| Boron (B) is the mineral element, it plays the important roles in the growth and development of high plants,In intact tissues of higher plants, this element occurs as both water soluble and water insoluble forms. The majority of the water soluble B seems to be localized in the apoplastic region as boric acid, the insoluble B is an important component in cell wall. Most of cell wall B is associated with RG-II (rhamnogalacturonan II). Analysis of the structure of the B-RG-II complex reveals that the complex is composed of boric acid and two chains of monomeric RG-II region through borate-diester bonding, thus forming a network of pectic polysaccharides in cell wall. These indicate that B deficiency can directly interfere in the cell expansion and division by influencing the structure of cell wall, further indirectly leads to various metabolic disorders and profoundly affects plant growth and development. We studied the effects of boron deficiency on the growth of cherry radish by using solution culture. We used RG-II antibody against RG-II to immunize the ultrathin sections of fleshy roots and observed the sections under light microscope and transmission electron microscopy. Microstructure was observed under light microscope by using toluidine blue-0(TBO) staining and periodic acid-Schiff reaction (Periodic acid Schiff reaction, PAS). Ultrastructure was observed under electron microscope. Boron concentration of the extract from leaves and fleshy roots was determined using a HPLC meters, the content of KDO and other relative index were determined by using different methods. The aim is to study the mechanism about boron deficiency lead to the hollow symptoms of fleshy root and inhibit the growth of cherry radish.The main results as follow:1. The shoot length of the B-deficient plants reduced significantly, The growth rate of cherry radish decreased from73.9%to39.6%, The sizes of mew leaves were very smaller because of B deficiency, but also their lamina unevenly expanded, so the apices of leaves were bent to hook-like structures, and the edges of new leaves curled. The symptoms of dry and loosen of fleshy roots was significant compared with B-sufficient plants.2. Boron deficiency caused the water insoluble boron content decreased very significant, but there were no significant difference on KDO. It seemed that there were not significant difference of immunofluorescence and colloidal gold particles between B-deficient plants and control, Immunocytochemical studies demonstrated perhaps B deficiency did not reduce the synthesis of RG-II of cherry radish, the B-deficiency inhibited the formation of B-RG-II, led to the increase of single RG-II, and then affected the structure of cell wall, so the cell shape and turgor could not be maintained, thus elongation of root was inhibited, cell morphology built and conducting tissue differentiation were affected. At the result, decreased ability of water absorb, water transport and water hold led to the hollow symptoms of fleshy root though it growed in the solution.3. As seen from the images of cross sections of young leaves under light microscopes and scanning microscopes, the leaf lamina thickened by35.7%, the cells of leaf widened by57.5%. the number of spongy parenchyma and palisade cells became bigger, but the intercellular spaces became smaller than normal, membrane of chloroplast thylakoids degraded, which inhibited photoreaction and carbon reaction of photosynthesis. Boron deficiency increased the sugar content compared to the leaves grown in the solution containing boron, which negative feedback inhibited photosynthesis.4. The cell walls of guard cells of plants grown in B-free solution became thicker compared to normal growth condition as other mesophyll cells did, Which affected the opening and closing of stomata, inhibited the gas exchange. At the result the stomatal conductance and transpiration rate decreased significantly, photosynthetic capacity decreased significantly.5. Boron-deficient leaves showed decreased protein content, SOD activity and POD activity, but the MDA content and the relative electrical conductivity of leaves increased compared to the leaves grown in the solution containing boron, which showed that boron deficiency increased the cell membrane permeability, reduced the activity of membrane protective enzymes, so had a bad effect on the antioxidant system of cherry radish.6. We found that the content of Ca, Mn, Mg element of fleshy roots increased compared to the control, and content of Fe?Cu?Zn?Mn element of leaves also increased significantly, but B content decreased. The pH of leaves decreased from6.18to5.75.The results showed that boron deficiency changed the stability of cell wall, and change the structure and ingredients of leaves, which made the pH of leaves decreased, induced the absorption of other divalent elements of Fe?Cu?Zn?Mn. These elements played important role in different metabolism for their different characteristics, so boron deficiency destroyed the balance of metabolic balance of mineral elements, led to the disordered metabolic and did harm to cherry radish.Therefore, we suggested that the reason of the harm of boron deficiency as follows: Pectin were not stable because there were not enough for RG-? to cross link with, which led to the unstable of cell walls. The mechanical change could be brought about by insufficient crosslinking of pectic polysaccharides at RG-? regions, as the B-RG-II complex significantly contributes to the wall tensile strength. If the cell wall becomes weaker and less resistant to turgor, then it may be difficult to maintain the shape of plant cells, so that the regulation on the volume of the guard cells may not function. Therefore, stomata of the plants exposed to B deficiency showed aberrant morphology with thicker guard cell walls compared with the control condition, indicating that the irregular guard cell walls severely affected the rhythms of stomatal closing and opening so that stomata could not open normally. Exchange of gas and water uptake, transport or loss were affected in cherry radish, and then affected the photosynthesis of plants, destroyed the balance of metabolic balance of mineral elements and destroyed the balance of antioxidant systems, ultimately affect the growth of cherry radishes. |
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