A Single-Cell-Squashing Technique for Observation of Organelle DNA Distribution in Tobacco Zygotes Effects of Four Chemical Regulators on Chilling Tolerance and Photosynthetic Characteristics of Tobacco Seedlings Effect of Different Light Qualities on Physiological Characteristics and Growth of Tobacco in vitro under Light Emitting Diodes(LEDs) Effects of mechanical wounding on nitrogen uptake and nicotine content in flue-cured tobacco plants