Gas exchange responses of two Eucalyptus species to salinity and waterlogging

Transcription

1 Tree Physiology 5, Heron Publishing-Victoria, Canada Gas exchange responses of two Eucalyptus species to salinity and waterlogging P. G. VAN DER MOEZEL, L. E. WATSON and D. T. BELL Department of Botany, The University of Western Australia, Nedlands, Western Australia 6009, Australia Received September 29, 1988 Summary The effects of salinity and waterlogging on stomata1 conductance, net photosynthesis and transpiration of 3-month-old Eucalyptus camaldulensis Dehnh and Eucalyptus lesouefii Maiden seedlings were studied under greenhouse conditions. Under non-saline conditions, waterlogging induced stomatal closure in both species. However, the stomata of E. camaldulensis reopened after five weeks, when adventitious roots were produced. Relative to that of controls, height growth of waterlogged seedlings was greater in E. camaldulensis than in E. lesouefii, as were rates of photosynthesis and transpiration. In a freely drained medium, high salinity reduced rates of seedling height growth and photosynthesis, relative to those in controls, less in E. lesouejl i than in E. camaldulensis. In both species, height growth, stomatal conductance and photosynthetic rate were lowest under conditions of saline waterlogging. Introduction Tolerance to salinity and waterlogging is associated with both morphological and physiological adaptations. The morphological changes are usually preceded by changes in stomata1 aperture, photosynthesis and transpiration (Kozlowski 1984). Long-term tolerance to salinity and waterlogging may therefore be related to the degree to which these physiological functions are affected. The worldwide occurrence of salinized land has led to the development of methods to increase productivity of agricultural wastelands (Dudal and Pumell 1986). In Australia, one approach is to plant trees in saline seeps for wood production, shelter and site reclamation through reduction of groundwater level. To assist in reclamation of saline seepages, which are often subject to periodic waterlogging, research is underway to micropropagate selected tolerant genotypes of Eucalyptus, Acacia, Casuarina and Meluleucu species (Kabay et al. 1986). Two tree species that might be suitable for planting in saline environments in Western Australia are Eucalyptus cumaldulensis Dehnh. and E. lesouefii Maiden. Eucalyptus cumuldulensis, which grows to a height of 30 m, occurs along seasonal streams in arid and semi-arid zones of Western Australia and other parts of Australia (Chippendale 1973). It has been reported to be tolerant of salinity (Sands 1981) and waterlogging (Karschon and Zohar 1975). Eucalyptus lesouefii, which grows to a height of 12 m, occurs in the semi-arid Goldfields area of Western Australia, a region with an annual average rainfall of mm. This species has some resistance to soil salinity (Chippendale 1973), but there are no reports of its tolerance to soil

2 252 VAN DER MOEZEL, WATSON AND BELL waterlogging. This study was undertaken to determine the effects of salinity and waterlogging on stomata1 conductance, net photosynthesis and transpiration of Eucalyptus camaldulensis and E. lesouefii as a guide to the potential of these species for reclaiming saline waterlogged sites. Materials and methods Seedlings of Eucalyptus camaldulensis and E. lesouefii were raised from seed collected from single trees in Western Australia. Three-month-old seedlings were transplanted to 25-cm diameter, plastic pots (four plants per pot) containing tine, white sand. Seedlings were acclimatized in a greenhouse for 2 weeks before treatments were started. Treatments were: non-saline drained control (C); non-saline waterlogged (W); saline drained (S); and saline waterlogged (SW). Sixteen seedlings per species were used in the S and SW treatments and eight seedlings in the C and W treatments. Seedlings were waterlogged by standing the pots in large tanks filled with tapwater to a level about 10 mm above the sand surface. Drained pots were drip irrigated three times daily, with each irrigation period providing one liter of solution per pot. Non-saline treatments consisted of applications of tapwater and one fifth strength Hoagland s No. 2 nutrient solution. Saline solutions were a mixture of NaCl, MgS04 and CaCL combined to give a Na/Mg/Ca ratio of 10/2/l by weight. Saline soils in Australia are dominated by NaCl with Mg, Ca and SO, occurring in significant amounts. The salinity of both drained and waterlogged treatments was increased weekly by 700 ms m- increments until a maximum of 4200 ms m- was reached. The maximum salinity was maintained for 6 weeks. The water and nutrient solutions of all treatments were changed weekly. Shoot heights were measured at the beginning of the experiment (week 1) and at 2-week intervals thereafter. Seedling survival was monitored weekly. Stomata1 conductance, transpiration rate and net photosynthetic rate were measured with a portable ADC, open-system photosynthesis apparatus. The first fully expanded leaf of each seedling was tagged, and measurements were taken from five, randomly selected, tagged leaves at each sampling time. Sampling began at the end of the first week and continued at 2-week intervals thereafter. All measurements were taken between 1200 and 1300 h. Air temperatures and radiant flux densities at the leaf surface were recorded on each sampling date (Table 1). Results Waterlogging with tap water for 11 weeks induced splitting of Eucalyptus camaldulensis stems immediately above and below the waterline. The split stem had a spongy consistency, indicative of aerenchyma tissue. Aerenchyma was also present in roots produced in waterlogged E. camaldulensis seedlings. These roots were located mainly in the subterranean root mass, and as adventitious roots arising from a portion of the stem just above the waterline. Aerenchymatous roots were not

3 RESPONSES TO SALINITY AND WATERLOGGING 253 Table 1. Average light and temperature conditions between 1200 and 1300 h inside the greenhouse at each sampling. Week Date number 1986 Temperature Pa Light (pm01 mm% ) 2 May May June June June July observed after 11 weeks in saline waterlogged E. camaldulensis seedlings or in any E. lesouefii seedlings. After 11 weeks, 25% of the saline waterlogged E. camaldulensis seedlings and 69% of the saline waterlogged E. lesouefii seedlings had died. All control and non-saline waterlogged seedlings survived. Height growth of waterlogged E. camaldulensis seedlings, relative to that of controls, declined initially, but after 5 weeks increased to about twice that of the control plants (Figure 1). Relative to that of controls, height growth of E. camaldulensis seedlings in a saline medium, whether drained or waterlogged, decreased throughout the experiment, and reached less than one tenth that of control seedlings between weeks 7 and 9. In waterlogged or saline waterlogged medium, height growth relative to that of controls was lower in Eucalyptus lesouefi than in E. camaldulensis, whereas in a drained, saline medium it was greater in E. lesouefii than in E. camaldulensis. Waterlogging with tap water caused stomata1 closure in E. camaldulensis seedlings, but stomata1 reopening was evident after five weeks (Figure 1). Stomata1 conductance was reduced immediately in saline waterlogged seedlings and after one week in seedlings in a drained, saline medium. Transpiration rates in E. camaldulensis control seedlings were variable, but similar to those of E. lesouefii control seedlings. Initially, waterlogged seedlings had marginally lower transpiration rates than control plants, but the difference was not apparent during the last three sampling periods. The lowest transpiration rates occurred in the saline and saline waterlogged seedlings. The net photosynthetic rates of control and waterlogged E. camaldulensis plants were extremely variable, which was probably a reflection of the variation in greenhouse environmental conditions from one sampling date to another (Table 1). At week 6, however, when light intensity and temperature increased, net photosynthetic rates of control and waterlogged plants were much higher than those of saline and saline waterlogged plants. Waterlogging with tap water or salt water greatly reduced stomata1 conductance in E. lesouefi seedlings (Figure 1). Stomata1 conductances of seedlings in a drained, saline medium were lower than those of control plants but higher than those of waterlogged and saline, waterlogged plants. There was a general decrease over time in the transpiration rate of control E. lesouefii plants (Figure 1). This decrease was

4 254 VAN DER MOEZEL, WATSON AND BELL 250 E. camaldulansis E. lesouefii *Control +-Waterlogged -0 - Saline +- Saline, waterlogged WEEK NUMBER Figure 1. The effect of salinity and waterlogging on height growth relative to the control (C), mean stomata1 conductance, transpiration and net photosynthesis between 1200 and 1300 h for Eucalyptus camuldulensis and E. lesouefii seedlings. C = non-saline, drained control; W = non-saline waterlogged; S = saline drained; SW = saline waterlogged. Bars indicate standard error of the mean. Growth at weeks 3, 5, 7, 9 and 11 correspond to the 2-week growth rate for weeks 1-3, 3-5, 5-7, 7-9 and 9-11, respectively.

5 RESPONSES TO SALINITY AND WATERLOGGING 255 correlated with declining temperature and radiant flux density (Table 1). There was little difference in transpiration rate between control seedlings and seedlings in a drained, saline medium, whereas waterlogged and saline waterlogged seedlings had lower transpiration rates. The net photosynthetic rates of waterlogged and saline waterlogged E. lesouefii seedlings were consistently lower than those of control seedlings, whereas plants in a drained saline medium had a lower net photosynthetic rate initially, followed by higher rates at the final three sampling periods. The higher rates may have been due to the different position of leaves relative to the plant apex at different times of the experiment. Initially all leaves sampled were near the top of the plant, but later, because of differences in growth rate among the treatments, sample leaves of control plants were further from the apex than sample leaves of plants in the saline treatments. Discussion The tolerance of Eucalyptus camaldulensis and E. lesouefii seedlings to waterlogging, salinity and a combination of waterlogging and salinity was correlated with changes in stomata1 aperture, net photosynthesis and transpiration. Tolerance to waterlogging in the absence of high salinity has been correlated with stomata1 conductance, photosynthesis and transpiration (Regehr et al. 1975, Pereira and Kozlowski 1977, Zaerr 1983). Consistently low stomata1 conductance, as in E. lesouejii, is characteristic of waterlogging-sensitive species, whereas stomata1 closure followed by reopening, as in E. camaldulensis, can occur in waterloggingtolerant species (Regehr et al. 1975, Pereira and Kozlowski 1977). As with E. camaldulensis, the timing of stomata1 reopening in flooded Fraxinus pennsylvanica seedlings coincided with the production of adventitious roots (Sena Gomes and Kozlowski 1980h). Tolerance of E. camaldulensis to flooding (Karschon and Zohar 1975, Sena Gomes and Kozlowski 1980a, Blake and Reid 1981) has been attributed to the formation of aerenchyma in adventitious roots and submerged stems. A reduction in net photosynthetic rate in flooded plants is common (Kramer and Kozlowski 1979). Moderately tolerant Liquidambar styraciflua seedlings had reduced net photosynthetic rates as well as lower transpiration rates over a 9-day flooding period (Pezeshki and Chambers 1985). Reduction in net photosynthetic rate of waterlogged plants can be attributed, initially, to declining stomata1 conductance, but in the longer term, net photosynthetic rate may be further affected by changes in carboxylation enzymes, hormonal changes and leaf chlorosis brought about by the anaerobic, waterlogged environment (Kozlowski 1984). Reduced stomata1 conductance and net photosynthetic rate as a result of salinity, in the absence of waterlogging, have been reported for a number of plant species (Downton 1977, Ball and Farquhar 1984, Longstreth et al. 1984, Pezeshki and Chambers 1986). In all cases the decrease in net photosynthetic rate was correlated with reduced plant growth. A reduction in net photosynthetic rate in saline conditions can be attributed to both stomata1 and non-stomata1 effects. Downton (1977) calculated that non-stomata1 factors contributed more to reduced photosynthesis in salt-

6 256 VAN DER MOEZEL, WATSON AND BELL stressed grapevines than stomata1 restriction of CO2 uptake. The decrease in photosynthetic activity of the salt-stressed grapevines occurred even when visual symptoms of salt damage were not apparent. The use of physiological characteristics as a measure of salt tolerance may lead to an improvement in current salt-screening procedures, which usually rely on visual symptoms of salt damage such as leaf chlorosis as a measure of tolerance. There have been few studies on the combined effects of salinity and waterlogging on plant physiological processes. Saline, hypoxic conditions resulted in severe dry weight yield reduction and low stomata1 conductance in sunflower seedlings (Kriedemann and Sands 1984). Low stomata1 conductance recorded for saline waterlogged mangrove species was attributed to increased root resistance and decreased water absorption (Naidoo 1985). The response of E. camaldulensis and E. lesouefii to flooding can be related to the habitat of the seed source. Eucalyptus camaldulensis occurs predominantly in areas subject to periodic inundation, whereas the habitat of E. lesouefii is rarely flooded. The salinity of the seed collection sites in this study were not measured so no conclusions can be made relating salt tolerance to site characteristics. Planting in highly saline, waterlogged seepage areas would adversely affect the physiological processes of E. camaldulensis and E. lesouefii seedlings, leading to slower growth and, ultimately, death. On the basis of seedling survival, E. camaldulensis would be more suited to moderately saline waterlogged areas than E. lesouefi. In saline soils without waterlogging, E. lesouefi would probably perform marginally better than E. camaldulensis. Eucalyptus camaldulensis would be the more suitable species for planting in areas subject to waterlogging by fresh water. Acknowledgments Support was provided under the National Biotechnology Program which is administered by the Commonwealth Department of Industry, Technology and Commerce. References Ball, M.C. and G.D. Farquhar Photosynthetic and stomata] responses of two mangrove species, Aegiceras corniculatum and Avicennia marina, to long term salinity and humidity conditions. Plant Physiol. 74: 16. Blake, T.J. and D.M. Reid Ethylene, water relations and tolerance to waterlogging of three Eucalyptus species. Aust. J. Plant Physiol. 8: Chippendale, G.M Eucalypts of the Western Australian Goldfields (and the adjacent wheat belt). Dept. Primary Industry, Forestry and Timber Bureau, Canberra, Australia. Downton, W.J.S Photosynthesis in salt-stressed grapevines. Aust. J. Plant Physiol. 4: Dudal, R. and M.F. Purnell Land resources: salt affected soils. Reclam. Reveg. Res. 5: 1-9. Kabay, E.D., L.A.J. Thomson, J.C. Doran, PG. Van der Moezel, D.T. Bell, J.A. McComb, I.J. Bennett, V.M. Hartney and N. Malajczuk Micropropagation of forest trees selected for salt tolerance. Aust. Salinity Newsletter 14:6&62. Karschon, R. and Y. Zohar Effects of flooding and of irrigation water salinity on Eucalyptus camaldulensis Dehn. from three seed sources. Agric. Res. Organ., Div. Forestry. lllanot Leaflet No. 54. Kozlowski, T.T Plant responses to flooding of soil. Bioscience 34: Kramer, P.J. and T.T. Kozlowski Physiology of woody plants. Academic Press, New York, 811 p. Kriedemann, P.E. and R. Sands Salt resistance and adaptation to root-zone hypoxia in sunflower. Aust. J. Plant Physiol. 11:

7 RESPONSES TO SALINITY AND WATERLOGGING 257 Longstreth, D.J., J.A. Bolanos and J.E. Smith Salinity effects on photosynthesis and growth in Alternantheraphiloxeroides (Mart.) Griseb. Plant Physiol. 75: Naidoo, G Effects of waterlogging and salinity on plant-water relations and on the accumulation of solutes in three mangrove species. Aquatic Bot. 22: Pereira, J.S. and T.T. Kozlowski Variations among woody angiosperms in relation to flooding. Physiol. Plant. 41: Pezeshki, S.R. and J.L. Chambers Stomata1 and photosynthetic response of sweet gum Liquidambar sfyraci&uz to flooding. Can. J. For. Res. 15: Pezeshki, S.R. and J.L. Chambers Effect of soil salinity on stomata1 conductance and photosynthesis of green ash Fravinus pennsy[vunica. Can. J. For. Res. 16: Regehr, D.L., EA. Bazzaz and W.R. Boggess Photosynthesis, transpiration, leaf conductance of Populus deltoides in relation to flooding and drought. Photosynthetica 9: Sands, R Salt resistance in Eucalyptus cumuldulensis Dehn. from three different seed sources. Aust. For. Res. 11: Sena Gomes, A.R. and T.T. Kozlowski. 1980a. Effects of flooding on Eucalyptus camuldulensis and Euculyprus globulus seedlings. Oecologia 16: Sena Gomes, A.R. and T.T. Kozlowski. 1980b. Growth response and adaptation of Fraxinus pennsylvan& seedlings to flooding. Plant Physiol. 66: Zaerr, J.B Short-term flooding and net photosynthesis in seedlings of three conifers. For. Sci. 29:71-78.

8