Rapid determination of comparative drought tolerance traits: Using an osmometer to predict turgor loss point

Background/Question/Methods

1. Across plant species, drought tolerance and distributions with respect to water availability are strongly correlated with physiological traits, the leaf water potential at wilting, i.e, turgor loss point (π_tlp), and the cell solute potential at full hydration, i.e., osmotic potential (π_o). We present methods to determine these parameters 30 times more rapidly than the standard pressure-volume (p-v) curve approach, making feasible community-scale studies of plant drought tolerance.

2. We optimized existing methods for measurements of π_o using vapor-pressure osmometry of freeze-thawed leaf discs from 30 species growing in two precipitation regimes, and developed the first regression relationships to accurately estimate pressure-volume curve values of both π_o and π_tlp from osmometer values_. 

Results/Conclusions

3. The π_o determined with the osmometer (π_osm) was an excellent predictor of the π_o determined from the p-v curve (π_pv, r²= 0.80). While the correlation of π_osm and π_pv enabled prediction, the relationship departed from the 1:1 line. The discrepancy between the methods could be quantitatively accounted for by known sources of error in osmometer measurements, i.e., dilution by the apoplastic water, and solute dissolution from destroyed cell walls. An even stronger prediction of π_pv could be made using π_osm, leaf density (r), and their interaction (r²= 0.85, all p < 2 × 10^-10).

4. The π_osm could also be used to predict π_tlp (r²= 0.86).  Indeed, π_osm was a better predictor of π_tlp than leaf mass per unit area (LMA; r²= 0.54), leaf thickness (T; r²= 0.12), r (r²= 0.63), and leaf dry matter content (LDMC; r²= 0.60), which have been previously proposed as drought tolerance indicators. Models combining π_osm with LMA, T, r, or LDMC or other p-v curve parameters (i.e., elasticity and apoplastic fraction) did not significantly improve prediction of π_tlp.

5. This osmometer method enables accurate measurements of drought tolerance traits across a wide range of leaf types and for plants with diverse habitat preferences, with a fraction of effort of previous methods. We expect it to have wide application for predicting species responses to climate variability, and to assess ecological and evolutionary variation in drought tolerance in natural populations and agricultural cultivars.