Epsilon Open Archive

Abstract

Leaves transpire large amounts of water through pores, stomata on their surfaces, which are held open to allow CO2 entry for photosynthesis. This loss must be balanced by water uptake in roots and further transport up the hydraulic pathway. In this thesis, water relations of willows (Salix spp.) were studied. Willows are used, increasingly, for biomass production on arable land where water supply limitation is more pronounced than in their native environments. Hopefully, by identifying physiological traits critical for efficient water use and high drought resistance, and by evaluating how these traits interact with traits of rapid growth, knowledge is gained that can assist breeders in improving willows for commercial use. Clones of contrasting ecological and geographical origins were characterised in different watering regimes. This included analysis of hydraulic conductances of roots and shoots, stomatal conductance, photosynthesis, water use efficiency of growth (WUE), resistance to drought-induced leaf wilting and xylem cavitation. The lattermost trait was assessed in relation to xylem anatomy and ultrasonic acoustic emissions arising from cavitations. The drought resistance of clones varied. It was positively related to the resistance of the xylem to cavitation, negatively related to maximal stomatal conductance and positively related to early stomatal closure, and negatively related to maximal growth rate. The resistance to xylem cavitation was positively related to wood density. The ability to acclimate to persistent, moderate drought also varied across clones. Generally, clones showed increases in WUE, whole-plant hydraulic conductance, root to shoot hydraulic conductance ratio, but decreases in stomatal conductance, osmotic potential, leaf area to leaf weight ratio, and leaf area to vessel internal cross-sectional area ratio. Drought acclimation with respect to resistance to xylem cavitation was only observed in one clone. In conclusion, water use and growth traits varied across clones and were in many cases inter-linked by trade-offs. Several of the critical traits could be assessed using rapid screening techniques.