The correct answer is 1, 2, and 3. Key Points Due to increased CO2 concentration, the rate of photosynthesis will increase and the stomatal conductance will decrease (due to partial closure of stomata). Thus, the transpiration rate may be reduced and consequently, water-use efficiency will increase. This may allow many species to grow successfully in regions of water scarcity. Under higher atmospheric CO2 conditions, plants shall allocate a greater proportion of photosynthate to roots. Greater root production is expected to enhance mycorrhizal development and fixation in root nodules, thereby enabling the plants to grow in nutrient-poor soils. However, under natural conditions, the natural effects of increased CO2 may not be actually realized because of the negative effects of global warming. Hence, all the options are correct. Additional Information The response of plants to elevated concentrations of CO2 is known as the Carbon dioxide fertilization effect. With a doubling of the atmospheric CO2 concentration, the growth of many plants, particularly the C3 species, under favorable conditions of water, nutrients, light, and temperature, could increase by about 30 percent on average, in the short-term (i.e., up to a few years).