If plants have less light, they photosynthesize more slowly. The speed of photosynthesis increases with greater light intensity, though it ultimately levels off once the plant has as much light as it needs. Carbon dioxide concentration is another factor that affects photosynthesis. In some cases, poor environmental conditions damage a plant directly. In other cases, environmental stress weakens a plant and makes it more susceptible to disease or insect attack. Environmental factors that affect plant growth include light, temperature, water, humidity and nutrition.