| Betel nut(Areca Catechu L),a member of the family Palmae, is tropical-economic-medicational crop, which has been the second rank of major industries. Due to outdate technology, extensively manage and deficiency of investigation, Betel nut industry has been restricted to low production. Hainan island is typical tropical ocean climate, with significantly rainy season and dry season.drought have significant effect on its growth and production, particularly during the efflorescence-from March to May every year.Therefor study on regularity of physical growth of betel nut have significant contribution to promote development of Betel nut industry by improving the management of threshold value。This study based on plant physiological ecology theory, annual growth betel nut was used as material in pot experiment, analysis its growing mechanism indexs such as seedling morphology,physical resistance and photosythese under different soil water stress fristly. Then establish the relationship model between soil relative moisture content,seedling photosynthesis and water utilization, finally analyzed the physiological datas and the models to find out the optimal and minimum threshold of soil relative moisture content, providing a reasonable basis for its water management. Finally finding the suitable relative soil water content and the minimum threshold limit relative soil water content for the finalization of betel nut seedling's growth, providing a reasonable basis for water management for the betel nut.In the dissertation, innovations conclusions were as follows:(1) Effect soil water stress on betel nut seedling growth and morphology. The results showed that betel nut seedlings'plant height, crown width, stem diameter, root number, root surface area and root volume decreased with the aggravation of water stress. Soil water stress significant effected water distribution in seedlings'body and it kept the rule as leaf> stem> root. According to soil water content increasing, root moisture content increased, leaf water content decreased,while no significant changes in stem water content. With the deepening of water stress betel nut seedling shoot ratio has increased firstly and then decreased.(2) Physiological resistance changes of betel nut seedling under soil water stress. The results indicated that soil moisture stress obviously enhanced the MDA content and osmotic adjustment substances in leaves and with the stress added,the MDA content, Pro content, SS content and SP content in leaves under different treatments were all increased in various degrees. Meanwhile, the Pro content had strong correlated to soil water stress time. The protective enzymes such as POD and SOD activity increased firstly and then decreased, while the CAT activity was in opposite changes under the soil water stress. Generally the POD and SOD activity in leaves would reduce as the extent of soil water stress increase. Betel nut seedlings'root activity would turn down when soil water stress increased. And the 75% relative soil water content was the turning point of its'root activity.(3) The relationship between soil water stress and photosynthetic capacity of betel nut seedling. It elaborated that with the water stress'time prolonged and stress level increased, chlorophyll content of betel nut decreased, the content of chl,cha,chb,caro,cha/b belong to betel nut seedling all dealing with 75% relative soil water content were distantly higher than others treatments. No matter how long the betel nut under the water stress, as soon as the soil water stress degree increased, betel nut seedling's Pn, Tr, cond all decreased, while seedlings'water efficiency increased. And in the prolonged stress period, the difference among different treatment would be weaken. betel nut seedlings'Fv, Fm, qP, Fv/Fm and Fv/Fo decreased under soil water stress, while Fo increased. It proved that water stress has inhibited the original PSⅡphotochemical efficiency, reduced photosynthetic electron transport capacity and the PSⅡphotochemical activity.(4) Analysis the correlation impacting factors of net photosynthetic rate and transpiration rate under different soil water stress, and fit stepwise regression equations under different soil water stress, establish the mathematics relationship models of the soil moisture content and net photosynthetic rate, and transpiration rate,and water use efficiency individually. The models as follow:Pn=-0.0018x2+0.2939x-5.5679, R2= 0.9981Tr=-0.0016x2+0.2652x-6.0083, R2= 0.9438WUE= 2E-06x4-0.0005x3+0.0404x2-1.4562x+20.54, R2= 0.8674(5)The research quantize the most suitable relative soil water content and the lowest soil moisture limits for betel nut seedling's growth for the first time. Through establishing the model and integrateing with physiological characters, it found that 45% relative soil moisture content was the lowest soil water condition for its growth. However considered the dry matter accumulation and water use efficiency, approximately 75% relative soil water content was more beneficial to betel nut seedling's growth.
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