Transpiration and root water uptake by olive trees

While the cultivated olive tree (Olea europaea L.) is known to be sclerophyllous and effective at tolerating drought, little is known of its short-term water-use dynamics for most studies have been based on longer-term, water-balance information. We present here, for the first time, heat-pulse measurements of the sap flux measured not only within the semi-trunk of an olive tree, but also within a root excavated close to the stump. One tree in the olive grove near Seville in Spain had regularly received basin irrigation during the summer, whereas the other, growing on this deep silt loam, had been without water for over 3 months. Following a flood irrigation of 730 L to a dyked area around the tree, the regularly-irrigated olive maintained a transpiration rate of 1.65 mm³ mm^–2 d^–1, on a leaf area basis, for only 3 days following the irrigation. This rate was maintained for a total consumption of 110 L. It then began again to limit its rate of water use with transpiration falling below that predicted for well-watered conditions by the Penman-Monteith equation. The flow of sap in the near-surface root dropped concomitantly. Meanwhile the unirrigated tree was using water at just 0.78 mm d^–1. Yet following an irrigation of 870 L it only lifted its consumption to 1.12 mm d^–1, on a leaf area basis. Neither did it recover its leaf water potential following this wetting because of an inability to refill cavitated vessels. These data again show olive to be a parsimonious and cautious consumer of soil water.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.     

Competition and habitat selection in Namib desert tenebrionid beetles

Summary We tested whether intra- and interspecific competition could affect habitat selection in the two most abundant tenebrionid beetles,Physadesmia globosa andOnymacris rugatipennis, in a dry riverbed in the Namib desert. The spatial distributions of these beetles at the microhabitat scale were negatively correlated. We performed a removal experiment, progressively removing first 25% and then a further 25% of the population of the most abundant species,P. globosa, under the trees where most of the preferred food of both species is concentrated. There was no response ofO. rugatipennis to this removal in the tree habitat. In the open, barely-vegetated habitat where mostO. rugatipennis are found, the number of this species caught in pitfall traps increased following both removals and decreased followingP. globosa replacement under the trees. It appears that intraspecific competition forces someP. globosa to occupy the open habitat. Interspecific competition betweenP. globosa andO. rugatipennis in the open habitat reduces the number ofO. rugatipennis that can co-exist withP. globosa there. Removal ofP. globosa under the trees allows conspecifics in the open habitat to move under the trees, releasingO. rugatipennis in the open habitat from competition. This then results in an increase in the numbers ofO. rugatipennis in the open habitat as a result of immigration from neighbouring areas. We found that differences in foraging efficiency, measured as giving-up times in artificial food patches, create a likely mechanism of co-existence that explains the distinct preferences of these two species for tree and open habitats.     

Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. II. Stomatal sensitivity to abscisic acid imported from the xylem sap

Sunflower plants [Helianthus annuus L.) were subjected to soil drought. Leaf conductance declined with soil water content even when the shoot was kept turgid throughout the drying period. The concentration of abscisic acid in the xylem sap increased with decreasing soil water content. No general relation could be established between abscisic acid concentration in the xylem sap and leaf conductance due to marked differences in the sensitivity of leaf conductance of individual plants to abscisic acid from the xylem sap. The combination of these results with data from Gollan, Schurr & Schulze (1992, see pp. 551–559, this issue) reveals close connection of the effectiveness of abscisic acid as a root to shoot signal to the nutritional status of the plant.     

Management and diversity of arthropods inCalluna heathland

Heathlands, dominated byCalluna vulgaris, are located along the western seaboard of Europe. Most undergo a cyclical succession as the heather plants pass through four definable growth-phases over a period of about 30 years. Examples of upland heathlands are drawn from the North York Moors National Park, an area of Jurassic rocks in the north of England (approximately 54°25N, 0°55W). Botanical diversity is low. In a survey of 10 heathlands selected to encompass the botanical variation of the heathlands of the National Park, only 40 species of higher plants and 31 species of mosses were recorded (2.2% and 4.5% of the British flora, respectively).However, diversity of some invertebrate groups is high. Totals of 54 species of carabid beetles and 127 species of spiders were recorded (15.3% and 20.4% of the British fauna, respectively). At least two environmental gradients appear to influence the species composition of the invertebrate assemblages: a wet-dry gradient and the height of the vegetation (corresponding to the growth-phase of the heather). Experimental areas on three heathlands were burnt and cut, the traditional and more modern methods of heather management. Analysis of both the spider and beetle data indicated that the species assemblages were primarily influenced by the growth-phase of the heather. Some of the nationally rarer species are associated with the open conditions of recently-cut or burnt heathland. An altitudinal effect was also inferred since heathland at 410 m above sea level tended to be less species-rich than heathlands at ca 260 m above sea level.These results present a paradox. Although botanical diversity is extremely low, and although it is a non-natural ecosystem maintained by fire and grazing management to prevent a succession to woodland, arthropod diversity can be extremely high. The discussion focuses on the importance of upland heathlands in the conservation of insect and spider diversity, as well as on the value of these upland areas for conservation more generally.C. vulgaris is seen to be a key species on which many other species depend. One conclusion is that there is insufficient effort, nationally and especially internationally, to conserve the heathlands of Europe.     

Are xylem-tapping mistletoes partially heterotrophic?

Summary Carbon isotope ratios, photosynthesis, and transpiration were measured on a xylem-tapping mistletoe (Phoradendron juniperinum) and its host (Juniperus osteosperma) in southern Utah, USA. For host tissues, the carbon isotope ratios agreed with theoretical values predicted from gas exchange observations. However, for mistletoe tissues, carbon isotope ratios deviated significantly from values predicted by gas exchange observations. This apparent discrepancy in mistletoe carbon isotope ratios can be resolved if one assumes that organic carbon dissolved in host xylem water was assimilated by the parasite. The mistletoes' high transpiration rates and low photosynthetic rates contributed to their heavy dependence on host xylem carbon. Two lines of evidence suggest that 62±2% of the carbon in the Utah mistletoe is derivated from the host and not from mistletoe autotrophic activities. Whereas xylem-tapping mistletoes have previously been characterized as wholly autotrophic parasites, we suggest that they may instead derive significant amounts of carbon from their hosts.     

Osmoregulation and the control of phloem-sap composition in Ricinus communis L.

Phloem-sap composition was studied in plants of Ricinus communis L. grown on a waterculture medium. The sap possessed a relatively high K⁺:Na⁺ ratio and low levels of Ca²⁺ and free H⁺. Sucrose and K⁺ (together with its associated anions) accounted for 75% of the phloem-sap solute potential (_s). In plants kept in continuous darkness, a decrease in phloem-sap sucrose levels over 24h was accompanied by an increase in K⁺ levels. Measurements of phloem-sap _s and xylem water potential () indicated that this resulted in a partial maintenance of phloem turgor pressure _p. In darkness there was also a marked decrease in the malate content of the leaf tissue, and it is possible that organic carbon from this source was mobilized for export in the phloem. The results support the concept of the phloem sap as a symplastic phase. We interpret the increase in K⁺ levels in the phloem in darkness as an osmoregulatory response to conditions of restricted solute availability. This reponse can be explained on the basis of the sucrose-H⁺ co-transport mechanism of phloem loading.Abbreviations water potential - _s solute potential - _p pressure potential