Growth characteristics of three Eucalyptus clonal hybrids in response to drought stress : the underlying physiology.
Previous research by Drew et al. (2009) on a SAPPI dendrometer trial, in northern KwaZulu-Natal, South Africa, yielded growth results for two Eucalyptus grandis clones (termed E. grandis x urophylla (GU) and E. grandis x camaldulensis (GC)). The GU clone was found to have a greater diameter than the GC clone, and further research has demonstrated that age, not environmental conditions, is the major determinant of tree growth for the GU and GC clone. The clones also showed different patterns of growth response following a rainfall event. In current study, young plants of three Eucalyptus grandis clones (two GU clones (GUA and GUW) and one GC clone) were grown in 80 L planting bags for 18 months at UKZN, Westville, South Africa. The experiments for this study entailed subjecting the three clones to three watering regimes. The trial was conducted using a randomised complete block (RCB) with a 33 factorial design (i.e. 9 treatments with 12 replicate plants in each treatment). The three watering regimes were monitored daily with a soil moisture probe and were a control (little or no water stress was applied), chronic water stress (mild, long-term, gradual water stress) and acute water stress (rapid, severe, cyclic water stress with periods of recovery from stress by re-watering). Physiological (photosynthesis, plant water relations and hydraulic conductance characteristics) and morphological (height, diameter and total biomass) measurements were performed. Two harvest periods determining Kh and total biomass at 9 and 18 months were undertaken, whereas morphological measurements were taken monthly throughout the trial. Considering that there were differing growth responses of clones in response to rainfall events (observed by Drew et al., 2009), the recovery of the plants from water stress was also studied (resistance to water flow in leaves, assimilation rates and stomatal conductance). Further investigation of leaf characteristics was performed to assess different aspects of the water transport system (stomatal density) and improvement of water use efficiency (WUE) in response to water stress by measurement of δ13C in leaf samples. The GC clone showed 30% greater height growth than the GU clones. Growth efficiency, root biomass and root:shoot were significantly greater in the GC clone. The GU clones showed significantly greater stem and leaf biomass, primarily due to the 25% greater total leaf area, after 18 months growth. Diameter of the plants subjected to the control, was 8% higher compared with water stress treatments (p = 0.036). Water stress significantly reduced tree volume by up to 10% and leaf area by 30%. Jmax and Vcmax were significantly lowered in plants subjected to acute stress at leaf wilting point (p < 0.001). After as little as 7 days re-watering however, Jmax and Vcmax were not different from the control. Plants subjected to chronic water stress showed moderately improved instantaneous WUE (8% increase compared with the control and acute stress). Long-term WUE (by measurement of δ13C in leaves, was significantly higher in leaves subjected to chronic water stress (p < 0.0001). Stomatal density was significantly different among clones, as the GUA clone showed complete stomatal absence on all upper leaf surfaces sampled (p < 0.001), although stomatal absence did not occur in leaves of the closely related GUW clone. Assimilation rate, stomatal conductance, Kh and total biomass were significantly positively correlated with one another. Recovery of plants subjected to acute stress differed among the GU and GC clones. An, gs and Rleaf (resistance to water flow in leaves) “recovered” (i.e. not significantly different from the control) by day 2 in the GC clone, but only by day 7 in the GU clones. There was hydraulic dysfunction in the GC clone which was suggested to be caused by collapse of the minor veins due to drought stress. The hydraulic dysfunction did not affect mesophyll tissue of the GC clone and thus hydraulic recovery was rapid. Although the GC clone was more drought tolerant (due to significantly greater root biomass), the selection of a GU clone would ensure improved wood productivity when planted commercially. The GUW clone showed enhanced traits of drought tolerance than the GUA clone including 20% less leaf dieback in response to water stress, as well as little to no variability of Kh in response to all watering regimes, and moderately improved WUE. Plants subjected to chronic stress showed long-term and instantaneous improvement in WUE, and greater diameters were maintained than plants subjected to acute stress. Perhaps the most important morphological and physiological parameter identified in the current study was that of leaf area. Leaf area differed significantly among eucalypt clones, in response to water stress and with tree age. Leaf area affected the expression of growth efficiency, hydraulic efficiency, total carbon assimilated and total biomass achieved. For the GU and GC Eucalyptus clones in the current study, the primary parameter driving physiological interactions and ultimately determining wood productivity could be considered to be leaf area.