adjustment is used to imply an adaptive response: a net increase in thequantity of osmotically active solute either by uptake or by synthesis. Osmoticadjustment is considered as one of the most important adaptations of plants to drought,because it allows plants to maintain water absorption, cell turgor and metabolic activityduring periods of drought stress, and also enables quick resumption of growth whenwater becomes available again. We chose Reaumuria soongorica as our target species,which is the dominant and constructive plant of desert shrubbery vegetations in China,with high ability to tolerate stressful environments, such as drought and salinity. Due toits potential role for ecosystem function in arid- and semi-arid regions studied, the presentwork explored the underlying mechanism of osmotic adjustment and major osmoticsolutes in R. soongorica suffering from different magnitude of stress regarding to droughtand salinity, mainly focusing on the content of the inorganic ions (K⁺,Na⁺ and NO3^-) andorganic solutes (proline and soluble sugar). The key results are as following:1. K⁺ and Na⁺ largely accumulated in the leaves of R. soongorica under droughtstress, especially for the Na⁺ content increasing continuously with drought time.Although R. soongorica is neither euhalophyte nor xero-halophyte such as Haloxylonammodendron, it does evolve the adaptive strategy to tolerate salinity. Through thecellular compartmentalization, R. soongorica is able to tolerate the high concentration ofNa⁺ in cell. Both K⁺ and Na⁺, especially Na⁺, play important roles in osmotic adjustmentof R. soongorica in this study. K⁺ and Na⁺ also tremendously accumulated in the stems,but their contents in roots did not have remarkable changes, especially the Na⁺ contentwas rarely affected by drought. Therefore, we conclude that when R. soongorica suffereddrought, stems could aid even act as surogate for leaves to perform many physiologicalfunctions to adapt severe environments. Leaves and stems of R. soongorica are the majororgans to accumulate K⁺ and Na⁺.2. NO₃^- greatly accumulated in the leaves and stems of R. soongorica, and its contentalways increased with drought time, especially in the stem, suggesting that the stem playsimportant role in the osmotic adjustment of R. soongorica and is the major organ to accumulate NO₃^-. In contrast to the leaves and stems, the NO₃^- content in rootssignificantly decreased with drought time, while in controlled plants it increased all thetime. Therefore, we presume that the root may be a store organ of NO₃^- in R. soongorica.When the water status is benign, NO₃^- can largely accumulate in the roots of R.soongorica, and then it can be transported up to shoots when water becomes rare, playingroles in the osmotic adjustment of leaves and stems and helping R. soongorica overcomesevere drought. This type of storage for NO₃^- is crucial for the desert plants, because thenitrogen content of the desert soil is very low, The addition of external KNO₃ alleviatingthe reduction of NO₃^- content in the root in this experiment, suggests the increasednitrogen content of soil could compensate the lost of NO₃^- in the root due to thetransportation to the shoot. The addition of external KNO₃ had much more effect on theNO₃^- content in R. soongorica than K⁺ and Na⁺, because probably the basal nitrogencontent of soil in which R. soongorica grows is lower than the optimal soil nitrogencontent required for its normal growth. The NO₃ content exhibited the increasing trend inthe leaf of R. soongorica under the salt stress.3. The proline content in the leaves of R. soongorica increased in the early 20 daysduring drought treatment, and then pronouncedly decreased. The content of the prolinedid increase at early stage in the leaves of R. soongorica under long-term water deficit,but it gradually reduced with drought time until lower than that of controlled plants.Moreover, the addition of external KNO₃ magnified the reduction of proline content. Theproline content in the stems of R. soongorica also increased remarkably under drought,and it did not have significant change after the addition of external KNO₃. For the rootsof R. soongorica, the proline content reduced notably under drought, and this decreasingtrend was not affected by the addition of external KNO₃. Therefore, proline may be notthe major osmotic solute in the osmotic adjustment of R. soongorica in droughtenvironment. The increased proline content in the leaves of R. soongorica at droughtearly stage may be just required to balance the inorganic ion concentration of vacuole incell. In our salt stress experiment, the proline content in the leave of R. soongoricaincreased significantly with the increasing salinity. The synthesis of proline is sufficientlynecessary for the considerable inorganic ions accumulated in the cellular vacuole.The soluble sugar content in the leaves of R. soongorica continuously decreased in the early 20 days of drought, and then distinctly increased, which had the opposite changeof the proline content, but was still lower than that of controlled plants. However, in thesalt stress experiment, the soluble sugar content in the leaves of R. soongorica increasedsignificantly with the increasing salinity. Therefore, proline and soluble sugar may be notthe major osmotic solutes of R. soongorica under drought stress, and inorganic ions suchas K⁺, Na⁺ and NO₃^-, especially Na⁺, are the major osmotic solutes. However, proline andsoluble sugar could play important roles in balancing the concentration of cytoplasm andthat of vacuole, in which numerous inorganic ions accumulate. Proline and soluble sugargreatly synthesize in the leaves of R. soongorica under salt stress. The avoidance of ionstoxicity in plants under salt stress has benefit from the highly increased organic solutes,thus the tolerance of plants to salinity is intensified.Totally, osmotic adjustment plays a critical role in the survival of R. soongorica inlong-term severely drought environments. Usually, when osmotic adjustment functionsmost water in soil has been depleted. Therefore, osmotic adjustment is just a survivalmechanism of plants suffering from long-term drought, not related to the growth,development or even yield (for crops) of plants, the only aim of which it to help plantssurvive in severe drought environment to wait for the water status improved. The twosaving energy mechanisms, accumulating ion and sugar as osmotic solutes, are veryimportant to R. soongorica, because more effective allocation of limited energyguarantees the survival of plants in severe environment. In addition, K⁺, Na⁺, NO₃^- andproline, especial NO₃^-, greatly accumulated in the stems of R. soongorica, thus the stemprobably could play important role in the tolerance to much severer drought, and has thecentral status in the strategies of R. soongorica in response to extreme drought.