The researchers confirmed there is a system in place that is working to limit the amount of water being lost from the leaf, but what’s causing it is not settled. They believe that water conduits, called aquaporins, located in the cell membranes are responsible. 

Researchers from The Australian National University and James Cook University have identified a “natural mechanism that helps plants limit their water loss with little effect on carbon dioxide (CO2) intake, which could mean more resilient crops that are capable of withstanding extreme weather events, including drought. The discovery is expected to help agricultural scientists and plant breeders develop more water-efficient crops.

Everyone knows about photosynthesis. Pores in the 'skin' of leaves draw in carbon dioxide, which is creating food for them and oxygen for us. But those same pores lose water, and plants require so much mater because for every unit of CO2 they bring in, plants typically lose 100X that in water. 

By examining humidity gradients in the air spaces of leaves via w_sat- w_a(Δw), they found plants could sustain the rate of CO2 assimilation when the relative humidity dropped to about 97% under mild Δw and as dry as around 80% when Δw was large.

That means plants could be genetically optimized to reduce water loss with little effect on CO2 uptake. That means less water needed by farmers and lower costs for people in more arid regions, and basically everywhere else also.