The roles of different plasma membrane aquaporins (PIPs) in leaf-level gas exchange of Arabidopsis thaliana were examined using knockout mutants. Since multiple Arabidopsis PIPs are implicated in CO₂ transport across cell membranes, we focused on identifying the effects of the knockout mutations on photosynthesis, and whether they are mediated through the control of stomatal conductance of water vapour (g_s), mesophyll conductance of CO₂ (g_m), or both. We grew Arabidopsis plants in low and high humidity environments and found that the contribution of PIPs to g_s was larger under low air humidity when the evaporative demand was high, whereas any effect of a lack of PIP function was minimal under higher humidity. The pip2;4 knockout mutant had 44% higher g_s than wild-type plants under low humidity, which in turn resulted in an increased net photosynthetic rate (A_net). We also observed a 23% increase in whole-plant transpiration (E) for this knockout mutant. The lack of functional plasma membrane aquaporin AtPIP2;5 did not affect g_s or E, but resulted in homeostasis of g_m despite changes in humidity, indicating a possible role in regulating CO₂ membrane permeability. CO₂ transport measurements in yeast expressing AtPIP2;5 confirmed that this aquaporin is indeed permeable to CO₂.

A population of eight open pollinated families of Pinus contorta was selected from sites varying in precipitation regimes and elevation to examine the possible role of aquaporins in adaptation to different moisture conditions. Five Pinus contorta aquaporins encoding PiconPIP2;1, PiconPIP2;2, PiconPIP2;3, PiconPIP1;2, and PiconTIP1;1 were cloned and detailed structural analyses were conducted to provide essential information that can explain their biological and molecular function. All five PiconAQPs contained hydrophilic aromatic/arginine selective filters to facilitate the transport of water. Transcript abundance patterns of PiconAQPs varied significantly across the P. contorta families under varying soil moisture conditions. The transcript abundance of five PiconPIPs remained unchanged under control and water-stress conditions in two families that originated from the sites with lower precipitation levels. These two families also displayed a different adaptive strategy of photosynthesis to cope with drought stress, which was manifested by reduced sensitivity in photosynthesis (maintaining the same rate) while exhibiting a reduction in stomatal conductance. In general, root:shoot ratios were not affected by drought stress, but some variation was observed between families. The results showed variability in drought coping mechanisms, including the expression of aquaporin genes and plant biomass allocation among eight families of Pinus contorta.