The stomatal conductance of photosynthetic apparatus is low.

What you want to ask is why the stomatal conductance of photosynthetic apparatus is low? The reason for this is the following：

1. Water limitation: The stomatal conductance is low because plants close their stomata when water is limited. Plants regulate the exchange of water and gas through stomata. When water supply is insufficient, plants will close their stomata to reduce water transpiration. In the case of drought and water shortage, the stomatal conductance of plants will decrease.

2. Temperature and light limitation: Stomatal conductance is also affected by environmental conditions. When the temperature is too high and the light is too strong, plants will actively reduce the stomatal opening to prevent excessive evaporation of water. In extreme high temperature and strong light environment, stomatal conductance of plants is low.