Refixation of xylem sap CO2 in Populus deltoides

Vascular plants have respiring tissues which are perfused by the transpiration stream, allowing solubilization of respiratory CO₂ in the xylem sap. The transpiration stream could provide a conduit for the internal delivery of respiratory CO₂ to leaves. Trees have large amounts of respiring tissues in the root systems and stems, and may have elevated levels of CO₂ in the xylem sap which could be delivered to and refixed by the leaves. Xylem sap from the shoots of three Populus deltoides trees had mean dissolved inorganic carbon concentrations (CO₂+H₂CO₃+HCO^?₃) ranging from 0. 5 to 0. 9 mM. When excised leaves were allowed to transpire 1 mM[¹⁴C]NaHCO₃, 99. 6% of the label was fixed in the light. Seventy-seven percent of the label was fixed in major veins and the remainder was fixed in the minor veins. Autoradiography confirmed that label was confined to the vasculature. In the dark, approximately 80% of the transpired label escaped the leaf, the remainder was fixed in the major veins, slightly elevating dark respiration measurements. This indicates that the vascular tissue in P. deltoides leaves is supplied with a carbon source distinct from the atmospheric source fixed by interveinal lamina. However, the contribution of CO₂ delivered to the leaves in the transpiration stream and fixed in the veins was only 0. 5% of atmospheric CO₂ uptake. In the light 90% of the label was found in sugar, starch and protein, a pattern similar to that found for atmospheric uptake of[¹⁴C]CO₂. Compared with leaves labelled in the light, leaves labelled in the dark had more label in organic acid, amino acid and protein and less label in sugar and starch. After a 5-s pulse the majority of the label fed to petioles in both the light and the dark was found in malate. The majority of the label was found in malate at 120 s in the dark; only 2% of the label was found in phosphorylated compounds at 120 s. The proportion of label found in phosphorylated compounds increased from 17% at 5 s to 80% at 120 s in the light. This suggests that CO₂ delivered to leaves in the light via the transpiration stream is fixed in the veins, a small portion through dark fixation into malate, the remainder by C-3 photosynthesis.     

Nitrogen assimilation and transport in carob plants

Most of the nitrate reductase activity (80%;) in carob ( Ceratonia siliqua L. cv. Mulata) is localised in the roots. The nitrate concentration in the leaves is relatively low compared to that in the roots, suggesting that nitrate influx into the leaf may be a major factor limiting the levels of nitrate reductase in the shoot. Transport of nitrate from root to shoot appears limited by the entrance of nitrate into the xylem. In order to study this problem, we determined the nitrate concentrations and nitrate reductase activities along the roots of nitrate-grown plants, as well as the composition of the xylem sap and the nitrate levels in the leaves. Some of the the bypocotyl, in order to bypass the loading of nitrate into the xylem of the roots. The results show that the loading of nitrate into the xylem is a limiting step.
The cation and anion concentrations of nitrate- and ammonium-fed plants were similar, showing almost no production of organic anions. In both nitrate- and ammonium-fed plants, the transport of nitrogen from root to shoot was in the form of organic nitrogen compounds. The nitrate reductase activity in the roots was more than sufficient to explain all the efflux of OH⁻ into the root medium of nitrate-fed plants. In carob plants the K-shuttle may thus be operative to a limited extent only, corresponding to between 11 and 27%; of the nitrate taken up. Potassium seems to be the cation accompanying stored nitrate in the roots of carob seedlings, since they accumulate nearly stoichiometric amounts of K⁺ and NO⁻₃.     

Characteristics of inorganic nitrogen assimilation of plants in fire-prone Mediterranean-type vegetation

A range of approaches was used to investigate how species within a fire-prone Banksia woodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline-dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow-rooted herbaceous, non-mycorrhizal or VAM-positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen-fixing species were active in reducing and transporting nitrate, others were virtually inactive in these respects.     

Mortality rates of desert vegetation during high-intensity drought at Uluru-Kata Tjuta National Park,Central Australia

Precipitation variability and heatwaves are expected to intensify over much of inland Australia under most projected climate change scenarios. This will undoubtedly have impacts on the biota of Australian dryland systems. However, accurate modelling of these impacts is presently impeded by a lack of empirical research on drought/heatwave effects on native arid flora and fauna. During the 2018–2021 Australian drought, many parts of the continent's inland experienced their hottest, driest period on record. Here, we present the results of a field survey in 2021 involving indigenous rangers, scientists and national parks staff who assessed plant dieback during this drought at Ulur u-Kata Tjut a National Park (UKTNP), central Australia. Spatially randomized quadrat sampling of eight common and culturally important plants indicated the following plant death rates across UKTNP (in order of drought susceptibility): desert myrtle (Aluta maisonneuvei subsp. maisonneuvei) (91%), yellow flame grevillea (Grevillea eriostachya) (79%), Maitland's wattle (Acacia maitlandii) (67%), waxy wattle (A. melleodora) (65%), soft spinifex grass (Triodia pungens) (53%), mulga (A. aneura) (42%), desert oak (Allocasuarina decaisneana) (22%) and quandong (Santalum acuminatum) (0%). The sampling also detected that seedling recruitment was absent or minimal for all plants except soft spinifex, while a generalized linear mixed model (GLMM) indicated two-way interactions among species, plant size and stand density as important predictors of drought survival of adult plants. A substantial loss of biodiversity has occurred at UKTNP during the recent drought, with likely drivers of widespread plant mortality being extreme multi-year rainfall deficit (2019 recorded the lowest-ever annual rainfall at UKTNP [27 mm]) and record high summer temperatures (December 2019 recorded the highest-ever temperature [47.1°C]). Our findings indicate that widespread plant death and extensive vegetation restructuring will occur across arid Australia if the severity and frequency of droughts increase under climate change.     

Relations between stomatal closure, leaf turgor and xylem vulnerability in eight tropical dry forest trees

This study examined the linkage between xylem vulnerability, stomatal response to leaf water potential (Ψ_L), and loss of leaf turgor in eight species of seasonally dry tropical forest trees. In order to maximize the potential variation in these traits species that exhibit a range of leaf habits and phenologies were selected. It was found that in all species stomatal conductance was responsive to Ψ_L over a narrow range of water potentials, and that Ψ_L inducing 50% stomatal closure was correlated with both the Ψ_L inducing a 20% loss of xylem hydraulic conductivity and leaf water potential at turgor loss in all species. In contrast, there was no correlation between the water potential causing a 50% loss of conductivity in the stem xylem, and the water potential at stomatal closure (Ψ_SC) amongst species. It was concluded that although both leaf and xylem characters are correlated with the response of stomata to Ψ_L, there is considerable flexibility in this linkage. The range of responses is discussed in terms of the differing leaf‐loss strategies exhibited by these species.     

The three-dimensional structure of vascular tissues in Agrobacterium tumefaciens-induced crown galls and in the host stems of Ricinus communis L.

The three-dimensional pattern of phloem and xylem in 10-d-to two-month-old tumors induced by Agrobacterium tumefaciens (C58) and in adjacent Ricinus communis L. stem tissues was studied in thick sections by clearing with lactic acid and by staining with lacmoid. The crown galls contained two types of vascular strands: treelike branched bundles, which developed towards the tumor surface in fast-growing regions, and globular bundles in the slowly developing parts. Both types of vascular bundles contained xylem and phloem and were continuous with the vascular system of the host plant. The tumor bundles were interconnected by a dense net of phloem anastomoses, consisting of sieve tubes but no vessels. These vascular patterns reflect the apparent synthesis sites, concentration gradients and flow pathways of the plant hormones additionally produced in the tumors upon expression of the T-DNA-encoded genes. The A. tumefaciens-induced crown gall affected vascular differentiation in the host stem. In the basipetal direction, the tumor induced more xylem differentiation directly below it, where the crown-gall bundles joined the vascular system of the host. In the centripetal direction, the crown gall caused the development of pathologic xylem characterized by narrow vessels, giant rays and absence of fibers. On the other hand, most probably as a consequence of its gibberellic acid content, the host plant stimulated a local differentiation of regenerative phloem and xylem fibers with unique ramifications, only at the base of the tumor. However, fibers were absent from the main body of the crown gall. The study shows that A. tumefaciens-induced crown galls are characterized by a sophisticated network of vascular tissues in the tumor and are accompanied by a perturbated vessel system in the host. The hormonal mechanisms controlling vascular differentiation in the tumor and neighboring host tissues are discussed. In addition, the gall constriction hypothesis is proposed for explaining the mechanism which gives priority in water supply to the growing gall over the host shoot.We thank Dr. Zs. Koncz (Max-Planck-Institut für Züchtungsforschung, Köln, Germany) for Agrobacterium strains and the Deutsche Forschungsgemeinschaft (SFB 199) for financial support to C.I.U.     

Dye-flow apparatus to measure the variation in axial xylem permeability over a stem cross-section

Abstract Equipment and methodology are described that allows the radial variation in axial xylem permeability (hydraulic conductivity) over a tree cross-section to be measured and the flow paths to be identified by the strictly controlled flow of dye through a specimen. The apparatus can be calibrated so that the point-to-point variation of absolute permeability over a xylem cross-section can be calculated from the dye-flow patterns, which otherwise show only relative variations in permeability. The effect of using different dyes and dye concentrations on the penetration time and the shape of the dye patterns was investigated. The penetration time through the wood of identical end-matched specimens is appreciably longer for fixing dyes than for non-fixing dyes, and for the fixing dyes it depends strongly on the dye concentration. However, the dye patterns of the end-matched specimens were indistinguishable with fixing and non-fixing dyes, and independent of dye concentration. The fixing dye toluidine blue at 0.25% to 0.5% (w/w) was found most suitable as it yields a clear permanent pattern.     

Effects of atrazine on the assimilation of inorganic nitrogen in cereals

The inclusion of sub-lethal amounts ofthe herbicide atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] in the nutrient solution supplied to maize and barley increased the growth of the root and shoot and the uptake of nitrate. The activities of nitrate and nitrite reductases, glutamine synthetase and glutamate synthase were enhanced and the amino acid and nitrate contents of the xylem sap increased. All these effects of atrazine were found only in plants grown with nitrate as the nitrogen source. The uptake of ¹⁵NO₃^? and its incorporation into protein in the root and shoot of maize and barley seedlings was significantly greater in the atrazine treated plants. However, a stimulation in the incorporation of leucine-[¹⁴C] into TCA-precipitable protein of detached leaves from 7-day-old barley seedlings was obtained only in the absence of a supply of combined nitrogen either in the culture medium or in the in vitro incubation mixture containing the labelled amino acid.     

Sonication: A new method for gene transfer to plants

Sonication is a novel method for gene transfer into plant protoplasts and intact plant cells. The mode of action of ultrasound and its chemical, biochemical and physiological effects are reviewed. The state of the art of acoustic transformation is presented and possible mechanisms are discussed.