Plant growth stimulants containing compounds which increase intracellular carbon dioxide as the main active component and optionally supplemented with specific nutrients have been found to enhance productivity of plants. Exemplary compounds include lower alcohols, such as methanol, ethanol, propanol and butanol, and amino acids, such as glycine, glutamate, and aspartate. Such growth stimulation appears to result, in part, from inhibition of stress induced photorespiration according to a previously unrecognized photosynthetic pathway. Exposure of the plants to sunlight or other sufficient illumination following treatment with the compositions results in enhanced fixation of carbon dioxide and turgor. Stimulant compositions may comprise from 5% to 50% by volume aqueous solutions of methanol or 0.1% to 10% by weight amino acids and are preferably applied by foliar spraying of plants and plant shoots.