| Alfalfa is widely used to establish pasture for improving environmental conditions and providing high protein forage for livestock in the western region of China. However, harsh environmental conditions in this region, such as drought, high salineness, extreme temperature, have threatened the pasture establishment of alfalfa due to low emergence and survival rate. On the other hand, alfalfa seed is dormant and post-mature. Therefore, it is an urgent to promote the seed germination of alfalfa and improve the survival rate of alfalfa seedling for solving the problems of the low emergence and survival rate in alfalfa. Seed priming not only increases the seed vigor and germination, but also enhances the growth of seedling and survival rate, sometimes, increases the biomass of many plants. Currently priming seed strategies for alfalfa have several problems including increasing the expense, handling the disposal, providing the beneficial conditions for bacteria propagation, separating the primed seeds from matrix particles, and causing the imbibition damage. The objective of this study is to develop a priming seed strategy with water, which should be simple, low-cost, low-risk approach, no imbibitiom damage, and then this developed strategy is tested systematically using the pot and field trials. The results of this study are as following.Firstly, the optimal of temperature, addition water and duration for priming seeds with water in alfalfa was determined by series experiments in this study. Total water weight (amount to volume) required by priming alfalfa seeds was 80% to 120% of original weight of seeds according to imbibition curve of seeds and seed germinations. Generally appropriate duration of hydro-priming is 2 to 9 days by measuring the initial times of seed germination resulting from the radicle emergence from the seed coat. The priming effectiveness differed from various combinations of temperatures, addition waters and durations when assessed by germination percentage or germination index. A new procedure for comprehensively assessing effectiveness of primed seeds with water was proposed by combining the germination percentage and the germination index. The comprehensive assessment indicated that 5℃+90%W+3D, 5℃+100%W+5D, 5℃+90%W+4D, 5℃+100%W+3D performed well. This study showed that the comprehensive assessment order was not consistent with any single indicator, neither germination index order nor germination rate order, so this study determined the optimal conditions of hydro-priming: temperature and addition water and duration was 5°C and was 90% to 100 % of original seed weight and 3 to 5 days. Other alfalfa cultivars were used to test the commonness of optimal conditions of hydro-priming (5℃+ 100W%+3D, 5℃+ 90%W+4D, 5℃+ 100%W+5D), and indicating that optimal conditions were suitable to all alfalfa cultivars.Secondly, this study disclosed the hydro-priming strategy impact on the physiological indicators of the alfalfa seeds, and indicating that hydro-priming strategy reduced the soluble sugar and malondialdehyde content and electrolyte leakage, and increased the proline content of seeds. The range of increase or decrease differed among alfalfa cultivars. This implied that hydro-priming strategy repaired the biological membrane, enhanced the stability of cellular structure, and improved the resistance to drought and extremely low temperature stress.Thirdly, the priming effectiveness of hydro-priming and PEG (Polyethylene glycol) priming was compared by the experiments carried out simultaneously and the results indicated that seeds of hydro-priming germinated faster, more uniformly, than that of PEG priming at the low temperature (10℃) conditions and serious drought condition (-0.6 MPa osmotic potential), however, the priming effectiveness was not significant difference between hydro-priming and PEG priming at the high temperature (15℃and 20℃) and light drought conditions (-0.3 MPa osmotic potential).Fourthly, Responses of the hydro-priming effectiveness of alfalfa seeds to temperature conditions and soil moistures were determined under the artificial simulation conditions. The emergence rate and survival rate of seedling at 30 d and 60 d of seeds primed by water were greater than that of non-primed seeds at 30% and 90% of field water capacity. At 50% to 70% of field water capacity, hydro-priming slightly reduced the emergence and survival rate of seedling of each cultivar. At the serious drought conditions, hydro-priming increased the plant height but decreased the shoots per plant, and the range of the increases in plant height and decreases in shoots per plant disappeared when the soil moisture was 60% of field water capacity. Therefore, hydro-priming had no effect on forage biomass per plant at the pot experiment. Root biomass of hydro-priming treatments was greater than that of the non-primed treatments at the serious drought conditions but smaller than that of the non-primed treatments when soil moisture was over 50% of field water capacity.Fifthly, the commonness of hydro-priming strategy was tested by four alfalfa seeds in the different conditions using the field trials. The results of this study indicated that seeds of hydro-priming strategy germinated faster, and more uniformly, than the non-primed seeds and survival rates of seedlings of primed seeds also was greater than that of the non-primed seeds at the field with the rainfall of 562.0 mm, 215.3 mm and 118.4 mm. The range of increases in survival rate varied with the rainfall and the cultivars.Finally, the priming effectiveness of alfalfa seeds at the different irrigation conditions was also discussed in this study. Hydro-priming had no effect on the initial time of emergence, uniformity, the biggest survival rate, but increased the final survival rate, which increased density and forage biomass.
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