We measured oxygen isotope ratios (delta18O) of xylem sap, phloem sap, leaves, wood and bark of Eucalyptus globulus Labill. growing in southwestern Australia. Carbon isotope ratios (delta13C) were measured in the dry matter of phloem sap, leaves and wood. Results were used to test several aspects of a mechanistic model of 18O enrichment and provided insights into post-photosynthetic variations in dry matter delta13C. Xylem water delta18O varied little within the tree crown, whereas variation at the landscape-level was more pronounced, with plantations near the coast being enriched by up to 3 per thousand compared with plantations less than 100 km inland. Phloem water was significantly enriched in 18O compared with xylem water in two of three sampling campaigns; mean enrichments were 0.5 and 0.8 per thousand. Phloem sap sugars exported from E. globulus leaves closely reflected observed leaf water enrichment when diurnal variation in photosynthesis was taken into account. Photosynthetic rates were higher in the morning than in the afternoon, whereas leaf water 18O enrichment increased to maximum values in the afternoon. A non-steady-state model of leaf water 18O enrichment accurately predicted observed values through a full diel cycle. Mean estimates of the proportion of organic oxygen effectively exchanging with xylem water during cellulose synthesis were close to 0.40 for both leaves and wood. Carbon isotope ratios of nascent xylem tissues did not differ from those of phloem sap sugars collected concurrently, whereas nascent leaf tissues were depleted in 13C by 2 per thousand compared with phloem sap sugars, suggesting that, in E. globulus, 13C enrichment of sink tissues compared with source leaves does not result from an enriching process within the sink tissue.

Exposure of dormant seed to cold smoke derived from burnt native vegetation had a positive influence on germination in one or more seed provenances in 45 out of 94 species of native Western Australian plants that are normally hard to germinate. When tested under controlled conditions some species showed earlier germination in smoke treatments than controls; in others smoke-treated seeds continued to germinate for several weeks after controls had achieved full germination. In the remainder, treated and control seeds germinated to similar time schedules. A group of 23 species which responded positively had previously been recorded as extremely difficult or impossible to germinate using conventional techniques. These included members of the genera Geleznowia (Rutaceae), Hibbertia (Dilleniaceae), Stirlingia (Proteaceae), Verticordia (Myrtaceae), Actinostrobus (Cupressaceae) and Pimelea (Thymelaeaceae). Both large- and small-seeded species were encountered amongst the positively responding taxa, which encompassed representatives of 15 families and 26 genera of dicotyledons, 5 families and 8 genera of monocotyledons and the gymnosperm Actinostrobus acuminatus. Sowing seeds on smoke-fumigated filter papers or watering with aqueous eluates of smoke elicited similar degrees of stimulation of germination, as did exposure to gaseous smoke in a readily germinating species Anigozanthos manglesii (Haemodoraceae) and the normally intractable species Lysinema ciliatum (Epacridaceae). Exposing recently burnt and unburnt natural bushland sites to smoke, smoked water or smoked dry sand elicited a significant germination response in 15 species. Over one third of the species sampled in the burnt site exhibited germination additional to that caused by the fire. Data are discussed in relation to previous germination studies on Australian and other taxa.

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4-6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.

No abstract

In this paper, we present an integrated account of the diurnal variation in the stable isotopes of water (

No abstract

No abstract

No abstract