In plants gaseous exchange is possible via stomata and lenticels. In addition to these a small amount of gases can be exchanged via cuticle. There is no
special system within plants for the transport of O2 and CO2
.These gases move entirely
by diffusion.
Mechanism of opening and closing of stomata
1. Water flows into guard cells by osmosis, increasing their turgor and causing them to expand and draw away from each other, opening the pore
2. Guard cells lose water, decreasing their turgor and causing their inner walls to become straighter, closing the pore
K+ influx hypothesis
Explains the mechanism of stomatal opening and closing
Stomatal opening
Guard cells actively accumulate K+ from neighbouring epidermal cells, lowering their water potential and leading to inflow of water by osmosis, increasing turgor and opening stomata
Stomatal opening
Requires energy from transfer of electrons during photosynthesis in guard cell chloroplasts
Stomatal closing
Loss of K+ from guard cells to neighbouring epidermal cells, leading to exosmosis of water from guard cells, decreasing turgor and closing stomata
Abscisic acid (ABA)
Plays a role in K+ influx mechanism
Role of ABA in stomatal closure in drought
1. ABA is produced in roots and leaves in response to water deficiency
2. Production of ABA leads to close the stomata by removal of K+ in guard cells
3. This prevents the wilting of the plant
Factors affecting stomatal action
Stomata open during day and mostly closed at night (light stimulates K+ accumulation in guard cells)
Decrease in CO2 concentration in substomatal cavity leads to open stomata
Internal clock in guard cells controlling daily rhythm of opening and closing
Environmental stresses like drought, high temperature and wind can cause stomata to close during the day
