| Soil salinization and nutrient deprivation, such as potassium shortage, are double stress factors affecting plant growth, which poses a serious threat to crop production and quality. It is important meaning for genetic improvement of abiotic stress resistance of the crops and herbages to explore the interactions between potassium nutrition and salinity tolerance and to reveal the physiological mechanism of low-potassium as well as saline-alkali resistance. In the present study, the effects of two K+ levels(0.01 and 2.5 m M K+)on the growth, photosynthetic gas exchange, photochemical characters of PS and ion absorption and distribution of Ⅱ Apocynum venetum were investigated under Na Cl stress by greenhouse pot experiment. The main results were as follows:1. The biomass production of low-K+ plants attained the equivalent level of normal-K+ plants under control conditions, this indicates that A. venetum has prominent potassium efficiency of uptake and utilization, and who exhibits the strong ability of resistance to low potassium. No stressful effect was found on growth after irrigating with 50 m M Na Cl for five weeks, which shows a strong salt tolerance in A. venetum. Under 200 m M Na Cl treatment, the biomass and plant height significantly decreased but an enhanced root/shoot ratio was noted, compared to the corresponding control, such behavior suggests that it ensures root growth by increasing the energy flow to roots for absorbing water and nutrients from soil. In addition, we also found that there was no significant difference in biomass and height between 0.01 and 2.5 m M K+ under the same salt treatment.2. Potassium deficiency in soil did not affect photosynthetic capacity of A. venetum seedlings in the absence of salt stress, there was no significant difference in photosynthetic gas exchange parameters(except Gs) between low-K+ plants and normal-K+ plants. The treatment of 50 m M Na Cl has no influence on the photochemical characters of PSⅡ, but chlorophyll fluorescence parameters such as Fv/Fm, Fv/Fo, Φ PSⅡ, Fv’/Fm’ and q P all declined significantly induced by 200 m M Na Cl stress. Under salinity stress, Ls significantly increased due to stoma tightening, which caused a noteworthy decline in Pn result from the restriction of CO2 into intracellular space. Moreover, improving water use efficiency through the regulation of stomatal closure to reduce water loss was beneficial to maintaining water balance in plants, and which is one of effective adaptation strategies of A. venetum responding to salinity stress.3. Na+ contents in organs of A. venetum raised significantly with the increase of external Na Cl concentration, and the increase magnitude in leaf was greater than that in root and stem. Compared with the control, K+ contents in leaf did not change significantly under both 0.01 and 2.5 m M K+ when seedlings were exposed to 50 m M Na Cl. Moreover, K+ contents decreased significantly but still sustained at a high level in shoot with the increase of external Na Cl concentration, resulting in high K+/Na+ ratio. K+ maintained stable in root after irrigating with Na Cl solution, which was determined by the high SA value. Meanwhile, A. venetum accumulated high contents of K+ in the absence of salt stress, especially in leaf where K+ contents were 15 times higher than that of Na+. In conclusion, enhancing the capacity of selective absorption and transport for K+, and hence maintaining high K+ contents and K+/Na+ ratio in leaf are the key adaptive mechanisms of salt-tolerance in A. venetum. |
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