MrRojos
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The accumulation of silicon (Si) in the shoots varies considerably among plant species, but the mechanism responsible for this variation is poorly understood. The uptake system of Si was investigated in terms of the radial transport from the external solution to the root cortical cells and the release of Si from the cortical cells to the xylem in rice, cucumber, and tomato, which differ greatly in shoot Si concentration. Symplasmic solutions of the root tips were extracted by centrifugation. The concentrations of Si in the root-cell symplast in all species were higher than that in the external solution, although the concentration in rice was 3- and 5-fold higher than that in cucumber and tomato, respectively. A kinetic study showed that the radial transport of Si was mediated by a transporter with a Km value of 0.15 mM in all species, but with different Vmax values in the order of rice>cucumber>tomato. In the presence of the metabolic inhibitor 2,4-dinitrophenol, and at low temperature, the Si concentration in the root-cell symplast decreased to a level similar to that of the apoplasmic solution. These results suggest that both transporter-mediated transport and passive diffusion of Si are involved in the radial transport of Si and that the transporter-mediated transport is an energy-dependent process. The Si concentration of xylem sap in rice was 20- and 100-fold higher than that in cucumber and tomato, respectively. In contrast to rice, the Si concentration in the xylem sap was lower than that in the external solution in cucumber and tomato. A kinetic study showed that xylem loading of Si was also mediated by a kind of transporter in rice, but by passive diffusion in cucumber and tomato. These results indicate that a higher density of transporter for radial transport and the presence of a transporter for xylem loading are responsible for the high Si accumulation in rice.
Key wordsPassive diffusion radial transport silicontransporter uptake xylem loading
Introduction
Silicon (Si) is a beneficial element for plant growth. Silicon helps plants to overcome multiple stresses including biotic and abiotic stresses (for a recent review, see Ma, 2004). For example, Si plays an important role in increasing the resistance of plants to pathogens such as blast on rice (Datnoff et al., 1997) and powdery mildew on cucumber (Miyake and Takahashi, 1982a, b). Silicon is effective in preventing lodging in rice by increasing the thickness of the culm wall and the size of the vascular bundles (Shimoyama, 1958), thereby enhancing the strength of the stems. Silicon also alleviates the effects of other abiotic stresses including salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalance, high temperature, and freezing (Epstein, 1999;Ma and Tahakashi, 2002; Ma, 2004). These beneficial effects are mostly expressed through Si deposition in the leaves, stems, and hulls, although other mechanisms have also been proposed (Ma, 2004). Therefore, the Si effect is characterized by a larger effect associated with a greater Si accumulation in the shoots.
However, Si accumulation in the shoots varies considerably among plant species, ranging from 0.1% to 10% Si in the dry weight (Ma and Takahashi, 2002). Takahashi and coworkers made an extensive survey on the Si concentrations of nearly 500 plant species from Bryophyta to Angiospermae, grown under similar soil conditions (for the summary, see Ma and Takahashi, 2002). The results showed that there is a characteristic distribution of Si accumulation in the plant kingdom. In higher plants, only plants in Gramineae and Cyperacea show high Si accumulation. Plants in Cucurbitales, Urticales, and Commelinaceae show intermediate Si accumulation, whereas most other plants species show low Si accumulation. The difference in Si accumulation has been attributed to the ability of the roots to take up Si (Takahashi et al., 1990). Silicon is taken up in the form of an uncharged molecule, silicic acid (Takahashi and Hino, 1978). Three different modes of Si uptake have been proposed for plants having different degrees of Si accumulation, that is, active, passive, and rejective uptake (Takahashi et al., 1990). Plants with an active mode of uptake take up Si faster than water, resulting in a depletion of Si in the uptake solution. Plants with a passive mode of uptake take up Si at a rate that is similar to the uptake rate of water; thus, no significant changes in the concentration of Si in the uptake solution are observed. By contrast, plants with a rejective mode of uptake tend to exclude Si, which is demonstrated by the increasing concentration of Si in the uptake solution. However, the mechanisms involved in the different uptake modes are not understood. The objective of this study was to examine the uptake systems of Si in rice, cucumber, and tomato, which represent high, intermediate, and low Si accumulation, respectively.
In a nut shell,some plants uptake and use silica based products differently.
foliar spray of silica is ideal for the sake of it not being a mobile nutrient amongst other benefits as well.
Key wordsPassive diffusion radial transport silicontransporter uptake xylem loading
Introduction
Silicon (Si) is a beneficial element for plant growth. Silicon helps plants to overcome multiple stresses including biotic and abiotic stresses (for a recent review, see Ma, 2004). For example, Si plays an important role in increasing the resistance of plants to pathogens such as blast on rice (Datnoff et al., 1997) and powdery mildew on cucumber (Miyake and Takahashi, 1982a, b). Silicon is effective in preventing lodging in rice by increasing the thickness of the culm wall and the size of the vascular bundles (Shimoyama, 1958), thereby enhancing the strength of the stems. Silicon also alleviates the effects of other abiotic stresses including salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalance, high temperature, and freezing (Epstein, 1999;Ma and Tahakashi, 2002; Ma, 2004). These beneficial effects are mostly expressed through Si deposition in the leaves, stems, and hulls, although other mechanisms have also been proposed (Ma, 2004). Therefore, the Si effect is characterized by a larger effect associated with a greater Si accumulation in the shoots.
However, Si accumulation in the shoots varies considerably among plant species, ranging from 0.1% to 10% Si in the dry weight (Ma and Takahashi, 2002). Takahashi and coworkers made an extensive survey on the Si concentrations of nearly 500 plant species from Bryophyta to Angiospermae, grown under similar soil conditions (for the summary, see Ma and Takahashi, 2002). The results showed that there is a characteristic distribution of Si accumulation in the plant kingdom. In higher plants, only plants in Gramineae and Cyperacea show high Si accumulation. Plants in Cucurbitales, Urticales, and Commelinaceae show intermediate Si accumulation, whereas most other plants species show low Si accumulation. The difference in Si accumulation has been attributed to the ability of the roots to take up Si (Takahashi et al., 1990). Silicon is taken up in the form of an uncharged molecule, silicic acid (Takahashi and Hino, 1978). Three different modes of Si uptake have been proposed for plants having different degrees of Si accumulation, that is, active, passive, and rejective uptake (Takahashi et al., 1990). Plants with an active mode of uptake take up Si faster than water, resulting in a depletion of Si in the uptake solution. Plants with a passive mode of uptake take up Si at a rate that is similar to the uptake rate of water; thus, no significant changes in the concentration of Si in the uptake solution are observed. By contrast, plants with a rejective mode of uptake tend to exclude Si, which is demonstrated by the increasing concentration of Si in the uptake solution. However, the mechanisms involved in the different uptake modes are not understood. The objective of this study was to examine the uptake systems of Si in rice, cucumber, and tomato, which represent high, intermediate, and low Si accumulation, respectively.
In a nut shell,some plants uptake and use silica based products differently.
foliar spray of silica is ideal for the sake of it not being a mobile nutrient amongst other benefits as well.
