COS 241-1 - Local adaptation in leaf water relations of a dominant aridland phreatophyte: Populus fremontii

Background/Question/Methods

Plant traits and water status can vary within a species, and seasonally within a plant. Leaf turgor loss point (p_tlp) is recognized as one of the most reliable traits for evaluating drought tolerance. Fremont cottonwood (Populus fremontii) is a dominant riparian tree species in the southwestern United States that relies on shallow groundwater for growth and survival. Although P. fremontii is generally drought intolerant, this species occurs across a vast climate gradient including where temperatures approach 50 °C. Previous results indicate that exposure to intense heat has resulted in the selection of warm-adapted P. fremontii for high midday canopy transpiration rates to enhance leaf cooling relative to cool-adapted genotypes. Given these patterns, we tested the hypotheses: 1) warm-adapted P. fremontii displays lower mean p<sub>tlp than cool-adapted genotypes, and 2) p<sub>tlp is correlated with higher midday stomatal conductance (g_s), lower midday leaf water potentials (Y_md) and higher leaf dry matter content (LDMC). The hypotheses were tested using genotypes planted in a common garden established in 2014 near Yuma, Arizona. Ten genotypes sourced from eight locations spanning an elevation gradient from 70 m to 1230 m were selected for the study. We measured leaf traits in spring and mid-summer in 2021.

Results/Conclusions

Mean p<sub>tlp varied from -2.45 MPa to -2.66 MPa in spring, and from -2.51 to -2.70 MPa in summer. However, contrary to hypothesis 1, there was no relation between p<sub>tlp and elevation of source population in either spring or summer. Across all populations, p<sub>tlp was marginally lower in summer than in spring (P=0.07). Likewise, from May to August, g_s increased 85% (P< 0.0001) and LDMC increased slightly by 6% (P< 0.0001). Unlike p<sub>tlp, seasonal changes in g_s was strongly negatively correlated with source population as a result of warm-adapted populations increasing g_s by over 150% from spring to the much warmer summer period. Mean p<sub>tlp was moderately negatively correlated with LDMC in May (R²=0.18, P< 0.0001), supporting our second hypothesis, but no relationship was detected between p<sub>tlp and other measured traits, including g_s and Y_md. LDMC was negatively correlated with g_s in spring (R²=0.86, P< 0.0001), but not by mid-summer. Results indicate that warm-adapted plants prioritize leaf cooling via increased mid-summer stomatal conductance, but leaf water use patterns did not appear to correspond with leaf hydraulic function in P. fremontii genotypes sourced across a broad temperature gradient.