Question

Using Fick's law and Darcy's law explain the biophysical links between photosynthesis, stomatal conductance and hydraulic conductance, Specifically: . Why is it necessary to increase hydraulic conductance in order to achieve greater maximum stomatal conductance? . Explain how xylem embolisms impact maximum potential stomatal conductance and transpiration? . Explain how decreases in soil moisture contribute to reductions in maximum potential photosynthesis.

Answer 1

• According to Darcy's law, hydraulic conductance is directly proportional to fluid velocity. So, leaf hydraulic conductance involves movement of water molecules accross the stomata. Stomatal conductance is defined as rate of water exiting through stomata. This shows direct correlation between hydraulic conductance and stomatal conductance. Thus, it is necessary to increase hydraulic conductance to achieve maximum stomatal conductance.
• Xylem embolism is a condition in which water flow is interrupted due to presence of air cavity. This reduced the hydraulic conductance leading to decrease in pressure gradient in the leaf. If there is no pressure gradient then there is no water flow (Darcy's law). Thus, transpiration rate will slow down. Accordingly stomatal conductance will also reduce.
• Reduction in water level in soil will decrease pressure gradient because of which water flow will not occur (Darcy's law). This reduces hydraulic conductance and induces the closure of stomata. Decrease in the area results in decreased diffusion of carbon dioxide (Fick's law). Absence of carbon dioxide affects photosynthesis because no food formation will occur. Thus decrease in soil moisture reduces maximum potential photosynthesis.