Factors influencing leaf vein selection in the lime aphid (Eucallipterus tiliae L.)

Summary The thickness of sclerenchyma around the veins of lime leaves varies with both size of leaf and size of vein and may provide a barrier to stylet penetration. Although small veins confer greater nutritional benefits and have no lignified barriers, there are advantages to feeding on the larger veins for those more mature aphids which can penetrate the sclerenchyma. Feeding on the large veins of the upper surface, which are situated in grooves, may provide protection against dislodgement. On the undersurfaces of small leaves, feeding on large veins promotes orientation towards the petiole and early warning of approaching predators. On the corresponding surfaces of large leaves the sclerenchyma is thicker but aphids can achieve orientation by feeding on the smallest veins.     

Ca2+ as developmental signal in the formation of Ca-oxalate crystal spacing patterns during leaf development inCarya ovata

Changes in the spacing patterns of Ca-oxalate crystals during enlargement ofCarya ovata Mill. leaves were quantified by computerized image-analysis. Single Ca-oxalate crystals form in the vacuoles of young mesophyll cells transformed into crystal cells Crystals are very small in newly induced crystal cells and increase in size throughout leaf development. Crystal patterns thus reflect both induction and relative age of crystal cells. Shortly after the emergence of young leaves from the bud, very small crystals are formed in the mesophyll at high density. As leaves expand, these crystals grow larger and become separated by increasing distances. New small crystals appear in the gaps between the older, larger crystals. Later crystal patterns consist of widely spaced, larger crystals only. Finally, clusters of small crystals are formed again in the gaps between large crystals. No crystals were observed in young leaves expanding in a moist chamber, but large numbers of crystal cells were induced experimentally in sections of immature leaves floating on 4 mM Ca-acetate. The observations support the following mechanism of crystal-pattern formation: Ca²⁺ carried into leaves with the transpiration stream acts as the developmental signal inducing transdifferentiation of a few mesophyll cells into crystal cells when apoplastic [Ca²⁺] rises. Crystal cells precipitate absorbed Ca²⁺ as oxalate and, acting as Ca²⁺ sinks, inhibit crystal-cell induction in their vicinity by depleting apoplastic Ca²⁺. This prevents close spacing of crystal cells. New crystal cells form in the gaps between the depletion zones of older crystal cells when these move apart during leaf expansion. Later changes in crystal patterns result from increasing sink strength of crystal cells, lowered inducibility of mesophyll cells, and increased Ca²⁺ influx into leaves during intensive transpiration. Throughout leaf development, spacing of crystal cells permits rapid secretion of apoplastic Ca²⁺ as Ca-oxalate. Dedicated to Professor Erwin Bünning, University of Tübingen, Germany, who pioneered the analysis of spacing patterns     

AGGREGATION IN APHIS FABAE SCOP. I. AGGREGATION ON PLANTS

A study of free infestations of Aphis fabae Scop. on the leaves of its host plants, spindle and sugar beet, showed that the clustered distribution of the aphids within the boundaries of each leaf was not a matter of chance but involved active aggregation. Experiments in small leaf cages showed that aggregation could take place independently of any differences between portions of a leaf, being brought about by truly gregarious reactions between the aphids. Experiments in which walking aphids were made to encounter other live or dummy aphids fixed to leaves, showed that the gregarious reactions were of at least two kinds. The aphids both probed more often, and remained inserted for longer, near other aphids than apart from them.
It appears that the unequal distribution of aphids among the leaves of a plant is due primarily to intrinsic differences between leaves, but is aided by gregariousness, while the aggregations observed on leaves are due primarily to gregariousness, and only secondarily to differences between portions of a leaf. It is suggested that the primary function of gregariousness is to concentrate the aphids on to the best feeding sites, both among the parts of a plant, and among whole plants; that is, it acts as an aid in host selection, even with aphids which do not aggregate so compactly as A. fabae. The functions of compact aggregation and other specialized forms of gregarious behaviour remain to be discovered.     

Aggregation in the lime aphid (Eucallipterus tiliae L.)

Summary Aggregation, shown by both nymphal and adult lime aphids, is not due to mutual tendency to select particular feeding sites, but is due to social aggregation, the aphids themselves being the attractive stimulus. In adults aggregation comes about through responses to visual stimuli from the wing patterns of other adults. Antennal contact between adults does not appear to be important in inducing settling, indicating that possibly visual or olfactory stimuli may be involved. With increased density closer spacing takes place resulting in increased antennal contact between adults and the possible significance of this is discussed. Closer spacing is not simply a result of limited space available at high densities but is self imposed by the adults themselves.     

Time allocation of the parasitoidAphidius colemani (Hymenoptera: Aphidiidae) foraging forAphis gossypii (Homoptera: Aphidae) on cucumber leaves

The time allocation of individualAphidius colemani female parasitoids foraging forAphis gossypii nymphs on cucumber leaves has been investigated. Apart from experiences on the current leaf (such as density of hosts on the current leaf, density of hosts on a neighboring leaf and encounters with hosts on the current leaf), the effect of previous leaf visits on the time allocation was studied. Behavioral records were analyzed by means of the proportional hazards model, to determine the tendency of leaving the current leaf. The leaving tendency decreased only on leaves with a high host density (100 aphids), thus increasing the giving up time since the latest encounter. Rejection of aphids had no influence on the leaving tendency. To assess the effect of the number of hosts encountered on the leaving tendency, we considered three classes: 0–30 encounters, 31–100 encounters, and 100 or more encounters with hosts. The effect of the number of hosts encountered differed at different aphid densities. When fewer than 10 aphids were present the leaving tendency was much greater after 30 encounters than beforehand. At a density of 100 aphids the leaving tendency was lower than at the other aphid densities and increased only after 100 encounters. The density of hosts on a neighboring leaf, ranging from 0 to 100 hosts, had a negligible effect on the leaving tendency. Repeated visits to leaves with 10 unparasitized aphids resulted in an increase in the leaving tendency after 10 visits. It is argued that the parasitoids have some innate expectancy of host availability and that they concentrate on high-density patches.     

The pathways of water movement in leaves modified into tents by bats

A number of species of bats modify leaves into tents, which they use as roost-sites. Through this process, some areas of the leaf lamina are damaged or become detached from the midrib. Such injuries do not cause death of the leaf or the detached areas, indicating that water supply to these areas must be maintained. We examined the anatomy of the vascular systems and water transport in the leaves of three species of plants: Heliconia pogonantha L., Manicaria plukenetii Griseb. & H. Wendl., and Cryosophila warcsewiczii (H. Wend.) Bartlett. In altered leaves of all three species, detached areas of the laminae were supplied with water by minor transverse veins branching from the first major parallel vein that remained intact next to the cut. These transverse veins conducted water through single xylem elements of narrow diameter (approximately 10 urn) previously thought to supply water only to mesophyll cells in their immediate vicinity. The short lengths of these veins compensates their high resistance to water flow (a consequence of their small diameter xylem elements), indicating that small transverse veins have a large capacity for water transport. Water typically flowed through transverse veins into detached major and minor parallel veins, filled these parallel veins in both directions (i.e. toward the midrib and the leaf edge), and continued on into subsequent transverse and parallel veins, thereby supplying water to the entire leaf. Water conduction through these small transverse veins could support large areas of leaf lamina, keeping the leaf-tent alive for at least several months. The maintenance of the flow of water and nutrients to areas of leaves detached by bats during the tent-making process increases the longevity and decreases the conspicuousness of leaf-tents, and is likely a key factor in the success of this roosting strategy.     

Water Transport in Impaired Leaf Vein Systems

Abstract: The subject of our investigation was the water regime of broad bean leaves ( Vicia faba L.), especially after having mechanically severed parts of the leaf blade and the leaf venation. Under moderate conditions, 18 - 22 °C temperature and 50 - 70 % relative humidity, the leaves remained viable even after extensive damage. Only if more than 90 % of the xylem cross sectional area of a leaf was severed, the leaf wilted. Lesser damage to the xylem cross-sectional area only resulted in a reduced rate of transpiration and assimilation, compared to intact leaves. The cuts in larger veins were bypassed into small or even very small veins, as shown by xylem transport of dyes. In intact leaves, small veins have a negligible task in long-distance transport. Here, however, transport velocity in small veins was severalfold increased compared to the measurement of transport velocity in veins of the same size in intact leaves. Thereby, water transport to leaf areas distal from the cut was ensured.     

THE DISTRIBUTION OF APHID INFESTATION IN RELATION TO LEAF AGE

Infestations of apterous Aphis fabae Scop, on potted sugar beets have been followed in detail for several weeks. The plants were somewhat stunted and their crowns presented an unusually complete series of leaf ages. Records were kept of the changing number and size of the leaves and of their stage of growth. Parallel records were kept of the changing population of aphids on every leaf, and the figures are analysed in various ways to show how suitability for the aphids varied through the life cycle of the leaves.
The leaves were very suitable when young, became unsuitable as they matured, became suitable again just after maturity and then unsuitable again as they senesced. But among leaves at any given stage, those which were growing or senescing rapidly were more suitable than those changing slowly, unless the rate of senescence was very high. The differences of population density on different-aged leaves were due largely to the preferences exercised by the apterous adults. The added effect of differences in the fecundity of these mothers while feeding on different leaves was not excluded, but could not be assessed. It is concluded that the physiological development of the plant as a whole determines, through the growth and senescence among its total complement of leaves, the progress and pattern of its aphid infestation.     

METHODS FOR MASS REARING AND INVESTIGATING THE HOST RELATIONS OF APHIS FABAE SCOP.

A method is described for producing some hundreds of apterous and thousands of alate virginoparous Aphis fabae every day continuously throughout the year. Broad beans are used as the host plants, in a 'production-line' arrangement of eleven ventilated breeding cages, with additional cages for starting subcultures and rearing alatae, all contained in a portion of a small greenhouse with supplementary lighting in winter. When running smoothly, the culture requires about half an hour's work by one man per day.
An 'aphid gun', being a modified type of aspirator catcher, is described, with which large numbers of aphids could be handled and counted rapidly without damaging them.
Amodified technique for host-transference experiments is described, which permits individual leaves to be compared one with another as host-units for the aphids. Two types of small metal cages are used, both easily attached to leaves in their natural positions on growing plants. One type confines the aphids to a part of a single leaf and is used mainly for determining relative reproduction rates on different leaves. The other type encloses equal areas of two different leaves between which the aphids can move freely, and is used for determining their behavioural preferences. The principles underlying the use of such cages, the experimental procedures and the interpretation of the results obtained are discussed.     

HOST FINDING BY APHIDS IN THE FIELD: APHIS FABAE SCOP. (GYNOPARAE) AND BREVICORYNE BRASSICAE L.; WITH A RE-APPRAISAL OF THE ROLE OF HOST-FINDING BEHAVIOUR IN VIRUS SPREAD

Aphis fabae gynoparae occurred in the same large proportion in simultaneous collections of all aphids alighting and probing on, and taking off from, a host plant (spindle) and a non-host (peach), and behaved similarly when approaching and leaving them in the same conditions. Most alighters took off again from leaves of both kinds within a few minutes, staying longer and probing more times on the host. In atmospheric conditions favouring local 'hovering' instead of dispersal, flying and alighting aphids became concentrated around host plants, not through any specific attraction to them, but apparently because more aphids had accumulated upon them and were now taking off.
Brevicoryne brassicae occurred in the same large proportion in simultaneous collections of aphids alighting on a host plant (cabbage) and a non-host (sugar beet). No satisfactory evidence was found of preferential alightment on cabbage and there were indications of preferential alightment on the non-host. A very small minority of the A. fabae and B. brassicae that alighted on their hosts stayed there long enough to larviposit. This minority was rather larger among alighters late in the day, but in the absolute sense, more colonization occurred during earlier periods when more aphids arrived.
The intensely dispersive type of host-finding behaviour in Myzus persicae, A. fabae and B. brassicae may be common among Aphididae. It seems ideal for the dissemination of non-persistent plant viruses, more particularly among the less-favoured host plants of each aphid. The tendency to commensal association between virus and vector provides an ecological framework which may govern the incidence of virus-vector specificity and symbiosis.     

The scaling of leaf area and mass: the cost of light interception increases with leaf size

For leaves, the light-capturing surface area per unit dry mass investment (specific leaf area, SLA) is a key trait from physiological, ecological and biophysical perspectives. To address whether SLA declines with leaf size, as hypothesized due to increasing costs of support in larger leaves, we compiled data on intraspecific variation in leaf dry mass (LM) and leaf surface area (LA) for 6334 leaves of 157 species. We used the power function LM=alpha LAbeta to test whether, within each species, large leaves deploy less surface area per unit dry mass than small leaves. Comparing scaling exponents (beta) showed that more species had a statistically significant decrease in SLA as leaf size increased (61) than the opposite (7) and the average beta was significantly greater than 1 (betamean=1.10, 95% CI 1.08-1.13). However, scaling exponents varied markedly from the few species that decreased to the many that increased SLA disproportionately fast as leaf size increased. This variation was unrelated to growth form, ecosystem of origin or climate. The average within-species tendency found here (allometric decrease of SLA with leaf size, averaging 13%) is in accord with concurrent findings on global-scale trends among species, although the substantial scatter around the central tendency suggests that the leaf size dependency does not obligately shape SLA. Nonetheless, the generally greater mass per unit leaf area of larger than smaller leaves directly translates into a greater cost to build and maintain a unit of leaf area, which, all else being equal, should constrain the maximum leaf size displayed.     

Vascular Anatomy of Angiospermous Leaves,with Special Consideration of the Maize Leaf

In this brief review an attempt has been made to discuss some of the important features of the vascular anatomy of angiospermous leaves, especially those related to assimilate transport. Accordingly, emphasis has been placed on the small or minor veins, which are closely related spatially to the mesophyll, and which play a major role in the uptake and subsequent transport of photosynthates from the leaf. The small veins are enclosed by bundle sheaths that intervene between the mesophyll and vascular tissues and greatly increase the area for contact with mesophyll cells. In the minor veins of dicotyledonous leaves, parenchymatic cells having organelle-rich protoplasts and numerous cytoplasmic connections with sieve elements dominate quantitatively. It is these so-called intermediary cells that apparently are directly involved with the loading of assimilates into the sieve elements. In the maize leaf the small and intermediate bundles have two types of sieve tubes, relatively thin-walled ones that have numerous cytoplasmic connections with companion cells, and thick-walled ones that lack companion cells but have numerous connections with vascular parenchyma cells. The companion cell-sieve tube complexes are virtually isolated symplastically from other cells of the vascular bundle and from the bundle sheath. Thick-walled sieve tubes similar to those in the maize leaf have been recorded in the leaves of other grasses.     

Distribution of hydroxamic acids in zea mays tissues

The hydroxamic acid content of leaves of cereals correlates well with resistance to aphids. In maize these compounds were absent from xylem exudates and guttation drops. Lateral veins of leaves of 7-day-old maize plants contained 8 mmol/kg fr. wt. while the entire leaf contained only 4.2 mmol/kg fr. wt. In leaves of 20-day-old plants, these amounts decreased by ca one-third. In mesocotyls, the cortex and central vascular cylinder contained 1.3 and 2.2 mmol/kg fr. wt, respectively. In 12-day-old wheat plants, the complete leaves and their veins contained 2.4 and 6.4 mmol/kg fr. wt respectively. Thus, the concentration of hydroxamic acid was always higher in the vascular bundles.     

Proteomics analysis reveals marker proteins for minor vein initiation in rice leaf

Leaf veins play a critical role in resource supplication and photosynthate translocation; thus, it is considered as an important agricultural trait for crop breeding. The rice minor veins are parallelly grown along all the parts of the leaf from base to tip. To understand the process of minor vein development, anatomy analysis was performed to reveal the initiation and development of minor veins in rice leaf. The frequency of minor vein initiation follows a decreased tendency from leaf base to tip. An iTRAQ-based proteomics analysis was performed in rice leaf sections. Photosynthesis- and carbon fixation-related proteins accumulated a high level in the middle part of leaves. Furthermore, marker proteins involved in sucrose degradation and starch synthesis were accumulated into initiation and mature parts of minor veins, respectively. It suggests a different source-sink activity in the initiation and mature parts of minor veins in terms of photosynthate translocation. The identified proteins are candidate markers for small vein initiation in rice leaves.     

Microbial impacts on plant-herbivore interactions: the indirect effects of a birch pathogen on a birch aphid

The role of indirect interactions in structuring communities is becoming increasingly recognised. Plant fungi can bring about changes in plant chemistry which may affect insect herbivores that share the same plant, and hence the two may interact indirectly. This study investigated the indirect effects of a fungal pathogen ( Marssonina betulae) of silver birch ( Betula pendula) on an aphid ( Euceraphis betulae), and the processes underpinning the interaction. There was a strong positive association between natural populations of the aphid and leaves bearing high fungal infection. In choice tests, significantly more aphids settled on leaves inoculated with the fungus than on asymptomatic leaves. Individual aphids reared on inoculated leaves were heavier, possessed longer hind tibiae and displayed enhanced embryo development compared with aphids reared on asymptomatic leaves; population growth rate was also positively correlated with fungal infection when groups of aphids were reared on inoculated branches. Changes in leaf chemistry were associated with fungal infection with inoculated leaves containing higher concentrations of free-amino acids. This may reflect a plant-initiated response to fungal attack in which free amino acids from the degradation of mesophyll cells are translocated out of infected leaves via the phloem. These changes in plant chemistry are similar to those occurring during leaf senescence, and are proposed as the mechanistic basis for the positive interaction between the fungus and aphid.     

Hydraulic architecture of leaf venation in Laurus nobilis L.

Veins are the main irrigation system of the leaf lamina and an understanding of the hydraulic architecture of the vein networks is essential for understanding leaf function. However, determination of leaf hydraulic parameters is challenging, because for most leaves the vein system is reticulate, contains a hierarchy of different vein sizes, and consists of leaky conduits. We present a new approach that allows for measurements of pressure differences between the petiole and any vein within the leaf. Measurements of Laurus nobilis leaves indicate that first‐ and second‐order veins have high axial conductance and relatively small radial permeability, thus allowing water to reach distal areas of the leaf with only a small loss of water potential. Higher order veins tend to be more hydraulically resistant and permit greater radial leakage. This design allows for a relatively equitable distribution of water potential and thus reflects the capacity of the venation to provide a relatively homogeneous water supply across the leaf lamina, with only the leaf margins being hydraulically disadvantaged relative to the rest of the leaf.     

Leaf water 18O and 2H maps show directional enrichment discrepancy in Colocasia esculenta

Spatial patterns of leaf water isotopes are challenging to predict because of the intricate link between vein and lamina water. Many models have attempted to predict these patterns, but to date, most have focused on monocots with parallel veins. These provide a simple system to study, but do not represent the majority of plant species. Here, a new protocol is developed using a Picarro induction module coupled to a cavity ringdown spectrometer to obtain maps of the leaf water isotopes (¹⁸O and ²H). The technique is applied to Colocasia esculenta leaves. The results are compared with isotope ratio mass spectrometry. In C. esculenta, a large enrichment in the radial direction is observed, but not in the longitudinal direction. The string‐of‐lakes model fails to predict the observed patterns, while the Farquhar–Gan model is more successful, especially when enrichment is accounted for along the radial direction. Our results show that reticulate‐veined leaves experience a larger enrichment along the axis of the secondary veins than along the midrib. We hypothesize that this is due to the lower major/minor vein ratio that leads to longer pathways between major veins and sites of evaporation.     

寄主植物叶片物理性状对潜叶昆虫的影响

潜叶昆虫广泛分布于鳞翅目、双翅目、鞘翅目和膜翅目中,其幼虫潜入叶片内部生活和取食,是一类用于研究植物-昆虫-天敌种间关系和协同进化的重要模式生物。有些潜叶昆虫是重要农林害虫。相比外食性昆虫,在叶内取食的潜叶昆虫幼虫更易受到叶片物理性状的直接影响。叶片的着生位置、朝向、大小、颜色和表皮毛等直接决定潜叶虫成虫的取食和产卵选择,从而影响幼虫的空间分布和寄主适应。叶片的某些物理性状也会直接影响幼虫的取食行为、生长发育和被寄生率。研究叶片物理性状的防御作用以及潜叶昆虫对这些防御的适应,有助于了解潜叶昆虫-寄主植物的协同进化。另一方面,外界环境和遗传育种都有可能改变植物叶片的物理特性,而对潜叶害虫产生抗性,从而实现潜叶害虫的可持续生态控制。     

Resource selection by female moths in a heterogeneous environment: what is a poor girl to do?

1. According to the preference-performance hypothesis, female insects select resources that maximize offspring performance. To achieve high fitness, leaf miner females should then adjust their oviposition behaviour in response to leaf attributes signalling high host quality. 2. Here we investigate resource selection in Tischeria ekebladella, a leaf-mining moth of the pedunculate oak (Quercus robur), in relation to two alternative hypotheses: (1) females select their resources with respect to their future quality for developing larvae; or (2) temporal changes in resource quality prevent females from selecting the best larval resources. 3. Specifically, we test whether females show the strongest selection at the levels at which quality varies the most (shoots and leaves); whether they respond to specific leaf attributes (leaf size, phenolic content and conspecific eggs); and whether female preference is reflected in offspring performance. 4. Female choice of leaves was found to be non-random. Within trees, the females preferred certain shoots, but when the shoots were on different trees the degree of discrimination was about four times larger than when they were on the same trees. 5. While females typically lay more eggs on large leaves, this is not a result of active selection of large leaves, but rather a result of females moving at random and ovipositing at regular intervals. 6. The females in our study did not adjust their oviposition behaviour in response to leaf phenolic contents (as measured by the time of larval feeding). Neither did they avoid leaves with conspecific eggs. 7. Female choice of oviposition sites did not match patterns of offspring performance: there was no positive association between offspring survival and counts of eggs. 8. We propose that temporal variation in resource quality may prevent female moths from evaluating resource quality reliably. To compensate for this, females may adopt a risk-spreading strategy when selecting their resources.