| Abstract: | This study describes the stomatal response occurring during water stress and subsequent recovery of three Eucalyptus grandis clonal hybrids. The aim was to investigate the degree to which stomatal conductance (g s) and stomatal density differ between the clonal hybrids across seasons and in response to water stress. Plants from one E. grandis × E. camaldulensis (GC) and two E. grandis × E. urophylla (GU1 and GU2) clones were grown for 18 months in 80 l planting bags. Plants were subjected to three watering treatments: control (100% field capacity), chronic water stress (maintained at 15% of field capacity) and acute water stress (cyclic water stress, where water was withheld until leaf wilting point, and a subsequent period of recovery followed). Stomatal conductance was measured after 6, 12 and 18 months growth. At 12 months of age, the recovery of g s 1, 2 and 7 d after rewatering (following acute water stress) was further investigated. The GC hybrid showed consistently higher g s than the GU clones at each measurement period. Stomatal conductance was 24–66% higher during winter (after 12 months growth) than during summer. The recovery of stomatal conductance from acute water stress was more rapid in the GC clone than the GU clones. Chronic water stress was shown to decrease g s in GU clones by up to 70%, but not in the GC clone. Water stress did not affect stomatal density or size. Remarkably, stomata were absent from the adaxial leaf surface of clone GU1 leaves, but not from the leaves of the other E. urophylla hybrid cross (GU2). Total biomass of the GC clone was significantly greater at 9 months growth, but after 18 months growth the GU1 clone had attained greater biomass accumulation (although not significantly). Measurement of g s, transpiration, stomatal density and total biomass in the GU1 clone indicated stomatal sensitivity to water stress, a favourable trait during periods of drought. The differing growth strategies of the GU and GC clones could be partially explained by their differences in stomatal sensitivity in response to water stress. |
