| At present, it is important to study plant eco-physiological mechanism for global ecology, bio-diversity conservation, restoration and reconstruction of graded eco-system and the sustainable development of eco-system due to a series of environmental problems, such as the CO2 concentration elevation, global climate warming and water resource shortage. Exploring the eco-physiological mechanism of Alnus cremastogyne Burk would not only make for understanding its photosynthetic ability, water ecological characteristic, and its response to changing environmental factors, also provide decision-making reference for early breeding, introduction, cultivation and commerical appliance.Alnus cremastogyne Burk, belonging to the genera Alnus in Betulaceae, and being a native species and one of the most important species among the total 11 species in. Alnus in China, has been used as one of the most important forestation tree species for sheltbelt, soil and water conservation forest and short-rotation industrial forest in Yangtze River Valley, whose commercial area has reached 2.67×107hm2. Study on the eco-physiological characteristics of different varieties of A. cremastogyne Burk would help people to understand its eco-physiological mechanism and their relation to growth volume at the early stage, especially under the current background of rising atmospheric CO2, globe climate warming, and the shortage of water resources. Exploring its eco-physiological mechanism would furthermore help us to understand its response to environmental changes. The major results are shown as follows:The diurnal and seasonal variation of net photosynthetic rate (Pn) between the upper and lower crown of four varieties of A. cremastogyne Burk showed obvious difference. The diurnal variation of Pn of the same variety showed two different types of curves, namely, the single or double-peak curve at different growing season, and different varieties did so at the same growing season. Even if they showed the same curve type, the peak value of Pn appeared at different time, and their size were different too. At the same growing season, the value of Pn of H12, J10 were higher than that of H1 and J5 at any time in the upper crown during the diurnal courses. While the seasonal variation of the mean monthly Pn values of these four varieties in the upper and lower crown, respectively, revealed a double-peak curve, H12 and J10 were higher than H1 and J5, i.e., H12 and J10 had a stronger photosynthetic ability. Highly significant difference of Pn was observed between the upper and lower crown or different month, or different varieties (p< 0.001), respectively. If ranked on the 5% significance level, J10 and H12 belonged to the first level, and J5 and H1 the second level.The remarkable difference of diurnal and seasonal variation of transpiration rate (Tr) of these four varieties were observed between the upper and lower crown. The diurnal variation of Tr was generally identical with a single-peak curve or double-peak one. The leaves at the lower crown mainly showed a single peak curve, belonging to the weak-transpiratory midday-depression plant, and the maximum of Tr exceeded 5 mmol·m-2·s-1. This indicated that A. cremastogyne Burk is a weak-transpiratory and strongly water-consumptive plant, needing adequate water. The mean monthly Tr values of these four varieties all displayed the same seasonal changing tendency with double-peak curves. The maximum and mean monthly values of Tr of H12 and J10 were higher than those of H1 and J5 in the upper and lower crown, respectively. Tr of these four varieties showed a highly significant difference (p<0.0001) at the upper and lower crown, and this was also true of different month or different varieties. The order of Tr mean among different varieties was H12 (1.8770)>J10 (1.8675)>J5 (1.3003)>H1 (1.2970).The changing trend of diurnal variation of water use efficiency (WUE) of the same variety was incompletely similar in different growing seasons, and different varieties did so at the same growing season. The seasonal variation of WUE mean monthly of these four varieties presented the form as "W". The WUE mean monthly of H1, H12, J5, J10 varied from-1.111 to 6.0,1.24 to 4.265,-0.404 to 5.80,-1.383 to 5.546, respectively, and the order of WUE maximum was H1 >J5>J10>H12. WUE of these four varieties between different positions or different varieties had no significant difference (p>0.05). And a highly significant difference between different months was observed (p=0.0001).When photosynthetically active radiation (PAR) ranged 0～1500μmol·m-2·s-1, Pn of these four varieties increased with the increase of PAR in the upper and lower crown. When PAR exceeded 1500μmol·m-2·s-1, Pn kept rather stable. While PAR continued to increase, Pn varied little, and decreased in certain cases. The apparent quanta efficiency of these four varieties varied from 0.0303 mol·mol'1 to 0.0495 mol·mol-1, and the light compensation point (LCP) of H1, H12, J5 and J10 ranged 7.36～34.69,16.3～37.9,3.16～31.95 and 11.48～50.61μmol·m-2·s-1, respectively, while their corresponding light saturation point (LSP) ranged 1204.72～1458.45,1374.42～1434.96,1224.95～1424.16,1216.69～1530.61μmol·m-2·s-1, respectively. These indicated that these four varieties possessed rather lower LCP and higher LSP, having a higher utilization efficiency on weak light and strong light, capable of adapting to a manifold light environment.Ci of these four varieties decreased with the increase of PAR in the upper and lower crown, and the response degree of Ci to PAR of different varieties were slightly different. Cond increased with the increase of PAR, but with the increase of PAR, the amplitude of Cond gradually became weaker, and the response of Cond of different varieties to the increase of PAR were different too. Tr in the upper and lower crown increased with the increase of PAR in the upper and lower crown, and there was a significantly positive correlation between them, with the correlation coefficient being 0.6893 in the upper crown. The increase amplitude, mean values of Tr of J10 and H12 were higher than those of J5 and HI with the increase of PAR. When PAR ranged 0～500μmol·m-2·s'1, WUE in the upper and lower crown rapidly increased with the increase of PAR. While PAR approached or exceeded 500μmol·m-2·s"1, WUE gradually increased, or reached the maximum. When PAR exceeded 1000μmol·m-2·s-1, WUE generally decreased.When the concentration of CO2 ranged 0～800μmol·mol-1, Pn of these four varieties increased with the increase of CO2 concentration in the upper and lower crown. When the CO2 concentration reached between 800 and 2000μmol·mol-1, Pn varied little, even reached the maximum. Tr decreased with the increase of CO2 concentration, but its changing amplitude was rather little. And the changing amplitude of Tr with the increase of CO2 concentration among different varieties were different, For H1, H12, J10 and J5, that was 0.76,0.55,1.10 and 0.6mmol·m-2·s-1, respectively. Within low CO2 concentration, WUE increased with the increase of CO2 concentration. When CO2 concentration increased from 400μmol·mol-1 to 800μmol·mol-1, WUE of these four varieties increased by 25.92%, and the maximum more than 209.22%. Ci in upper and lower crown increased with the increase of CO2 concentration, and linear positive correlation between them were observed, and the difference among these varieties varied little (p >0.05). Cond decreased with the increase of CO2 concentration, and showed manifold changing type:gradually decrease, first increase and late decrease, or significantly fluctuate, etc. Vapor pressure deficit at the leaf surface (Vpdl) slightly increase with the increase of CO2 concentration.The eco-physilogical factors affecting Pn in upper, lower crown or whole plant were different, and as far as different varieties were concerned, it did so. the major factors affecting Pn in upper crown were Cond, Ci, PARi, Tleaf and Vpdl, while a little difference were observed among different varieties. The major factors affecting the lower crown were Cond, Ci, PARi, Tleaf, Tair, Vpdl and CO2R, and rather great difference were observed among different varieties. The major factors affecting whole plant were Cond, Ci, PARi, Tair and Vpdl, and there were positive correlation among Pn, Cond and PARi.The eco-physiological factors affecting Tr in upper, lower crown or whole plant were different, and among different varieties, the mainly affecting factors were different too. The main factors affecting the diurnal variation of Tr were Cond, Tair, Tleaf, Vpdl, PARi, H2OR and RHR, and among these factors, the influence of Cond, Vpdl, Tleaf and PARi were greater than that of others. The influence of Cond, Tair, PARi and Vpdl on Tr in lower crown were rather greater, and Cond, Tair, RHR on the whole plant were the greatest, especially, the influence of Cond was the most remarkable.The eco-physiological factors affecting WUE of the upper, lower or whole plant were different, and different varieties did so. The major factors affecting the diurnal variation of WUE in upper crown were Ci, Tair, CO2R, RHR,Vpdl,Tleaf and Cond, and among these factors, the affect of Ci,Cond, Vpdl,CO2R were greater than that of other factors. The major factors affecting WUE in lower crown were Ci, Tair, H2OR, RHR, Vpdl, CO2R, PARi, and among these factors, the affect of Ci,PARi,Vpdl,CO2R were greater than that of others. The major factors affecting WUE of the whole plant were Cond, Ci, Vpdl, CO2R, Tair, Tleaf, RHR, Tair and PARi, and among these factors, the affect of Ci,Cond were greater than that of others. In conclusion, Ci was the most important affecting factor on WUE, and there were significantly negative correlation between Ci and WUE. The order of Chi content of these four varieties was J5 (43.31)>H1 (42.63)>H12 (42.62) >J10 (38.74), and Chi content among different varieties had highly significant difference (p≤0.0001). Significant positive correlation were observed between Chi content and Pn, or seedling height, growth volume (p≤0.05). The order of leaf area, seedling height and biomass all was H12>J10>J5>H1, and leaf area among different varieties had significant difference (p≤0.05), seedling height and biomass did so. Pn had a terribly significantly positive coefficient with seedling height and biomass (p≤0.05), The accumulated contribution rate of Pn to individual growth volume reached 80.56%, to individual height 48.48%.
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