Amram Eshel

Interspecific Competition among Phloem‐Feeding Insects Mediated by Induced Host‐Plant Sinks

The role of interspecific interactions among herbivorous insects is considered to be limited, especially in specialist communities. In the current study we report on exploitative interspecific interaction between two closely related phloem—feeding species of gall—forming aphids (Homoptera; Pemphigidae; Fordinae), mediated by the supply of photoassimilates from the host plant. Geoica sp. forms a spherical gall on the leaflet midrib of Pistacia palaestina (Anacardiaceae), while Forda formicaria forms crescent—shaped galls on the leaflet margin of the same host plant. Using 14C labeling, we were able to trace the food supply (assimilated carbohydrates) from the leaves to galls of each species. We found that Geoica galls are strong sinks. These galls divert the normal phloem transport of the plant and reduce the amount of assimilates imported by F. formicaria, especially when they are located on the same leaflet. By the end of the season Geoica caused death of 84% of F. formicaria galls that were located on the same leaflet, and reduced reproductive success in the surviving galls by 20%. This is because the presence of Geoica causes early senescence (but not abscission) of the leaflet it is on (whether or not F. formicaria is present). The interaction is asymmetrical: F. formicaria did not affect reproductive output of Geoica nor did it cause visible damage to the leaflets. To our knowledge, this it the first demonstration of exploitation competition for plant assimilates between two insect—induced sinks. This exploitative competition, mediated by manipulation of plant phloem transport, stands in contrast to the absence of interference competition for galling sites between the two aphid species. Although their spatial distributions partly overlapped, the niche breadth of each species (measured from gall positions on leaves along the shoot axis) was not affected by the presence of the other. Moreover, when both species were located on the same leaf, they formed galls independently on the same or different leaflets, and there was no indication of interference competition over galling sites.

Towards developmental modelling of tree root systems

Abstract Knowledge of belowground structures and processes is essential for understanding and predicting ecosystem functioning, and consequently in the development of adaptive strategies to safeguard production from trees and woody plants into the future. In the past, research has mainly been concentrated on growth models for the prediction of agronomic or forest production. Newly emerging scientific challenges, e.g. climate change and sustainable development, call for new integrated predictive methods where root systems development will become a key element for understanding global biological systems. The types of input data available from the various branches of woody root research, including biomass allocation, architecture, biomechanics, water and nutrient supply, are discussed with a view to the possibility of incorporating them into a more generic developmental model. We discuss here the main focus of root system modelling to date, including a description of simple allometric biomass models, and biomechanical stress models, and then build in complexity through static growth models towards architecture models. The next progressive and logical step in developing an inclusive developmental model that integrates these modelling approaches is discussed.

The ability of Porphyra linearis (Rhodophyta) to tolerate prolonged periods of desiccation

Porphyra lineraris, is the uppermost intertidal macroalga on the eastern Mediterranean coast. It is able to regain photosynthetic activity upon reimmersion, even after drying in air to a stable water content of ca. 5% of fully hydrated plants for a period of three weeks. During desiccation experiments the plants continued net emersed photosynthesis at a maximal rate until their relative water content (RWC) dropped to 60%. Net photosynthetic gas exchange of plants with only 10% RWC was still positive. Photosynthetic rates, however, decreased during the drying process more rapidly than did the respiration rates. During rehydration, plants previously kept dry (5% RWC) for short periods (up to 24 h) regained net photosynthetic capability only when their RWC reached about 35%

Photosynthesis of Ulva sp. I. Effects of desiccation when exposed to air

Abstract The photosynthetic performance of intertidal Mediterranean Ulva sp. during emergence has been investigated and compared with certain field desiccation levels of this alga. Net photosynthetic rates were constant from the onset of desiccation up to a water loss of 20%; at higher desiccation levels rates decreased. Positive net photosynthesis was maintained until a relative water content of ≈ 35%. Comparing these results with maximal natural dehydration rates in the summer, it may be predicted that net photosynthesis in the morning is positive for ≈90 min after exposure, while the corresponding time during midday is 30 min only. Ulva growing in the lower mid-littoral zone, was not exposed enough time to cause a reduction in photosynthetic rates. At higher levels plants did desiccate to a point where photosynthetic rates would be adversely affected, but net rates were still positive during the whole day. Plants growing at the highest level were dehydrated to such an extent as not to allow for positive net photosynthesis for many hours of the day. No apparent barrier for water loss from Ulva fronds was found. The ability, however, to perform positive net photosynthesis when mildly desiccated may be a feature permitting this plant to grow in the intertidal zone.

Photosynthesis of Ulva sp. II. utilization of CO2 and HCO−3 when submerged

Abstract The photosynthetic capacity of submerged Ulva sp. when utilizing CO 2 and HCO − 3 as exogenous carbon forms has been investigated and compared with ambient carbon concentrations in sea water. Saturating concentrations of HCO − 3 and CO 2 were 1200 and 100 μM, respectively at saturating light, and photosynthetic rates under such conditions averaged 700 μmol O 2 · g DW −1 · h −1 . The HCO − 3 concentration of sea water (≈2500μM), was thus found to be saturating for photosynthesis of Ulva . At the CO 2 concentration of sea water (≈ 10 μM), the contribution of this carbon form to photosynthesis could be 27% at the most. Under conditions of slow water movement, the relative importance of CO 2 utilization would probably be minimized in favour of HCO − 3 utilization. It is concluded that HCO − 3 uptake is not limiting photosynthesis for Ulva under natural conditions.

Spatial variation of seedling distribution in an east Mediterranean pine woodland at the beginning of post-fire succession

Most of the area in pine woodlands is occupied by perennial seeders that regenerate from seeds in the first winter after the fire and by annuals. Control of the germination in the regenerating vegetation after wildfire is therefore a primary ecological component of the post-fire succession in this ecosystem. The aim of the study presented here was to determine the distribution of Pinus, Cistus and other plants seeds around burned Pinus halepensis trees, and to measure the conditions related to seed germination in the upper soil layers in the same locations. The study was carried out in a 50-year old planted Pinus halepensis woodland that was burned down by a wildfire in July 1995. The variation of seedbank density was determined by collecting samples under the canopies of burned trees and in a nearby open area. Pine seedbank density decreased and that of Cistus and annuals increased with increasing distance from the burned trunks. Most pine seeds were present in the ash layer while those of the other plants were in the soil. In situ germination experiments showed that seedling density decreased with distance from the burned trunks while the proportion of pines in the seedling population increased. This was a result of seedbank variation and germination inhibition by the high pH conditions caused by the ash. The establishment of sparse pine seedling under the dead tree canopies insured their rapid development without interference by other plants and played a key role in the regeneration and stability of the pine woodland community. The concomitant mass germination of the perennial seeders in the rest of the area prevented invasion by annuals.

The development of halophyte-based agriculture: past and present

BACKGROUND Freshwater comprises about a mere 2·5% of total global water, of which approximately two-thirds is locked into glaciers at the polar ice caps and on mountains. In conjunction with this, in many instances irrigation with freshwater causes an increase in soil salinity due to overirrigation of agricultural land, inefficient water use and poor drainage of unsuitable soils. The problem of salinity was recognized a long time ago and, due to the importance of irrigated agriculture, numerous efforts have been devoted towards improving crop species for better utilization of saline soils and water. Irrigating plants with saline water is a challenge for practitioners and researchers throughout the world. SCOPE Recruiting wild halophytes with economic potential was suggested several decades ago as a way to reduce the damage caused by salinization of soil and water. A range of cultivation systems for the utilization of halophytes have been developed, for the production of biofuel, purification of saline effluent in constructed wetlands, landscaping, cultivation of gourmet vegetables, and more. This review critically analyses past and present halophyte-based production systems in the context of genetics, physiology, agrotechnical issues and product value. There are still difficulties that need to be overcome, such as direct germination in saline conditions or genotype selection. However, more and more research is being directed not only towards determining salt tolerance of halophytes, but also to the improvement of agricultural traits for long-term progress.

The contribution of aerenchymal CO2 to the photosynthesis of emergent and submerged culms of Scirpus lacustris and Cyperus papyrus

Abstract In this work it was investigated whether sediment-derived aerenchymal CO 2 could be utilized for photosynthesis in the culms of the two emergent aquatic macrophytes Scirpus lacustris L. (a C 3 plant) and Cyperus papyrus L. (a C 4 plant). Aerenchymal CO 2 concentrations within the submerged parts of the culms were found to be 30 000–50 000 μl l −1 , and ca. 800 μl l −1 in the emergent parts of Scirpus lacustris and 2000 μl l −1 in Cyperus papyrus . These concentrations tended to be lower during the day in Cyperus , while no clear diurnal pattern was observed for Scirpus . Photosynthetic rates based on fixation of external or internal CO 2 were measured in situ by providing 14 CO 2 either externally or from the aerenchyma (by supplying 14 C-labelled CO 2 through test-tubes attached to excised culms). The results showed that the contribution of aerenchymal CO 2 to the total photosynthesis of emergent culms was less than 0.25% in both species. This has a rationale in that photosynthetic rates of both species were saturated at the ambient air CO 2 concentration, but it remains unclear why CO 2 does not diffuse towards the photosynthesizing tissues. By contrast, internal CO 2 appeared to be the only source of inorganic carbon used for photosynthesis of young submerged green culms. It is thus suggested that the aerenchyma, in addition to other functions, is important in providing sediment-derived CO 2 for photosynthesis in young shoots or culms if growing submerged, before they reach the water surface.

Differences in ion uptake among roots of various types

Abstract Ion uptake by tap and by lateral roots of peas (Pisum sativum L.) was investigated. The roots were either of intact seedlings or of seedlings whose young shoots or cotyledons had been removed. Tap roots seem to before responsive to temperature, and more susceptible to ion competition, than lateral roots. This indicates that the contribution of the metabolic uptake system, in tap roots, is dominant over that of the non‐metabolic one. The uptake capability of both types of roots was lowered by removal of the shoots. Apparently, under such treatments, the contribution of the metabolic component, to the total uptake, was reduced. Studies of plant mineral nutrition should take into account such differences in the physiological and morphological characteristics that exist among the various root types.

Clustering of allergenic pollen on the basis of skin responses of atopic patients by matrix analysis

The responses of 148 atopic patients to some 43 different extracts of allergenic pollen were tested by prick tests. The measure of dissimilarity was introduced and calculated for all pairs of allergens. The investigated allergens were clustered into groups, according to their unbiased greatest similarity, by a matrix‐structuring method. Results indicate that subgroups of allergens can be distinguished even within groups of closely related pollen allergens that were believed to be fully cross‐reactive. A few cases are demonstrated for various varieties of olives, pecans, date palms, and turf grasses and for some wild chenopods and amaranths. The usefulness of the suggested solution for allergy research and for clinical practice is discussed.