Ecological Effects Of Urban-rural Environmental Gradient On Cinnamomum Camphora And The Stomatal Controlling Foundation

Obvious environmental gradients between urban and rural areas brought by rapid urbanization progress all around the world strongly impact the growth and development of plant. Stomata are the "gateways" of substances and signals intercommunion between the interior of the leaf of plant and the atmosphere, with the functions of regulating plant physiological processes such as photosynthesis, respiration and transpiration so much as the biomass producing. Nowadays, there are great arguments on the effects of urbanization on plant growth, and it is even little known about the controlling foundations and mechanisms.Hangzhou, a middle city with rapid urbanization progress in China, was taken as the investigation area and Cinnamomum camphor a L. Presl, a broadleaved evergreen at subtropical zone was taken as the study object in this research. The plant biomass growth at different urbanization levels along time series were investigated with the utilization of tree ring width analytical method, and at meanwhile, the stomata distribution patterns were quantified, the development pathways were traced and the ecological functions of stomata cluster on leaf surface were primary discussed. The main results and conclusions are as follows:1. Effects of urbanization on plant growth shifted from negative to positive.By surveying the tree ring width index of C. camphor a, the data between 1986 and 2004 were analyzed, and the results indicated that the impacts of urbanization on tree growth was found shifting from negative to positive during the urbanization progress. And the effects on plant growth of urbanization shifted from -0.10 to 0.13. What's more, the plant phenophases in urban sites were about 20 days ahead of the time at rural areas.2. The stomatal clusters distribute along the leaf veins and increase gradually from the newly generated parts to older parts, and they were developed by the clustered arranged meristemoids directly, or by means of the divisions of neighbor cells to form new stomata contacted presented stomata or little stomata pushing the epidermal cells between them aside on leaves of C. camphora.Stomata are usually scattered in the leaf epidermis and separated from one another by a stomata-free region preventing direct contact between the stomata in most wild-type plant species. However, stomata on leaf epidermis in C. camphora havenot been found distributed completely scattered, rather a few of them are arranged adjacently to form stomatal clusters. The study results of observing the stomata distribution pattern over the abaxial surface of the whole leaves showed that stomatal clustering occurs along the veins and stomatal cluster increased gradually from the apex to the base and the middle part to the margin part. The first two pathways have been documented by many researchers based on the observations of Arabidopsis thaliana L. Heynh. And the third one which had a strongly impact on the development as well as the pattern of stomatal clusters.is brought forward by us to explain the phenomenon that the stomatal cluster index (SCI) gradually increased while the stomata index (SI) decreased along with leaf development series.3. The stomatal clusters on leaf lower surface of C. camphora have the ecological function of "increasing local air humidity and conserving plant water" , and favor to improve plant water use efficiency (WUE).It is well known that when molecules pass though a pore, molecules which reach the edge will "spill over" and in effect have a shorter diffusion distance to get away from the pore, then water evaporates most rapidly around the outer edge, the perimeter of a pore. Water evaporates more rapidly from many small pores than from a few large ones, because the size of the perimeters added together is much greater. Compared with the one stomatal spacing pattern on leaf surface, the efficientperimeter and the diffusion shell surface area is much lower at stomatal clusters with stomata contacted each other directly, and then the water lose through stomata was reduced. And the effects of stomata on diffusion of CO2 are much weaker than that on water. Then we hypothesized that the WUE may be increased on leaves with more stomatal clusters.The evidences indicated that the leaves with the highest stomatal cluster density(SCD) and SCI have the highest WUE, while the stomatal density (SD) changedlittle, additionally, the SCD had negative correlation with soil moisture. Then weconcluded that the structure of stomatal cluster is favor for plant water conservation.These results may validate our hypothesis to a certain degree.The results of urbanization at different urbanization levels have different effects on plant growth may account for the arguments on the effects of urban environments on plant growth. Hypothesises of development pathway of stomatal cluster and their ecological function of "increasing local air humidity and conserving plant water" are based on our experimental phenomenon, it need more exactitude tracing observations to testify. If they are attested to be true, the hypothesises will not only indicate the physiological mechanism of stomatal cluster distribution pattern in nature, but also provide a new idea for biological water conservation, namely discovering and cloning plant high water use efficiency genes by modern biological techniques, and using the correlation genes to cultivate new high water use efficiency crop species, and then the problem of low agriculture water use efficiency will be settled.