Sequoia sempervirens and Sequoiadendron giganteum seedlings show different leaf tissue water relations properties in response to soil drought and rewetting

Background/Question/Methods

Previous results from a soil dry-down and rewetting greenhouse experiment showed that young coast redwood (Sequoia sempervirens) and giant sequoia (Sequoiadendron giganteum) trees exhibited contrasting drought-response strategies that most closely matched the environmental conditions of their native habitats. Coast redwood had poor stomatal control, more negative shoot water potentials, greater sensitivity of growth, and higher vulnerability to xylem embolism than giant sequoia in response to soil water deficit.  However, drought-stressed seedlings of both species responded rapidly to rewetting and returned to levels similar to control plants for many of the physiological traits measured, including embolism. We extended the findings from the abovementioned experiment by comparing coast redwood and giant sequoia leaf tissue water relations properties. PV (pressure/volume) curves were measured on control (well watered) and drought-stressed (water withheld for 6 weeks) seedlings from northern and southern populations for both species, and then again 2 weeks after rewatering.

Results/Conclusions

On average, Sequoia showed more negative osmotic potentials at full turgor (Ψ_SFL) and at the turgor loss point (Ψ_TLP), and more rigid tissues (higher maximum bulk elastic modulus) than Sequoiadendron. Sequoia responded to drought by decreasing both Ψ_SFL and Ψ_TLP through a net increase of osmotically active substances, and in so doing it maintained Ψ_TLP values similar to values measured at the peak of the drought even upon rewatering. In contrast, Sequoiadendron did not adjust significantly any of the measured leaf tissue traits in response to the drought and rewatering treatments. Also, we did not find significant intra-specific differences. We conclude that osmoregulation played an important role in lowering the water potential of Sequoia leaf tissues under drought conditions, and likely played a role in facilitating xylem refilling in this otherwise drought-sensitive species. In contrast, the relatively high tissue elasticity might have helped in maintaining favorable tissue water status in Sequoiadendron.