The meeting of stomata, trichomes and xylem: Are leaf and wood anatomy coordinated to control water loss and transport in olive?

BACKGROUND AND AIMS: Photosynthetic tissues of terrestrial plants require a continuous water supply to avoid desiccation while allowing CO2 to diffuse to chloroplasts. This imposes a functional coordination between water-conducting tissues and stomatal regulation. What is the anatomical basis of this coordination? METHODS: We assessed anatomical traits of stomata, trichomes, and xylem vessels as well as branch allocation patterns in 30 olive varieties grown under common environmental conditions in Southern France. Theoretical conductances of leaves and stems were calculated and compared to maximal leaf conductance measured in the field. We also tested whether wood density could serve as a proxy for stem conductance. RESULTS: Stomata, trichomes, and xylem vessels showed positive covariation in size, but not in density. Consequently, no coordination was observed between the total stem vessel area and the total stomatal pore area, nor between the theoretical conductances of stems and leaves. However, we found a positive association between the stomatal pore area and the lumen area when values were scaled at the branch level. Maximal leaf conductance was negatively related to trichome area, but not to theoretical stomatal conductance. Wood density correlated negatively with the lumen fraction in stems, but not with stem conductance. CONCLUSIONS: We conclude that coordination between water transport and loss likely occurs at the branch level, involving allocation patterns beyond anatomy, and that the role of trichomes in water regulation deserves further study in pubescent-leaved species.