| Leaf photosynthetic and chlorophyll fluorescence characteristics of Ulmus elongata were studied firstly in this paper, then its phylogeographic relationships were studied by using Fourier transform infrared spectroscopy (FT-IR). Finally we discussed protection methods based on leaf traits and survival status.1. Diurnal variations of endangered plant one year and current year U. elongata seedlings were studied by using Li-6400XT portable photosynthesis system with the aim of protecting and providing technical approaches. It was found that both current and one year growth U. elongata seedlings had one single peak, which had no "midday depression". For current year seedlings, the average net photosynthetic accumulation (5.64μmol·m-2·s-1) was greater than the accumulation of net photosynthesis (5.05μmol·m-2·s-1) of one year seedlings. The diurnal variations of intercellular CO2concentration (Ci) were opposite to the net photosynthesis. The diurnal variations of stomatal conductance (Gs) and transpiration rate (Tr) had no significant certain law. Light use efficiency (LUE) were lowest at12:00, water use efficiency (WUE) were highest at13:00. Light saturation point (LSP) of one year seedlings was1620.75μmol·m-2·s-1, and1123.07μmol·m-2·s-1for current year seedlings, the light compensation point (LCP) of both were very low,8.33μmol·m-2·s-1and9.89μmol·m-2·s-1, respectively. It indicated that U. elongata seedlings would be adapted to a large scale of light. Partial correlation analysis showed that the major environmental factors of net photosynthetic rates were the photosynthetic active radiation (PAR) and air temperature (Ta).2. Diurnal variations of chlorophyll fluorescence parameters and rapid light curves of endangered plant U. elongata seedlings were determined by using Li-6400XT portable photosynthesis system. The results showed that the actual photochemical efficiency (ΦPSⅡ), electron transport rate (ETR) of photosystem Ⅱ (PSⅡ) were stable throughout the daytime, and decreased significantly at18:00pm. Photochemical quenching (qP) increased firstly at morning and then decreased after midday, non-photochemical quenching (NPQ) showed the opposite changes, the minimum at noon, indicating the light use efficiency of U. elongata seedlings were high. The rapid light curves showed that actual photochemical efficiency (ΦPSⅡ) and photochemical quenching (qP) decreased while the photosynthetic active radiation (PAR) increased, electron transport rate (ETR) and non-photochemical quenching (NPQ) increased while the photosynthetic active radiation (PAR) increased.The power function could fit well the actual photochemical efficiency (ΦPSⅡ) and electron transport rate (ETR) with the light intensity changes, while the logarithmic function could fit well the actual photochemical quenching (qP) and non-photochemical quenching (NPQ) with the light intensity changes.3. The phylogenetic relationship between U. elongata trees growing in different geographic regions was studied by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis, and differential thermal technology. We found that the infrared absorbance values of FTIR spectra differed significantly between U. elongata samples from seven distributed regions. We defined four genetic groups (clades) of U. elongata by differences in16characteristic absorbance peaks. The trees (named by regions) were sorted as follows:Shexian Anhui (AHSX) and Lin’an Zhejiang (ZJLA) in clade I, Kaihua (ZJKH), Suichang Zhejiang (ZJSC), and Wuning Jiangxi (JXWN) in (clade Ⅱ); Songyang Zhejiang (ZJSY) in clade Ⅲ, and Nanping Fujian (FJNP) clade Ⅳ. Thermogravimetric and differential thermal analysis revealed that the leaves of U. elongata had two obvious exothermic pyrolysis stages, and reached maximum mass loss near550℃. TG and DT curves between350-550℃were also used to elucidate the phylogenic relationship between U. elongata samples.4. Identifying the environmental factors affecting the leaf traits of U. elongata is important for understanding its endangered mechanisms. Specific leaf area (SLA), leaf nitrogen content, and leaf carbon isotope discrimination (δ13C) were determined in this study, and temperature and light were found to be the main environmental factors influencing SLA, indicating that U. elongata is heliophilous. The maximum and minimum δ13C values of the plant species were-26.33‰and-29.64%o, respectively (the mean was-28.09%o), suggesting that it is of the C3photosynthetic type. Compared with the altitude, annual sunshine duration, annual precipitation, and mean annual temperature accounted for36%of the variance in δ13C, which is the main environmental factor affecting13C fractionation. The foliar δ13C value of U. elongata was significantly more negative than that of U. pumila, a widely distributed elm tree in northern China (P<0.05), but its leaf nitrogen content was somewhat higher (33.30mg g-1) than that of the latter species (28.18mg g-), with the difference being statistically insignificant (P>0.05). These findings indicate that U. elongata has less photosynthetic stomatal limitation and higher carbon assimilation rates. Interestingly, across latitude gradients, the leaf δ13C value of U. elongata became even more negative but the soil δ13C value of the distribution sites became more positive, demonstrating low water use efficiency and abundance of C4plant species in warm and humid areas. SLA was observed to increase with increasing annual average temperature; the decrease in leaf δ13C value indicates that water use efficiency lowers when water loss and transpiration strengthen in a high-temperature environment; these data revealed that the population of U. elongata may diminish under global warming and drought stress. At last, we proposed several conservation strategies based on leaf characters and habitat traits.5. The main distribution areas were Anhui, Zhejiang, Jiangxi and Fujian provinces according to our two years’survey in China, and we concluded that habitats of U. elongata have been fragmented severely; the main causes of its extinction were natural and human interferences. The main forms of human interferences were habitat destruction caused by mining and tourism, the demand for firewood from seed trees by local farmers and the priority development of economic forest. Natural interferences mainly were extreme weather, such as blizzards in2008, which caused huge destruction to U. elongata in Suichang county Zhejiang province, but the impact of natural interferences was less than human interferences. U. elongata was mainly distributed at an altitude of600-900m in sunny or half-sunny slope; Suichang and Kaihua County in Zhejiang province were the glacial refuge and had the largest populations because of their special terrain. Spearman correlation test showed that the correlations of the18biological and environmental factors to each other were low significant or very low significant (P>0.05) because of serious habitat fragmentation of U. elongata, indicating the communities of U. elongata were at different stages of succession. Principal component analysis showed the biological and environmental factors of habitats were:light, soil nutrients, aspect and elevation. We extracted4principal components and found that the principal component1"light and energy factor" of environmental factors explained the variance28.408%of the total variance, principal component2"soil factor" was27.031%, principal component3"slope" was19.063%, principal component4"elevation" was12.221%, all of them explained86.723%of the total variance."Light" was the main factor affecting populations of U. elongata because it was a heliophilous plant. U. elongata was mainly distributed on the slopes at higher elevations to avoid excessive human destruction. We concluded that U. elongata was highly endangered based on the characteristics of habitats and distribution, which easily led to extinction. The protection grade of U. elongata should be enhanced; national grade I is more suitable for protection. To carry out the protection of U. elongata were imminent needs, and different regions should adopt different protection strategies, it was better to unite the in-situ and ex situ conservation and propaganda of the scientific knowledge of U. elongata. The prior strategy was ex situ conservation for Songyang Zhejiang, Wuning Jiangxi and Nanping Fujian provinces due to their small populations, and active introduction and cultivation were also needed; the core areas of protected areas should be expanded for Shexian Anhui, Lin’an, Kaihua, Suichang Zhejiang to avoid following extinction, and human destruction must be prohibited. To our knowledge, the habitat fragmentation was a chief reason for endangering; human interferences were the primary reason for the extinction of U. elongata. However, the physiological and ecological effects of habitat fragmentation on populations of U. elongata needed further study. In addition, it also needed to illustrate the evolutionary status of U. elongata as an Ulmus and Ulmaceae plant in order to define priority conservation populations. |
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