Growth chamber studies were conducted to investigate the effects of silicon (Si) on the distribution of Cd in rice seedlings (Oryza sativa L., cv. Qiu Guang) grown hydroponically under toxic level of cadmium (Cd). Si added significantly alleviated the toxicity of Cd in aerobic rice seedlings. Si partly overcame the reduction in growth due to Cd. This amelioration was correlated with a reduction in Cd uptake. Si increased Cd accumulation in the roots and restricted the transport of Cd from roots to shoots, where the distribution of Cd in the shoots decreased by 33%. Si reduced the transport of Cd and the apoplastic fluorescence tracer PTS (tri-sodium-8-hydroxy-1, 3, 6-pyrenesulphonate) from roots to shoots by 23 and 36%, respectively. Energy-dispersive X-ray analysis (EDX) showed Cd was mainly deposited in the vicinity of the endodermis and epidermis, Si deposition was heavier in the vicinity of the endodermis than in the epidermis. Although the tracing result of fluorescein isothiocyanate-dextrans showed Si did not change epidermal wall porosity, the significant reduction of apoplastic PTS transport in +Si plants suggested that the heavy deposition of silica in the vicinity of endodermis might offer possible mechanisms by which silicon did at least partially physically block the apoplast bypass flow across the roots, and restrained the apoplastic transport of Cd. In addition, the effect of Si on the subcellular distribution and chemical form of Cd was investigated by fractionation. Si decreased the concentrations of Cd in shoots and roots, but did not remarkably change the distribution ratio of Cd in symplasm and apoplast. Mechanisms by which Si alleviates the toxicity of Cd in rice seedlings are discussed.

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[1] Temperature profiles derived from Global Positioning System (GPS) Radio Occultation (RO) data from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission are compared with those from four types of radiosonde systems from 12 to 25 km to assess the performance of these radiosonde systems in the upper troposphere and lower stratosphere. Results show that temperature measurements from the Vaisala-RS92 and Shanghai radiosonde systems agree well with those of COSMIC with a close-to-zero mean difference. Large temperature biases are shown for the MRZ and VIZ-B2 radiosonde systems relative to COSMIC, which are probably caused by diurnal radiative effects. In addition, we show that the temperature measurements from a new Chinese radiosonde system are improved compared to those of an older system through a comparison with COSMIC measurements.

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