| Physiological responses of potted seedings of Cupressus chengia, Pinus radiate, Juglans regia, Ulmus pumila and Acer laxiflorum to a progressive water deficit imposed by stopping irrigation and of Cupressus chengia, Pinus radiate, Juglans regia and Ulmus pumila to the fall of natural temperature from Oct.19 in 2009 to Jan.19 in 2010 were analyzed. The results were as follows:(1) Water relations were effected by different water stress treatments significantly. Pre-dawn leaf water potential (φpd) and leaf relative water content (RWCpd) decreased progressively from control to severe water stress. Ratio of leaf water loss in 24h and 48h of conifer species were less than broadleaved species, it showed that the ability of holding water of conifer species were stronger than broadleaved species.(2) Drought stress effected gas exchange of the plants significantly. Net CO2 assimilation (AN), stomatal conductance (gs) and transpiration rate (E) of the four tree species decreased with increasing drought stress. Maximum net photosynthetic rate (Amax), rubisco carboxylation rate (Vcmax) and electron transport rate (Jmax) were changed under water stress significantly because drought induced the change of A-Ci response curves.(3) Different water stress treatments had significant effect on chlorophyll fluorescence parameters. Maximum quantum yield of PSII measured at pre-dawn (Fv/Fm), actual quantum yield of PSII (ΦpsⅡ) and electron transport rate (ETR) decreased significantly at drought stress. Fv'/Fm/ showed different change from control to moderate water stress, followed by a larger decrease at severe water stress compared to control.(4) Stomatal closure and the consequent reduction in Ci were the major reason for reducing photosynthetic rates under mild drought conditions; however, under severe drought condition photosynthesis may also be restricted by non-stomatal effects.(5) Threshold value of stomatal conductance of Acer laxiflorum to photosynthetic recovery was 0.03 mol·m-2·s-1. So photosynthetic capacity never or just partly recovered if gs lower than that under water stress.(6) Electrolytic conductivity of all the species increased as the air temperature dropped, but the extent was not large due to they were exposed to their slight injury.(7) The proline content of broadleaved species increased firstly, then decreased when acclimated to the low temprature, but the content increased when tempature further deceased. Compared to broadleaved species, conifer species changed steadily during the air temperature decreased, which indicated that broadleaved species were more sensitive than conifer species to cold stress.(8) The Malondialdehyde content in Ulmus pumila and Juglans regia seedlings increased significantly with temperature decreased, especially the MDA content of Juglans regia was the highest during the decrease of air temperature, while that in Cupressus chengia and Pinus radiate seedlings changed slightly.(9) The four tree species increased the contents of soluble sugars and decreased the content of starch as the temperature decreased, but broadleaved species decreased dramatically and consumed more starch compared to conifer species. As a result, the soluble sugars/starch ratio increased dramatically in low-temperature-stressed plants but conifer species decreased in Jan.(10) Comprehensive analysis indicated that the drought resistance of seedings of four tree species in the order of Pinus radiate> Cupressus chengia> Ulmus pumila> Juglans regia; the cold resistance of seedings ranked in the following order:Cupressus chengia> Pinus radiate> Ulmus pumila> Juglans regia.
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