Regulated irrigation of woody ornamentals to improve plant quality and precondition against drought stress

Regulated irrigation has the potential to improve crop quality in woody ornamentals by reducing excessive vigour and promoting a more compact habit. This research aimed to compare the effectiveness and the mode of action of two techniques, regulated deficit irrigation (RDI) and partial root drying (PRD), when applied to container-grown ornamentals through drip irrigation. Results showed that RDI and PRD reduced growth in Cotinus coggygria 'Royal Purple', but in Forsythia x intermedia 'Lynwood', significant reductions were recorded only with RDI. Physiological measurements in Forsythia indicated that reductions in stomatal conductance (g(s)) occurred in both treatments, but those in the RDI tended to be more persistent. Reduced g(s) in PRD was consistent with the concept that chemical signals from the root can regulate stomatal aperture alone; however, the data also suggested that optimising the growth reduction required a moderate degree of shoot water deficit (i.e. a hydraulic signal to be imposed). As RDI was associated with tissue water deficit, it was used in a second experiment to determine the potential of this technique to precondition container-grown plants against subsequent drought stress (e.g. during retail stages or after planting out). Speed of acclimation would be important in a commercial context, and the results demonstrated that both slow and rapid imposition of RDI enabled Forsythia plants to acclimate against later drought events. This article discusses the potential to both improve ornamental plant quality and enhance tolerance to subsequent adverse conditions through controlled, regulated irrigation.

Are differences in life history parameters of the pine beauty moth Panolis flammea modified by host plant quality or gender?

Pine beauty moth (Panolis flammea D&S, Lepidoptera: Noctuidae) were reared individually from egg hatch to pupation on one of three host plants, Pinus sylvestris (native host plant), Pinus contorta (Central Interior seed origin - good quality introduced host) and P. contorta (Alaskan seed origin - poor quality introduced host). After emerging from the pupae the adult moths were confined to a Skeena River seed origin of P. contorta. Female pupal weight and adult life span were significantly higher on P. sylvestris than on the two lodgepole pine seed origins. Development time was, however, not significantly different between treatments, but larval mean relative growth rate was found to be negatively correlated with birth weight and positively correlated with pupal weight. The time to emerge from the pupa was also not significantly different between treatments. However, there were marked differences between the genders. Male moths lost a significantly greater proportion of their weight over the pupal stage but lived significantly longer as adults than the females. Female moths emerged from the pupal stage significantly sooner than male moths. There was no apparent advantage of lai-ge birth size when looked at in terms of subsequent performance. These results are discussed in light of current life history theory.

Plant genotype affects the quality of oilseed rape (Brassica napus) for adults and larvae of the pollen beetle (Meligethes aeneus)

Plant quality is one of the main factors influencing the fitness of phytophagous insects. Plant quality can vary not only among genotypes of the same host plant species, but also relative to the insect sex or its life stage. In the present study, the performance of larvae and adults of the pollen beetle (Meligethes aeneus F., Coleoptera: Nitidulidae), a major insect pest of oilseed rape crops, is compared on six genotypes of oilseed rape (Brassica napus). All of the traits that are measured vary among genotypes, and comprise larval developmental duration, life span of unfed emerging adults and survival time of field-sampled adults fed with pollen from the different genotypes. No correlation is found between insect performance and quantity of food available, showing that the quality of the food (i.e. pollen) is the fitness determinant for this insect species. Additionally, the performance of larvae and adults is also not correlated despite use of the same plant genotypes, suggesting that the determinants of pollen quality differ at least partially between both life stages. It is hypothesized that this may be a result of extensive differences in diet breadth between the life stages: larvae are specialists of brassicaceous plants, whereas adults are generalists. Finally, it is suggested that the manipulation of plant quality to increase pollen beetle development time may comprise a valuable strategy for favouring biological control by natural enemies of this pest; for example, as a result of extending the vulnerability window of larvae to attack by parasitoids.

Influence of host plant heterogeneity on the distribution of a birch aphid

1. The spatial and temporal abundance of the aphid Euceraphis betulae was investigated in relation to heterogeneity in host plant ( Betula pendula) vigour and pathogenic stress. The performance of aphids feeding on vigorous and stressed foliage was also examined. 2. The plant stress and plant vigour hypotheses have been suggested as opposing ways in which foliage quality influences herbivore abundance. In many plants, however, vigorous growing foliage co-exists with stressed or damaged foliage. 3. There was a negative correlation between branch growth ( vigour) and branch stress ( leaf chlorosis), with the most vigorous branches displaying little or no stress, and the most stressed branches achieving poor growth. There was a similar negative correlation between vigour and stress at the level of individual trees, which themselves represented a continuum in quality. 4. At the beginning of the season, E. betulae were intermittently more abundant on vigorous branches than on branches destined to become stressed, but aphids became significantly more abundant on stressed branches later in the season, when symptoms of stress became apparent. Similar patterns of aphid abundance were seen on vigorous and stressed trees in the following year. 5. Euceraphis betulae performance was generally enhanced when feeding on naturally stressed B. pendula leaves, but there was some evidence for elevated potential reproduction when feeding on vigorous leaves too. 6. Overall, plant stress probably influences E. betulae distribution more than plant vigour, but the temporal and spatial variability in plant quality suggests that plant vigour could play a role in aphid distribution early in the season.

Regulation of plant growth in container-grown ornamentals through the use of controlled irrigation

The aim of this research was to determine whether shoot growth could be regulated and plant quality improved through two controlled irrigation techniques: Regulated Deficit Irrigation (RDI) or Partial Root Drying (PRD). An additional benefit of such techniques is that they would also improve the efficiency of irrigation application and reduce the volume of water used on commercial nurseries. Results from two ornamental woody plant species (Cotinus and Forsythia) demonstrated that plant quality could be significantly improved when RDI was applied at ≤ 60% of potential evapo-transpiration (ETp). Stomatal closure and reduced leaf and internode growth rates were associated with both the RDI and PRD techniques, but reduced leaf water potential was only recorded in the RDI system. Changes in xylem sap pH and ABA concentrations were correlated with changes in shoot physiology, and thought to be generated by those roots exposed to drying soil. By adopting such controlled irrigation systems on commercial holdings it is estimated that water consumption could be reduced by 50 to 90%.

Between-population outbreeding affects plant defence

Between-population crosses may replenish genetic variation of populations, but may also result in outbreeding depression. Apart from direct effects on plant fitness, these outbreeding effects can also alter plant-herbivore interactions by influencing plant tolerance and resistance to herbivory. We investigated effects of experimental within- and between-population outbreeding on herbivore resistance, tolerance and plant fitness using plants from 13 to 19 Lychnis flos-cuculi populations. We found no evidence for outbreeding depression in resistance reflected by the amount of leaf area consumed. However, herbivore performance was greater when fed on plants from between-population compared to within-population crosses. This can reflect outbreeding depression in resistance and/or outbreeding effects on plant quality for the herbivores. The effects of type of cross on the relationship between herbivore damage and plant fitness varied among populations. This demonstrates how between-population outbreeding effects on tolerance range from outbreeding depression to outbreeding benefits among plant populations. Finally, herbivore damage strengthened the observed outbreeding effects on plant fitness in several populations. These results raise novel considerations on the impact of outbreeding on the joint evolution of resistance and tolerance, and on the evolution of multiple defence strategies.

The impact of plant diversity and fertilization on fitness of a generalist grasshopper

In many environments land use intensification is likely to result in a decrease in species richness and in an increase in eutrophication. Although the importance of both factors for higher trophic levels such as insect herbivores is well documented, their impact has rarely been studied in combination. Herbivorous insects have a strong impact on the functioning of ecosystems and it is therefore important to understand how they are affected by eutrophication in high or low diversity environments. We used a grassland biodiversity experiment to investigate the combined effect of fertilization and plant diversity loss on the fitness of the generalist grasshopper Chorthippus parallelus by rearing grasshopper nymphs for four weeks in cages on unfertilized or fertilized (NPK) subplots across a species richness gradient from 1 to 60 plant species. Survival, the number of oothecae, body mass and the number of hatchlings were measured separately for each cage. Plant diversity had no effect on any of the grasshopper fitness measures, neither in unfertilized nor in fertilized plots. NPK-fertilization reduced grasshopper survival but increased body mass of males and reproductive success of the surviving females. Fertilization effects were not mediated by plant community structure, productivity or composition, suggesting that higher food plant quality was one of the main drivers. There was no interaction between plant diversity and fertilization on any of the measures. In conclusion, an increase in eutrophication, in both species-rich and species-poor grasslands, could lead to higher reproductive success and therefore higher abundances of herbivorous insects including insect pests, with fertilization effects dominating plant diversity effects.

Sequence of arrival determines plant-mediated interactions between herbivores

Induced changes in plant quality can mediate indirect interactions between herbivores. Although the sequence of attack by different herbivores has been shown to influence plant responses, little is known about how this affects the herbivores themselves. We therefore investigated how induction by the leaf herbivore Spodoptera frugiperda influences resistance of teosinte (Zea mays mexicana) and cultivated maize (Zea mays mays) against root-feeding larvae of Diabrotica virgifera virgifera. The importance of the sequence of arrival was tested in the field and laboratory. Spodoptera frugiperda infestation had a significant negative effect on colonization by D. virgifera larvae in the field and weight gain in the laboratory, but only when S. frugiperda arrived on the plant before the root herbivore. When S. frugiperda arrived after the root herbivore had established, no negative effects on larval performance were detected. Yet, adult emergence of D. virgifera was reduced even when the root feeder had established first, indicating that the negative effects were not entirely absent in this treatment. The defoliation of the plants was not a decisive factor for the negative effects on root herbivore development, as both minor and major leaf damage resulted in an increase in root resistance and the extent of biomass removal was not correlated with root-herbivore growth. We propose that leaf-herbivore-induced increases in feeding-deterrent and/or toxic secondary metabolites may account for the sequence-specific reduction in root-herbivore performance. Synthesis. Our results demonstrate that the sequence of arrival can be an important determinant of plant-mediated interactions between insect herbivores in both wild and cultivated plants. Arriving early on a plant may be an important strategy of insects to avoid competition with other herbivores. To fully understand plant-mediated interactions between insect herbivores, the sequence of arrival should be taken into account. © 2011 The Authors. Journal of Ecology © 2011 British Ecological Society.

Ant-caterpillar Antagonism At The Community Level: Interhabitat Variation Of Tritrophic Interactions In A Neotropical Savanna.

Ant foraging on foliage can substantially affect how phytophagous insects use host plants and represents a high predation risk for caterpillars, which are important folivores. Ant-plant-herbivore interactions are especially pervasive in cerrado savanna due to continuous ant visitation to liquid food sources on foliage (extrafloral nectaries, insect honeydew). While searching for liquid rewards on plants, aggressive ants frequently attack or kill insect herbivores, decreasing their numbers. Because ants vary in diet and aggressiveness, their effect on herbivores also varies. Additionally, the differential occurrence of ant attractants (plant and insect exudates) on foliage produces variable levels of ant foraging within local floras and among localities. Here, we investigate how variation of ant communities and of traits among host plant species (presence or absence of ant attractants) can change the effect of carnivores (predatory ants) on herbivore communities (caterpillars) in a cerrado savanna landscape. We sampled caterpillars and foliage-foraging ants in four cerrado localities (70-460 km apart). We found that: (i) caterpillar infestation was negatively related with ant visitation to plants; (ii) this relationship depended on local ant abundance and species composition, and on local preference by ants for plants with liquid attractants; (iii) this was not related to local plant richness or plant size; (iv) the relationship between the presence of ant attractants and caterpillar abundance varied among sites from negative to neutral; and (v) caterpillars feeding on plants with ant attractants are more resistant to ant predation than those feeding on plants lacking attractants. Liquid food on foliage mediates host plant quality for lepidopterans by promoting generalized ant-caterpillar antagonism. Our study in cerrado shows that the negative effects of generalist predatory ants on herbivores are detectable at a community level, affecting patterns of abundance and host plant use by lepidopterans. The magnitude of ant-induced effects on caterpillar occurrence across the cerrado landscape may depend on how ants use plants locally and how they respond to liquid food on plants at different habitats. This study enhances the relevance of plant-ant and ant-herbivore interactions in cerrado and highlights the importance of a tritrophic perspective in this ant-rich environment.

Larval diet affects number of femoral 'brush' scales in male Helicoverpa punctigera moths (Lepidoptera : Noctuidae)

Males of Helicoverpa punctigera (Wallengren) show considerable variation in the number of femoral scales on the prothoracic legs. Such intraspecific variation in adult morphology could indicate the presence of undetected sibling species, or it may be related to larval diet. Helicoverpa putactigera is polyphagous, and different host plant species are likely to represent diets of different quality. Femoral lengths and the numbers of femoral scales on the prothoracic legs were therefore determined from: (i) individuals that had been collected as larvae from various host species in the field; and (ii) individuals that had been laboratory-reared, in split-family tests, on different diets, namely cotton, lucerne, sowthistle and artificial diet. Host plant species (and therefore presumably diet quality) influenced femoral length of H. punctigera males and, perhaps in conjunction with this, the number of femoral scales on the fore leg. The rearing experiment indicated, in addition, that the effect of host plant quality varies with larval stage, and that the pattern of this variation across the immature stages is dependent on host plant species. The recorded variation in the morphology of field-collected H. punctigera males is therefore most readily explained as a consequence of different individuals developing (at least for most of their larval life) on different host plant species, with diet quality varying significantly with species. The relevance of these results for insect developmental studies and evolutionary interpretations of host relationships is outlined.

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Studies aiming at explaining specialization along latitudinal gradients of plant-herbivore interactions have, to date, yielded inconclusive results. Here we propose the use of steep altitudinal gradients for dissecting factors driving evolution of polyphagy in insect herbivores. First, we test whether colonization of high elevation environment favours increased niche-breadth in two disparate insect groups - the wood-boring beetles and the pollinator group of bees - and show increased polyphagy at higher altitudes in both groups. We then assess classic assumptions transferred from the 'latitude-niche-breadth hypothesis', particularly the increase in environmental variability at high, compared to low, altitude. Finally, we discuss alternative mechanisms shaping the observed pattern of increased polyphagy in altitude, including variation in plant quality and predator pressure at different altitudes. We thus suggest evidence for the 'altitude niche-breadth hypothesis', in which both abiotic and biotic conditions, including increased variability and an increase of the potential feeding niche-breadth, promote evolution for increased insect polyphagy in altitude.

Strawberry yield submitted to different root pruning intensities of transplants

This study aimed to evaluate the growth of plants and the precocity of strawberry production under different root pruning intensities at planting time. Bare roots plants with 12 millimeters crown diameter produced in nurseries from the Patagonia region, Argentina were used. The planting was carried out on May 12th 2010 into experimental plots with non-fumigated soil. The treatments consisted of three cultivars (Camarosa, Florida Festival and Camino Real) and three pruning intensities (1/3, 2/3 and no pruning) on the total root length of the plants. The experimental design used was a randomized block design in a 3x3 factorial arrangement with three replications and 12 plants per plot and density of 11.1 plants m-2. Mature fruits were harvested from July 15th to December 14th 2010 and the production of fresh fruit was determined. There was no significative interaction between cultivars and pruning intensity. 'Camarosa' and 'Florida Festival' plants showed precocity and had the most abundant and heavier fruits during the precocity period. The different root pruning intensities did not affect the assessed variables. It was concluded that, in order to facilitate strawberry planting of the cultivars Camarosa, Florida Festival and Camino Real root pruning is possible, with no damages on plant growth and development, precocity and early fruit production.

Growth and flowering of petunia and impatiens: effects of competition and reduced water content within a container

The primary objective of this research was to determine how the presence of more than one plant and more than one species in a container influence plant quality, particularly when the volume of water given to the container is reduced. Petunia xhybrida 'Hurrah White' and Impatiens 'Cajun Violet' were chosen as typical bedding plant species. Plants were grown in 2 1 containers either under "100% ETp" (i.e., replacing all the water lost by evapotranspiration in the previous 24 h) or under a moisture-restrictive regime of "25% ETp," in which plants received 25% of the "100% ETp" value. An ancillary experiment investigated whether low watering resulted in floral buds being aborted. Results demonstrated that watering requirements of Petunia under "100% ETp" (i.e., replacing all the water lost by evapotranspiration in the previous 24 h) were on average 30% greater than those of Impatiens. However, when two Petunia plants were growing in the same container, the volume of water required to maintain soil moisture content at container capacity was on average only 10% greater than for a single plant. Under a "25% ETp" regime in which plants received 25% of the "100% ETp" value, flower number, plant height, and flower size were reduced by 50%,33%, and 13%,respectively,in Petunia compared with "100% ETp." For example, flower numbers decreased from an average of 71 to 33 flowers per plant in "100% ETp" and "25% ETp," respectively. Petunia plants in the "25% ETp" regime, however, were more efficient at producing both biomass and flowers in relation to the volume of water applied. Petunia plants that experienced both competition from other plants in the container and lower irrigation rates had enhanced efficiency of flower production (i.e., more flowers per unit biomass). For Impatiens, however, the growing of single plants at "25% ETp" was plausible, but the addition of a Petunia plant at "25% ETp" was detrimental to plant quality (Impatiens flower numbers reduced by 75%).

Regulated deficit irrigation - a means to control growth in woody ornamentals

Improving plant quality and the uniformity of a crop are major objectives for growers of ornamental nursery stock. The potential to control excess vigour and to improve quality through regulated deficit irrigation (RDI) was investigated using a range of woody ornamental species. RDI regimes reduced vegetative growth consistently across different species and growing seasons. Plants adapted to reduced water supplies primarily via stomatal control, but also by osmotic adjustment when grown under the most severe RDI regimes. Only plants exposed to <= 25% of potential evapo-transpiration demonstrated any evidence of leaf injury, and the extent was slight. Growth inhibition increased as the severity of RDI increased. Improvements in quality were attained through a combination of shorter internodes and final shoot lengths, yet the number of 'formative' primary shoots remained unaffected. Compact, well-branched plants could be formed without a requirement for mid-season pruning. In addition to severity, the timing of RDI also influenced growth responses. Applying 50% ETp for 8 weeks during July-August resulted in the formation of good quality plants, which retained their shape until the following Spring. Re-positioning irrigation drippers within the pots of well-watered plants, in an attempt to induce a partial root drying (PRD) treatment, reduced growth, but not significantly. The adoption of irrigation scheduling, based on 50-100% ETp, has the potential to improve commercial crop quality across a range of ornamental species.

Relative importance of fertiliser addition to plants and exclusion of predators for aphid growth in the field

Herbivore dynamics and community structure are influenced both by plant quality and the actions of natural enemies. A factorial experiment manipulating both higher and lower trophic levels was designed to explore the determinants of colony growth of the aphid Aphis jacobaeae, a specialist herbivore on ragwort Senecio jacobaea. Potential plant quality was manipulated by regular addition of NPK-fertiliser and predator pressure was reduced by interception traps; the experiment was carried out at two sites. The size and persistence of aphid colonies were measured. Fertiliser addition affected plant growth in only one site, but never had a measurable effect on aphid colony growth. In both habitats the action of insect predators dominated, imposing strong and negative effects on aphid colony performance. Ants were left unmanipulated in both sites and their performance on the aphid colonies did not significantly differ between sites or between treatments. Our results suggest that, at least for aphid herbivores on S. jacobaea, the action of generalist insect predators appears to be the dominant factor affecting colony performance and can under certain conditions even improve plant productivity.