How does an increase in CO2 concentration affect plant growth?

An increase in CO2 concentration primarily enhances photosynthesis rates in plants. Carbon dioxide is a critical component of the photosynthetic process, where plants convert CO2 and sunlight into glucose and oxygen. The more carbon dioxide available to plants, the more they can increase their rate of photosynthesis, leading to enhanced growth and productivity. This response is particularly beneficial in environments where other factors, such as nutrients and water, are sufficiently available to support plant growth.

The other options focus on aspects that either do not directly relate to the immediate effects of increased CO2 or respond negatively to its increase. For instance, while some may think that increased CO2 might inhibit nutrient uptake or reduce humidity levels, these effects are not directly correlated with photosynthesis and plant growth in the same positive manner. Thus, the correct answer reflects the beneficial role of CO2 in enhancing the photosynthetic process, thereby promoting better growth outcomes for plants.