Some physiological and growth responses of three eucalyptus clones to soil water supply.

Abstract

The response of three Eucalyptus spp. clones (GC550, GU210 and TAG14) to water availability was assessed in terms of growth, plant water status, leaf gas exchange, whole plant hydraulic characteristics (both root and shoot), stem xylem vulnerability. Furthermore, to experimentally assess the influence of hydraulic conductance on leaf physiology and plant growth, specimens of two of the clones were subjected to long-term root chilling. Prior to harvesting data were collected on the diurnal variation in leaf water potential (ΨL), transpiration rate (E), stomatal conductance (gs) and net CO2 assimilation rate (A). Main stem xylem vulnerability was assessed using ultrasonic acoustic emissions (UAE). Vulnerability of the main stem was assessed as the leaf water potential corresponding to the maximum rate of acoustic emissions (ΨL, EPHmax), and as the critical water potential triggering cavitation events, calculated as the mean of the water potentials of data points lying between 5 and 10% of the total accumulated emissions (ΨCAV,cUAE,%). Hydraulic conductance was measured on roots and shoots using the high-pressure flow meter (HPFM). Root data were expressed per unit root dry mass (Kr/trdw) and per unit leaf areas (Kr/LA), shoot data expressed per unit shoot dry mass (Ks/tsdw) and per unit leaf area (Ks/LA), and whole plant conductance was expressed per unit leaf area (KP/LA). Soil-to-leaf hydraulic conductance was also assessed as the inverse of the slope of the relationship between leaf water potential and transpiration rate (the evaporative flux, EF, method). A field study was undertaken on three month old TAGl4 and GU210 plants. Diurnal values of leaf water potential ΨL, E and gs were consistently higher in TAG14 than GU210, but A did not differ among the clones. Main stem xylem vulnerability (ΨCAV, cUAE,%) was higher in TAG14 than GU210. In both clones midday ΨL fell below ΨcAv,cUAE,%, suggesting lack of stomatal control of xylem cavitation. Kr/LA was higher in TAGl4 than GU210, whereas, Ks/LA and Ks/tsdw was higher in GU210 than TAGI4. A greater proportion of hydraulic resistances resided in the roots, particularly in GU210. Kp/LA was higher in TAGl4 than GU210 clone, although the significance was marginal (P=0.089). However, all the physiological measurements, were consistent with the concept of higher hydraulic conductances in TAGl4 leading to lower leaf level water stress. Above ground biomass was higher in TAG14 than GU210, in agreement with this concept, although this clone was more vulnerable than GU210. Material grown for 14 months in 25 l pots clones showed no differences in ΨSoil between the high and low watering supply, indicating that even the 'high' supply was inadequate to prevent water stress. In accordance with this, diurnal values of ΨL, gs, E and A did not differ significantly between treatments and clones. Early stomatal closure was apparent, maintaining ΨL constant during the middle of the day. Stem xylem vulnerability, assessed as both ΨL,EPHrnax and ΨCAV,cUAE,% showed that the main stem of GC550 was more vulnerable than other two clones, and that low watered plants were more resistant to xylem cavitation than those receiving high water. Midday ΨL fell below the vulnerability values assessed by both measures across treatments and clones, suggesting lack of stomatal control preventing stem xylem cavitation. There was no relationship between stem xylem cavitation and the shoot hydraulic conductances. Root pressures did not differ between either treatment or clones. Kr/LA was marginally higher in high watered plants, and Ks/LA and Ks/tsdw were higher in low watered plants, possibly by adjustment of leaf hydraulic architecture, and there were no clonal differences. Kr/LA was much lower than Ks/LA. Kp/LA did not differ between the watering treatment, but there was a clonal effect. Growth in dry mass was higher in high watered than low watered plants, but there were no differences among clones. As KP/LA was not affected by watering treatment there was no relationship between KP/LA and growth in total biomass. In plants grown for 21 months in 85 l pots low water treatment decreased midday ΨL, gs, E and A relative to high watered plants. Interclonal differences occurred at midday. Stem xylem vulnerability assessed as ΨCAV,cUAE,% and as ΨL,EPHrnax show similar trends as in the 14 months saplings, clonal differences being significant in ΨL, EPHmax. There was a 1:1 relationship between minimum leaf water potential and ΨL, EPHmax, suggesting that the water potential developed was limited by stem vulnerability. This implies stomatal control to reduce transpiration rates to prevent extensive cavitation occurring. These plants did not develop positive root pressures, indicating that recovery from xylem cavitations occurred through some other process. Kr/LA was higher in high watered plants than those receiving low water, and clonal differences were observed in Kr/trdw. There was no treatment effect in KS/LA and KS/tsdw, but a clonal effect was apparent. KP/LA was significantly different between treatment, and was reduced by low water in two clones, and increased by this in TAGI4. Reduced water availability reduced biomass production, with a greater effect on roots than shoots, such that low watering reduced root:shoot ratios. There was a weak but significant relationship between whole plant hydraulic conductance and maximum stomatal conductance, and between plant conductance and total biomass produced; these data are consistent with the concept of some hydraulic limitation to growth. Root chilling (achieved through chilling the soil) of two of the clones was used to experimentally manipulate hydraulic conductance to test the hydraulic limitation hypothesis. Short-term root chilling decreased both Kr/LA and KP/LA in both clones, but had marginal effects on leaf gas exchange. With long-term chilling the decrease in Kr/LA was observed only in GU210, with TAGl4 showing some adjustment to the treatment. As the roots constitute the major hydraulic resistance, KP/LA largely reflected those of the roots. Long-term root chilling significantly affected leaf physiological characteristics, despite the lack of effect on hydraulic conductance in TAGI4. Long term chilling decreased the whole plant dry mass, but the effect was smaller in TAGI4, and this clone also showed morphological adjustment, in that root growth was less adversely affected than shoot growth. The data from GU210 support the hydraulic limitation hypothesis; because of the morphological and physiological adjustment to long-term root chilling in TAGI4, the data are unsuitable to directly assess the hypothesis.