The effect of silicon (Si) nutrition on lowlevel cadmium (Cd) toxicity symptoms was investigated in hydroponically-grown rice seedlings (Oryza sativa L.). Silicon (0.0, 0.2, or 0.6 mM) was added when seedlings were 6 or 20 days old representing early (Si E ) or late (Si L ) Si treatment, respectively. Cadmium (0.0 or 2.5 μM) was added when seedlings were 6 days old. Measurements included generation of CO 2 and light response curves; chlorophyll fluorescence analysis; growth; and tissue-element content analysis. Our results showed that low-level Cd treatment generally inhibited growth and photosynthesis. However, the addition of 0.2 or 0.6 mM Si E or Si L significantly reduced root-and leaf-Cd content. Consequently, the addition of 0.6 mM Si L significantly alleviated lowlevel Cd-induced inhibition of growth. Furthermore, 0.2 mM Si treatment significantly reduced g s compared to 0.0 or 0.6 mM Si without inhibiting A, especially in +Cd plants, suggesting an increase in instantaneous water-use-efficiency (IWUE). Additionally, in +Cd plants, the addition of 0.6 mM Si E significantly reduced F o but increased F v /F m , while treatment with 0.2 mM Si L significantly increased q P , suggesting an increase in light-use-efficiency. We thus, propose that 0.6 mM Si L treatment is required for the alleviation of low-level Cd-mediated growth inhibition. Furthermore, we suggest that 0.2 mM Si concentration might be close to the optimum requirement for maximum Siinduced increase in IWUE in rice plants, especially when under low-level Cd-stress. Our results also suggest that Si alleviates low-level Cd toxicity by improving light-use-efficiency. Keywords Chlorophyll fluorescence . Instantaneous water-use-efficiency . Low-level cadmium . Silicon . Stomatal conductance Abbreviations A net CO 2 assimilation rate A max maximum net CO 2 assimilation rate C a ambient CO 2 concentration C E carboxylation efficiency C i intercellular CO 2 concentration E transpiration rate F m maximum chlorophyll fluorescence yield in a dark-adapted state F o minimum chlorophyll fluorescence yield in a dark-adapted state F o /F m basal quantum yield of non-photochemical processes in PS2 in a dark-adapted state F v maximum variable fluorescence yield in a dark-adapted state F v /F m quantum efficiency of open PS2 centers in a dark-adapted state Plant Soil (2008) 311:73-86

The best known silicon (Si)-accumulating plant, rice (Oryza sativa L.), stores most of its Si in leaves, but the importance of Si has been limited to a mechanical role. Our initial studies showed that Si-induced cadmium (Cd) tolerance is mediated by the enhancement of instantaneous water-use-efficiency, carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO), and light-use-efficiency in leaves of rice plants. In this study, we investigated changes in the rice leaf proteome in order to identify molecular mechanisms involved in Si-induced Cd tolerance. Our study identified 60 protein spots that were differentially regulated due to Cd and/or Si treatments. Among them, 50 were significantly regulated by Si, including proteins associated with photosynthesis, redox homeostasis, regulation/protein synthesis, pathogen response and chaperone activity. Interestingly, we observed a Si-induced up-regulation of a class III peroxidase and a thaumatin-like protein irrespective of Cd treatment, in addition to a Cd-induced up-regulation of protein disulfide isomerase, a HSP70 homologue, a NADH-ubiquinone oxidoreductase, and a putative phosphogluconate dehydrogenase, especially in the presence of Si. Taken together, our study sheds light on molecular mechanisms involved in Si-induced Cd tolerance in rice leaves and suggests a more active involvement of Si in plant physiological processes than previously proposed.

Silicon (Si)‐induced cadmium (Cd) tolerance in rice (Oryza sativa L.) was investigated by analyzing Cd uptake, growth, and physiological parameters. Silicon treatments (0.0, 0.2, or 0.6 mM) were added to 6 d–old seedlings, and Cd treatments (0.0 or 5.0 μM) were added to 20 d–old seedlings. Parameters determined included: maximum net CO2 assimilation (Amax), stomatal conductance (gsmax), and transpiration (Emax) rates at varying intercellular CO2 concentrations (Ci). Also measured were chlorophyll fluorescence, growth, and Cd‐uptake parameters. Results showed a Si‐induced inhibition of Cd uptake. However, 0.2 mM or 0.6 mM Si treatment concentrations did not differentially inhibit Cd uptake or differentially alleviate Cd‐induced growth inhibition, despite a significant increase in tissue Si concentration due to 0.6 mM Si treatment compared to 0.2 mM Si treatment. Additionally, photosynthesis and chlorophyll‐fluorescence analysis showed that treatment with Cd significantly inhibited photosynthetic efficiency. Interestingly, the addition of 0.2 mM Si, more so than the addition of 0.6 mM Si, significantly alleviated the inhibitory effects of Cd toxicity on photosynthesis and chlorophyll‐fluorescence parameters. Our results suggest that 0.2 mM Si could be close to an optimum Si‐dose requirement for the alleviation of toxicity symptoms mediated by moderate (5 μM) Cd exposure.

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In response to water stress, Portulacaria afra (L.) Jacq. (Portulacaceae) shifts its photosynthetic carbon metabolism from the Calvin-Benson cycle for CO2 fixation (C3) photosynthesis or Crassulacean acid metabolism (CAM)-cycling, during which organic acids fluctuate with a C3-type of gas exchange, to CAM. During the CAM induction, various attributes of CAM appear, such as stomatal closure during the day, increase in diurnal fluctuation of organic acids, and an increase in phosphoenolpyruvate carboxylase activity. It was hypothesized that stomatal closure due to water stress may induce changes in internal CO2 concentration and that these changes in CO2 could be a factor in CAM induction. Experiments were conducted to test this hypothesis. Wellwatered plants and plants from which water was withheld starting at the beginning of the experiment were subjected to low (40 ppm), normal (ca. 330 ppm), and high (950 ppm) CO2 during the day with normal concentrations of CO2 during the night for 16 days. In water-stressed and in well-watered plants, CAM induction as ascertained by fluctuation of total titratable acidity, fluctuation of malic acid, stomatal conductance, CO2 uptake, and phosphoenolpyruvate carboxylase activity, remained unaffected by low, normal, or high CO2 treatments. In well-watered plants, however, both low and high ambient concentrations of CO2 tended to reduce organic acid concentrations, low concentrations of CO2 reducing the organic acids more than high CO2. It was concluded that exposing the plants to the CO2 concentrations mentioned had no effect on inducing or reducing the induction of CAM and that the effect of water stress on CAM induction is probably mediated by its effects on biochemical components of leaf metabolism.Various species of succulent plants have been shown to shift their photosynthetic carbon metabolism from C32 to CAM due to water stress (8,12,16,23). This shift, which was initially studied as a response to salinity and, hence, thought to be a direct salt effect (23), was later shown to occur also as a response to water stress (14,16,17,19,22 MATERIALS AND METHODSPlant Material. Portulacaria afra (L.) Jacq. (Portulacaceae) plants were propagated by cuttings from a parent plant, rooted in vermiculite, and replanted in 1-gal pots in a standard peat loam soil mix in a greenhouse. Plants were watered 3 times per week or as needed to maintain the soil moist to touch and were fertilized twice a month with half-strength Hoagland solution (10). Greenhouse temperature was kept at an average of 18°C during the night and 29°C during the day. All leaves used were fully expanded and mature. All plants were healthy, mediumsized plants when used (ca. 0.50 m tall).Exposure of Plants to Various Levels of CO2 and Water Stress. Plants were exposed to the following conditions for a period of 16 d: (a) well-watered, ambient C02; (b) well-watered, 40 ppm CO2 during the day; (c) well-watered, 950 ppm CO2 during the day; (d) water-stressed, normal C02; (e) water-stressed, 40 ppm CO2 during the d...