Response And Mechanism Of Physiological Integration In Clonal Plants To Heterogeneity Of UV-B Radiation

Ultraviolet-B radiation can result in deleterious effects on many plant processes. In clonal plants, connected ramets often share resources and hormones. However, little comprehensive study is known about whether and how clonal integration influences the ability of clonal plants to respond to UV-B radiation. In this paper, the changes in intensity of physiological integration, anatomic characteristic parameters, photo synthetic efficiency, survival and growth and of physiological parameters under homogeneous and heterogeneous ultraviolet-B radiation (280-320nm) were measured in order to test the hypothesis that in addition to resource integration a defensive integration in clonal plants might exist as well. The contents of NO and H2O2were measured in order to study the mechanism of physiological integration in clonal plants to heterogeneity of UV-B radiation. We conducted an green house experiment in which ramet pairs of Trifolium repens, Duchesnea indica and Glechoma longituba were subjected to four treatments:(1) connected ramet pairs under a homogeneous condition (both of ramets received only natural background radiation, ca.0.6kJ m-2d-1), i.e. pairs of ramets were kept under no additional UV-B conditions and the stolon connection between the ramets remained intact;(2) disconnected ramet pairs under the same condition as in treatment1;(3) connected ramet pairs under a heterogeneous condition (one of the ramet received only natural background radiation and the other was exposed to supplemental UV-B radiation,2.54kJ m-2d-1), i.e. one of the ramets was exposed under additional UV-B radiation and the stolon connection between the ramets remained intact;(4) disconnected ramet pairs under the same condition as in treatment3. Changes in intensity of water and nutrient integration were followed with acid fuchsin dye and15N-isotope labeling of the xylem water transport; Anatomic characteristic parameters were studied by the paraffin and the freehand sectioning; In order to assess the patterns of physiological integration contents of chlorophyll, Ultraviolet-B absorbing compounds, soluble sugar and protein were determined and activities of superoxide dismutase (SOD) and peroxidase (POD) measured; Gas exchange and chlorophyll fluorescence parameters were evaluated to study the photosynthetic efficiency in clonal plants; Biomass and number of ramet to study the survival and growth; The contents of NO and H2O2were measured in order to study the mechanism of physiological integration. The results were as follows: 1. When ramets were connected and exposed to heterogeneous UV-B radiation, the velocity of water transportation from the UV-B treated ramet to its connected sister ramet was markedly lower and the percentage of15N left in labelled ramets that suffered from enhanced UV-B radiation was higher and their transfer to unlabelled ramets lower in Trifolium repens L, The direction of UV-B radiation and15N-isotope labeling had no significant effect on the result in Duchesnea indica. It indicated that physiological integration existed in clonal plants under either homogeneous or heterogeneous UV-B radiation, and the intensity of water and nutrient integration were decreased under heterogeneous UV-B radiation.2. When ramets were severed and exposed to heterogeneous UV-B radiation, stomatal length and the thickness of palisade tissue in Trifolium repens L increased. The results in connected group were different, indicated that physiological integration played the role.3. In comparison with clones under homogeneous ultraviolet-B radiation, the contents of chlorophyll, Ultraviolet-B absorbing compounds, soluble sugar and activities of SOD and POD increased notably if ultraviolet-B stressed ramets were connected to untreated ramets. Chlorophyll and UV-B absorbing compounds were shared between connected ramets under heterogeneous UV-B radiation. This indicated that physiological connection improved the performance of whole clonal plants under heterogeneous ultraviolet-B radiation in Trifolium repens L. the contents of chlorophyll in Duchesnea indica under UV-B radiation decreased in homogeneous ultraviolet-B radiation.4. Stomatal conductance (gs), intercellular CO2concentration (Ci) and transpiration rate (E) showed no significant differences in connected and severed ramets under homogenous and heterogeneous UV-B radiation, however, net photosynthesis rate (PN) and maximum photosynthetic rate (Pmax) of ramets suffered from supplemental UV-B radiation increased and that of its connected sister ramet decreased significantly. Moreover, additive UV-B radiation resulted in a notable decrease of the minimal fluorescence of dark-adapted state (Fo), the electron transport rate (ETR) and photochemical quenching coefficient (qp) and an increase of non-photochemical quenching (NPQ) under supplemental UV-B radiation, while physiological connection reverse the results in Trifolium repens L. PN, gs and Ci were lower and E were higher under enhanced UV-B radiation in UV-B stressed part, and Ci in UV-B unstressed part were reversed in Duchesnea indica. Moreover, in UV-B unstressed part, maximum photochemical efficiency of PSâ…¡ (Fv/Fm) and qp had no significant difference, ETR were lower and NPQ were higher, and in UV-B stressed part, additive UV-B radiation resulted in a notable decrease of Fv/Fm, ETR and qp and an increase of NPQ, stolon connection made Fv/Fm, ETR, qP and NPQ changed more in the connected group. 5. Stolon connection resulted in an increase in biomass and number of ramets in the UV-B stressed part of the clonal fragments at the expense of UV-B unstressed part, fullcompensatory growth existed in Duchesnea indica.6. The contents of NO and H2O2showed no significant differences in connected and severed ramets under homogenous and heterogeneous UV-B radiation in Glechoma longituba. The contents of NO were significant higher under enhanced UV-B radiation in UV-B stressed part, however, the contents of NO were also significant higher in UV-B unstressed part, indicated that physiological integration played the role. The contents of H2O2suffered from supplemental UV-B radiation increased in severed group, while the results were different in connected group, that stolon connection will mitigate the stress. The chlorophyll fluorescence contents of NO and H2O2in UV-B stressed part were lower in homogenous UV-B radiation than heterogeneous UV-B radiation. While in heterogeneous UV-B radiation, the chlorophyll fluorescence contents of NO and H2O2in UV-B unstressed part were also higher, it was related to physiological integration, and to study the mechanism of physiological integration in clonal plants to heterogeneity of UV-B radiation.In all, this indicated that physiological connection improved the performance of whole clonal plants under heterogeneous ultraviolet-B radiation. Under heterogeneous UV-B micro-habitat, clonal integration markedly reduced the stress effects imposed by enhanced UV-B radiation with a cost, and clonal plants are able to optimize the efficiency to maintain their presence in less favourable sites. As a kind of detrimental radiation, UV-B has been regarded to have an impact on plant growth and physiochemical process. Additionally, it is generally believed that the strategy selected by clonal plants is to finally reinforce the holistic fitness, Since all the UV-B absorbing compounds, soluble sugar and protein, and anti-oxidant enzymes are of defensive/protective in stress resistance, this kind of physiological integration is more veracious to be regarded as defensive integration instead of resources integration. The results could be helpful for further understanding of the function of heterogeneous UV-B radiation on growth regulation and microevolution in clonal plants.