The root:shoot relationship changes with plant size so allometric analyis should be used. 13C discrimination is a good integrated measure of water use efficiency, the relationship between 13C dsicrimination and growth and/or gas exchange measurments are needed to see if variation in 13C discrimination is due to variation in photosythesis or conductance. Osmotic potential and turgor potential can be estimated using pressure;volume curves. And of course growth rate following a series of imposed drought is clearly important.

Almost impossible to answer for (genetic) adaptation to drought depends on further environmental conditions (e.g. hot temperature (summer) drought, cold temperature (=frost) drought). Besides anatomical feature (e.g. vessel size, reduced numbers of stomata, position of stomata - in cavities) there are lots of morphological adaptations (reduced leaf sizes, canopy shapes (e.g.) for shading the ground to reduce ground evaporation, or slim canopies to reduce light absorption etc. etc.). Or seasonal adaptations, like drought deciduousness (bosque seco in N-Peru !!!) or winter deciduousness (e.g. in Larix to avoid frost dessication) etc... They all have a genetic background....

It does seem that too many trajectories for adaptation exist to find strong bivariate relationships between a morphological trait and drought survivability. We can relate a morphological trait to a particular gene or gene expression, however. And that gene may play a role in drought survivability. Hopefully we can begin to unlock the hidden insights gene by gene.

You might want to contact Neil Pederson at Columbia University for insight into this question.

what about the genes that are involved in the formation of secondary wall pits..