UV radiation effects on pathogens and insect pests of greenhouse-grown crops

Validating Methods To Eradicate Plant-Pathogenic Ralstonia Strains Reveals that Growth In Planta Increases Bacterial Stress Tolerance

Plant-pathogenic bacteria in the Ralstonia solanacearum species complex (RSSC) cause highly destructive bacterial wilt disease of diverse crops. Wilt disease prevention and management is difficult because RSSC persists in soil, water, and plant material. Growers need practical methods to kill these pathogens in irrigation water, a common source of disease outbreaks. Additionally, the R. solanacearum race 3 biovar 2 (R3bv2) subgroup is a quarantine pest in many countries and a highly regulated select agent pathogen in the United States. Plant protection officials and researchers need validated protocols to eradicate R3bv2 for regulatory compliance. To meet these needs, we measured the survival of four R3bv2 and three phylotype I RSSC strains following treatment with hydrogen peroxide, stabilized hydrogen peroxide (Huwa-San), active chlorine, heat, UV radiation, and desiccation. No surviving RSSC cells were detected after cultured bacteria were exposed for 10 min to 400 ppm hydrogen peroxide, 50 ppm Huwa-San, 50 ppm active chlorine, or temperatures above 50°C. RSSC cells on agar plates were eradicated by 30 s of UV irradiation and killed by desiccation on most biotic and all abiotic surfaces tested. RSSC bacteria did not survive the cell lysis steps of four nucleic acid extraction protocols. However, bacteria in planta were more difficult to kill. Stems of infected tomato plants contained a subpopulation of bacteria with increased tolerance of heat and UV light, but not oxidative stress. This result has significant management implications. We demonstrate the utility of these protocols for compliance with select agent research regulations and for management of a bacterial wilt outbreak in the field. IMPORTANCE Bacteria in the Ralstonia solanacearum species complex (RSSC) are globally distributed and cause destructive vascular wilt diseases of many high-value crops. These aggressive pathogens spread in diseased plant material and via contaminated soil, tools, and irrigation water. A subgroup of the RSSC, race 3 biovar 2, is a European and Canadian quarantine pathogen and a U.S. select agent subject to stringent and constantly evolving regulations intended to prevent pathogen introduction or release. We validated eradication and inactivation methods that can be used by (i) growers seeking to disinfest water and manage bacterial wilt disease outbreaks, (ii) researchers who must remain in compliance with regulations, and (iii) regulators who are expected to define containment practices. Relevant to all these stakeholders, we show that while cultured RSSC cells are sensitive to relatively low levels of oxidative chemicals, desiccation, and heat, more aggressive treatment, such as autoclaving or incineration, is required to eradicate plant-pathogenic Ralstonia growing inside plant material.

Technological Advancements and Economics in Plant Production Systems: How to Retrofit?

Plant production systems such as plant factories and greenhouses can help promote resilience in food production. These systems could be used for plant protection and aid in controlling the micro- and macro- environments needed for optimal plant growth irrespective of natural disasters and changing climate conditions. However, to ensure optimal environmental controls and efficient production, several technologies such as sensors and robots have been developed and are at different stages of implementation. New and improved systems are continuously being investigated and developed with technological advances such as robotics, sensing, and artificial intelligence to mitigate hazards to humans working in these systems from poor ventilation and harsh weather while improving productivity. These technological advances necessitate frequent retrofits considering local contexts such as present and projected labor costs. The type of agricultural products also affects measures to be implemented to maximize returns on investment. Consequently, we formulated the retrofitting problem for plant production systems considering two objectives; minimizing the total cost for retrofitting and maximizing the yearly net profit. Additionally, we considered the following: (a) cost of new technologies; (b) present and projected cost for human labor and robotics; (c) size and service life of the plant production system; (d) productivity before and after retrofit, (e) interest on loans for retrofitting, (f) energy consumption before and after retrofit and, (g) replacement and maintenance cost of systems. We solved this problem using a multi-objective evolutionary algorithm that results in a set of compromised solutions and performed several simulations to demonstrate the applicability and robustness of the method. Results showed up to a 250% increase in annual net profits in an investigated case, indicating that the availability of all the possible retrofitting combinations would improve decision making. A user-friendly system was developed to provide all the feasible retrofitting combinations and total costs with the yearly return on investment in agricultural production systems in a single run.

Investigation of UV Dye-Sensitized Solar Cells Based on Water Electrolyte: A New Insight for Wavelength-Selective Greenhouse

The optimization of the photoactive electrode based on TiO2 with a complex architecture for UV dyes along with water-based electrolyte has successfully allowed us (i) to obtain a photovoltaic efficiency of the dye-sensitized solar cell with 1.45 times higher than the best efficiency reported for synthetic dye and 3 times for curcumin dye so far; (ii) transparency on the entire Photosynthetic Active Radiation domain; (iii) preserving high efficiency for lighting 1 sun (summer) and shading, especially for 60 mW/cm2, which represents the maximum illumination in the rest of the seasons. Our water-based dye-sensitized solar cells loaded with synthetic and natural UV dyes have revealed that the implementation of a dye-sensitized solar cell in autonomous greenhouses is a viable and inexpensive concept.

First Multi-Target Application of Exclusion Net in Nectarine Orchards: Effectiveness against Pests and Impact on Beneficial Arthropods, Postharvest Rots and Fruit Quality

Simple Summary

Recently, in fruit orchards, some well-established Integrated Pest Management (IPM) programs are losing their effectiveness and may be compromised by chemicals required to control new invasive pests. In this contest, exclusion nets represent an alternative sustainable control strategy. The use of a photoselective exclusion net was investigated in semi-field conditions as a tool to protect nectarine orchards against different pests in NW Italy. The presence and the abundance of key and new invasive pest populations, as well as the damage they caused on fruits, were evaluated. Moreover, any possible effect of the net on beneficial arthropods, postharvest rots and fruit quality and nutraceutical parameters were considered. The exclusion net significantly reduced pest populations allowing the production of healthier fruits due to a strong reduction of insecticide treatments. Moreover, no negative impact on postharvest rots, neither fruit quality nor nutraceutical properties were recorded.

Abstract

Over the past few years, there has been an increasing interest in the development of alternative pest control strategies to reduce environmental impact. In this contest, exclusion nets have been evaluated as a sustainable alternative to pesticides. In this study, the use of a photoselective exclusion net was investigated in semi-field conditions as a potential strategy to protect nectarine orchards from different pests (i.e., fruit moths, Halyomorpha halys and Drosophila suzukii) in NW Italy. The presence and abundance of pest populations inside and outside the net, as well as the damage they caused on fruits, were evaluated. Moreover, any possible effects of the net on beneficial arthropods, postharvest rots and fruit quality and nutraceutical parameters were considered. The exclusion net significantly reduced pest populations. At harvest, fruit damage caused by Grapholita molesta and H. halys in netted plots was reduced up to 90% and to 78%, respectively, compared with insecticide-treated plots. The exclusion net allowed the production of healthier fruits with a strong reduction of insecticide treatments (up to seven less) and of their related costs without any negative impact on postharvest rots, neither fruit quality nor nutraceutical properties.

UV-LED lights enhance the establishment and biological control efficacy of Nesidiocoris tenuis (Reuter) (Hemiptera: Miridae)

The zoophytophagous mirid Nesidiocoris tenuis (Hemiptera: Miridae) is one of the biological control agents against the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae), a major pest of greenhouse crops. The successful establishment of a biological control agent and its co-occurrence with the target pests increases the efficacy of biological control programs in greenhouses. In this study, we explored the effects of different wavelengths of LED light on establishment of N. tenuis in laboratory condition, with the goal of enhancing the biological control of B. tabaci in greenhouse crops. Nesidiocoris tenuis was most strongly attracted by LED light at a wavelength of 385 nm. This same wavelength was also highly attractive to B. tabaci in Y-tube experiments with lights of specific wavelengths provided is each arm of the apparatus. In trials in growth chambers, we verified the attraction of N. tenuis to 385 nm wavelength. When LED light at a wavelength of 385 nm was used in a growth chamber for 6 hours out of 24 hours, it significantly increased the remaining number of N. tenuis in growth chamber and level of predation compared to treatment with white LED light or without LED light. In conclusion, UV-LED light at a wavelength of 385 nm attracts both B. tabaci and N. tenuis. Thus, it would be used for enhancing early establishment of this mirid bug, better spatial congruence of both mirid bug and whitefly, and better control of the whitefly.

New understanding of the direct effects of spectral balance on behaviour in Myzus persicae

The study of insect responses to colour has mainly focused on flying species and morphs, however colour cues are likely to be important for insect positioning within the canopy. We examine the role of illumination colour in canopy positioning of apterous Myzus persicae (Sulzer) using both a field experiment, utilising various UV-manipulating optical filters, and a laboratory experiment using video tracking of individuals illuminated by a variable intensity UVA-Blue-Green LED-array. In the field experiment, approximately twice as many aphids were located on exposed leaf surfaces under UV-deficient environments compared to UV-rich environments. The lab experiment showed all three M. persicae photoreceptors were involved in a visually-mediated feeding/avoidance behaviour. Highly UV-rich, green-deficient environments were up to 3 times as likely to trigger an avoidance behaviour compared to UV-absent, green-rich environments such as those found below the leaf surface. We show that apterous M. persicae use this, in addition to other cues, in order to locate feeding positions that minimise exposure to direct sunlight. This has relevance to both the fundamental understanding of photoprotective behaviour in Hemiptera as well as to applied research of crop production environments that disrupt pest behaviour.

A perspective on ecologically relevant plant-UV research and its practical application

Plants perceive ultraviolet-B (UV-B) radiation through the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8), and initiate regulatory responses via associated signalling networks, gene expression and metabolic pathways. Various regulatory adaptations to UV-B radiation enable plants to harvest information about fluctuations in UV-B irradiance and spectral composition in natural environments, and to defend themselves against UV-B exposure. Given that UVR8 is present across plant organs and tissues, knowledge of the systemic signalling involved in its activation and function throughout the plant is important for understanding the context of specific responses. Fine-scale understanding of both UV-B irradiance and perception within tissues and cells requires improved application of knowledge about UV-attenuation in leaves and canopies, warranting greater consideration when designing experiments. In this context, reciprocal crosstalk among photoreceptor-induced pathways also needs to be considered, as this appears to produce particularly complex patterns of physiological and morphological response. Through crosstalk, plant responses to UV-B radiation go beyond simply UV-protection or amelioration of damage, but may give cross-protection over a suite of environmental stressors. Overall, there is emerging knowledge showing how information captured by UVR8 is used to regulate molecular and physiological processes, although understanding of upscaling to higher levels of organisation, i.e. organisms, canopies and communities remains poor. Achieving this will require further studies using model plant species beyond Arabidopsis, and that represent a broad range of functional types. More attention should also be given to plants in natural environments in all their complexity, as such studies are needed to acquire an improved understanding of the impact of climate change in the context of plant-UV responses. Furthermore, broadening the scope of experiments into the regulation of plant-UV responses will facilitate the application of UV radiation in commercial plant production. By considering the progress made in plant-UV research, this perspective highlights prescient topics in plant-UV photobiology where future research efforts can profitably be focussed. This perspective also emphasises burgeoning interdisciplinary links that will assist in understanding of UV-B effects across organisational scales and gaps in knowledge that need to be filled so as to achieve an integrated vision of plant responses to UV-radiation.

Light and Microbial Lifestyle: The Impact of Light Quality on Plant–Microbe Interactions in Horticultural Production Systems—A Review

Horticultural greenhouse production in circumpolar regions (>60° N latitude), but also at lower latitudes, is dependent on artificial assimilation lighting to improve plant performance and the profitability of ornamental crops, and to secure production of greenhouse vegetables and berries all year round. In order to reduce energy consumption and energy costs, alternative technologies for lighting have been introduced, including light-emitting diodes (LED). This technology is also well-established within urban farming, especially plant factories. Different light technologies influence biotic and abiotic conditions in the plant environment. This review focuses on the impact of light quality on plant–microbe interactions, especially non-phototrophic organisms. Bacterial and fungal pathogens, biocontrol agents, and the phyllobiome are considered. Relevant molecular mechanisms regulating light-quality-related processes in bacteria are described and knowledge gaps are discussed with reference to ecological theories.

Joint effects of habitat, zooplankton, host stage structure and diversity on amphibian chytrid

Why does the severity of parasite infection differ dramatically across habitats? This question remains challenging to answer because multiple correlated pathways drive disease. Here, we examined habitat-disease links through direct effects on parasites and indirect effects on parasite predators (zooplankton), host diversity and key life stages of hosts. We used a case study of amphibian hosts and the chytrid fungus, Batrachochytrium dendrobatidis, in a set of permanent and ephemeral alpine ponds. A field experiment showed that ultraviolet radiation (UVR) killed the free-living infectious stage of the parasite. Yet, permanent ponds with more UVR exposure had higher infection prevalence. Two habitat-related indirect effects worked together to counteract parasite losses from UVR: (i) UVR reduced the density of parasite predators and (ii) permanent sites fostered multi-season host larvae that fuelled parasite production. Host diversity was unlinked to hydroperiod or UVR but counteracted parasite gains; sites with higher diversity of host species had lower prevalence of infection. Thus, while habitat structure explained considerable variation in infection prevalence through two indirect pathways, it could not account for everything. This study demonstrates the importance of creating mechanistic, food web-based links between multiple habitat dimensions and disease.

Reduced ultraviolet light transmission increases insecticide longevity in protected culture raspberry production

High tunnels are large protective structures used for season extension of many crops, including raspberries. These structures are often covered in plastic films to reduce and diffuse ultraviolet light transmission for pest and disease control, but this may also affect the photodegradation and efficacy of pesticides applied under these tunnels. We compared the residue levels of ten insecticides under three tunnel plastics with varying levels of UV transmission and open field conditions. Raspberry plants placed in research-scale tunnels were treated with insecticides and residues on fruit and foliage were monitored for one or two weeks in early 2015 and early and late 2016. Plastics that reduce UV transmission resulted in 50% greater residues of some insecticides compared to transparent plastics, and 60% compared to uncovered tunnels. This increased persistence of residues was evident within 1 day and remained consistently higher for up to 14 days. This pattern was demonstrated for multiple insecticides, including bifenthrin, esfenvalerate, imidacloprid, thiamethoxam, and spinosad. In contrast, the insecticide malathion degraded rapidly regardless of the plastic treatment, indicating less sensitivity to photodegradation. Bioassays using insecticide-treated leaves that were under UV-blocking plastic revealed higher mortality of the invasive fruit pest, Drosophila suzukii, compared to leaves that were uncovered. This indicates that the activity of pesticides under high tunnels covered in UV-reducing plastics may be prolonged, allowing for fewer insecticide applications and longer intervals between sprays. This information can be used to help optimize pest control in protected culture berry production.

Ecological turmoil in evolutionary dynamics of plant-insect interactions: defense to offence

Available history manifests contemporary diversity that exists in plant-insect interactions. A radical thinking is necessary for developing strategies that can co-opt natural insect-plant mutualism, ecology and environmental safety for crop protection since current agricultural practices can reduce species richness and evenness. The global environmental changes, such as increased temperature, CO₂ and ozone levels, biological invasions, land-use change and habitat fragmentation together play a significant role in re-shaping the plant-insect multi-trophic interactions. Diverse natural products need to be studied and explored for their biological functions as insect pest control agents. In order to assure the success of an integrated pest management strategy, human activities need to be harmonized to minimize the global climate changes. Plant-insect interaction is one of the most primitive and co-evolved associations, often influenced by surrounding changes. In this review, we account the persistence and evolution of plant-insect interactions, with particular focus on the effect of climate change and human interference on these interactions. Plants and insects have been maintaining their existence through a mutual service-resource relationship while defending themselves. We provide a comprehensive catalog of various defense strategies employed by the plants and/or insects. Furthermore, several important factors such as accelerated diversification, imbalance in the mutualism, and chemical arms race between plants and insects as indirect consequences of human practices are highlighted. Inappropriate implementation of several modern agricultural practices has resulted in (i) endangered mutualisms, (ii) pest status and resistance in insects and (iii) ecological instability. Moreover, altered environmental conditions eventually triggered the resetting of plant-insect interactions. Hence, multitrophic approaches that can harmonize human activities and minimize their interference in native plant-insect interactions are needed to maintain natural balance between the existence of plants and insects.

Molecular and physiological effects of environmental UV radiation on fungal conidia

Conidia are specialized structures produced at the end of the asexual life cycle of most filamentous fungi. They are responsible for fungal dispersal and environmental persistence. In pathogenic species, they are also involved in host recognition and infection. Conidial production, survival, dispersal, germination, pathogenicity and virulence can be strongly influenced by exposure to solar radiation, although its effects are diverse and often species dependent. UV radiation is the most harmful and mutagenic waveband of the solar spectrum. Direct exposure to solar radiation for a few hours can kill conidia of most fungal species. Conidia are killed both by solar UV-A and UV-B radiation. In addition to killing conidia, which limits the size of the fungal population and its dispersion, exposures to sublethal doses of UV radiation can reduce conidial germination speed and virulence. The focus of this review is to provide an overview of the effects of solar radiation on conidia and on the major systems involved in protection from and repair of damage induced by solar UV radiation. The efforts that have been made to obtain strains of fungi of interest such as entomopathogens more tolerant to solar radiation will also be reviewed.

Growth under visible light increases conidia and mucilage production and tolerance to UV-B radiation in the plant pathogenic fungus Colletotrichum acutatum

Light conditions can influence fungal development. Some spectral wavebands can induce conidial production, whereas others can kill the conidia, reducing the population size and limiting dispersal. The plant pathogenic fungus Colletotrichum acutatum causes anthracnose in several crops. During the asexual stage on the host plant, Colletototrichum produces acervuli with abundant mucilage-embedded conidia. These conidia are responsible for fungal dispersal and host infection. This study examined the effect of visible light during C. acutatum growth on the production of conidia and mucilage and also on the UV tolerance of these conidia. Conidial tolerance to an environmentally realistic UV irradiance was determined both in conidia surrounded by mucilage on sporulating colonies and in conidial suspension. Exposures to visible light during fungal growth increased production of conidia and mucilage as well as conidial tolerance to UV. Colonies exposed to light produced 1.7 times more conidia than colonies grown in continuous darkness. The UV tolerances of conidia produced under light were at least two times higher than conidia produced in the dark. Conidia embedded in the mucilage on sporulating colonies were more tolerant of UV than conidia in suspension that were washed free of mucilage. Conidial tolerance to UV radiation varied among five selected isolates.

Impact of UV-A radiation on the performance of aphids and whiteflies and on the leaf chemistry of their host plants

Ultraviolet (UV) radiation directly regulates a multitude of herbivore life processes, in addition to indirectly affecting insect success via changes in plant chemistry and morphogenesis. Here we looked at plant and insect (aphid and whitefly) exposure to supplemental UV-A radiation in the glasshouse environment and investigated effects on insect population growth. Glasshouse grown peppers and eggplants were grown from seed inside cages covered by novel plastic filters, one transparent and the other opaque to UV-A radiation. At a 10-true leaf stage for peppers (53 days) and 4-true leaf stage for eggplants (34 days), plants were harvested for chemical analysis and infested by aphids and whiteflies, respectively. Clip-cages were used to introduce and monitor the insect fitness and populations of the pests studied. Insect pre-reproductive period, fecundity, fertility and intrinsic rate of natural increase were assessed. Crop growth was monitored weekly for 7 and 12 weeks throughout the crop cycle of peppers and eggplants, respectively. At the end of the insect fitness experiment, plants were harvested (68 days and 18-true leaf stage for peppers, and 104 days and 12-true leaf stage for eggplants) and leaves analysed for secondary metabolites, soluble carbohydrates, amino acids, total proteins and photosynthetic pigments. Our results demonstrate for the first time, that UV-A modulates plant chemistry with implications for insect pests. Both plant species responded directly to UV-A by producing shorter stems but this effect was only significant in pepper whilst UV-A did not affect the leaf area of either species. Importantly, in pepper, the UV-A treated plants contained higher contents of secondary metabolites, leaf soluble carbohydrates, free amino acids and total content of protein. Such changes in tissue chemistry may have indirectly promoted aphid performance. For eggplants, chlorophylls a and b, and carotenoid levels decreased with supplemental UV-A over the entire crop cycle but UV-A exposure did not affect leaf secondary metabolites. However, exposure to supplemental UV-A had a detrimental effect on whitefly development, fecundity and fertility presumably not mediated by plant cues as compounds implied in pest nutrition - proteins and sugars - were unaltered.

Control of cucurbit viruses

More than 70 well-characterized virus species transmitted by a diversity of vectors may infect cucurbit crops worldwide. Twenty of those cause severe epidemics in major production areas, occasionally leading to complete crop failures. Cucurbit viruses' control is based on three major axes: (i) planting healthy seeds or seedlings in a clean environment, (ii) interfering with vectors activity, and (iii) using resistant cultivars. Seed disinfection and seed or seedling quality controls guarantee growers on the sanitary status of their planting material. Removal of virus or vector sources in the crop environment can significantly delay the onset of viral epidemics. Insecticide or oil application may reduce virus spread in some situations. Diverse cultural practices interfere with or prevent vector reaching the crop. Resistance can be obtained by grafting for soil-borne viruses, by cross-protection, or generally by conventional breeding or genetic engineering. The diversity of the actions that may be taken to limit virus spread in cucurbit crops and their limits will be discussed. The ultimate goal is to provide farmers with technical packages that combine these methods within an integrated disease management program and are adapted to different countries and cropping systems.

Management of air-borne viruses by "optical barriers" in protected agriculture and open-field crops

The incurable nature of viral diseases and the public awareness to the harmful effects of chemical pest control to the environment and human health led to the rise of the integrated pest management (IPM) concept. Cultural control methods serve today as a central pivot in the implementation of IPM. This group of methods is based on the understanding of the complex interactions between disease agents and their vectors as well as the interactions between the vectors and their habitat. This chapter describes a set of cultural control methods that are based on solar light manipulation in a way that interferes with vision behavior of insects, resulting in a significant crop protection against insect pests and their vectored viruses.

Control methods of virus diseases in the Mediterranean basin

Viral pathogens form an important group of obligatory parasites of plants. About 977 plant viruses have been described and classified in 14 families and 70 genera. This group of pathogens has complex interactions with their host plants and vectors due to their integration in the molecular mechanisms of living cells, interfering with our ability to manage the malfunctions of virus infected plants by curing means. These constraints led to the perception that the best protection from virus diseases is by prevention. Many cultural procedures used for virus control are aimed at eradicating or altering one or more of the primary participants in the transmission process (vector, virus source plants, and the crop) or preventing their coming together. Part of these control measures were devised to reduce to a minimum, the number of inoculative vector individuals that are active in the crop or interfere with the transmission process at any of its phases, thereby arresting virus spread. Advances in plant virology and a better understanding of plant vector interactions provide strategies based on the formation of mechanical and optical barriers that interfere with the ability of the viral pathogen or its vector to reach the plant and initiate an epidemic.

UV-B irradiation changes specifically the secondary metabolite profile in broccoli sprouts: induced signaling overlaps with defense response to biotic stressors

Only a few environmental factors have such a pronounced effect on plant growth and development as ultraviolet light (UV). Concerns have arisen due to increased UV-B radiation reaching the Earth's surface as a result of stratospheric ozone depletion. Ecologically relevant low to moderate UV-B doses (0.3-1 kJ m(-2) d(-1)) were applied to sprouts of the important vegetable crop Brassica oleracea var. italica (broccoli), and eco-physiological responses such as accumulation of non-volatile secondary metabolites were related to transcriptional responses with Agilent One-Color Gene Expression Microarray analysis using the 2×204 k format Brassica microarray. UV-B radiation effects have usually been linked to increases in phenolic compounds. As expected, the flavonoids kaempferol and quercetin accumulated in broccoli sprouts (the aerial part of the seedlings) 24 h after UV-B treatment. A new finding is the specific UV-B-mediated induction of glucosinolates (GS), especially of 4-methylsulfinylbutyl GS and 4-methoxy-indol-3-ylmethyl GS, while carotenoids and Chl levels remained unaffected. Accumulation of defensive GS metabolites was accompanied by increased expression of genes associated with salicylate and jasmonic acid signaling defense pathways and up-regulation of genes responsive to fungal and bacterial pathogens. Concomitantly, plant pre-exposure to moderate UV-B doses had negative effects on the performance of the caterpillar Pieris brassicae (L.) and on the population growth of the aphid Myzus persicae (Sulzer). Moreover, insect-specific induction of GS in broccoli sprouts was affected by UV-B pre-treatment.

Ecological responses to UV radiation: interactions between the biological effects of UV on plants and on associated organisms

Solar ultraviolet (UV)-B radiation (280-315 nm) has a wide range of effects on terrestrial ecosystems, yet our understanding of how UV-B influences the complex interactions of plants with pest, pathogen and related microorganisms remains limited. Here, we report the results of a series of experiments in Lactuca sativa which aimed to characterize not only key plant responses to UV radiation in a field environment but also consequential effects for plant interactions with a sap-feeding insect, two model plant pathogens and phylloplane microorganism populations. Three spectrally modifying filters with contrasting UV transmissions were used to filter ambient sunlight, and when compared with our UV-inclusive filter, L. sativa plants grown in a zero UV-B environment showed significantly increased shoot fresh weight, reduced foliar pigment concentrations and suppressed population growth of green peach aphid (Myzus persicae). Plants grown under a filter which allowed partial transmission of UV-A radiation and negligible UV-B transmission showed increased density of leaf surface phylloplane microbes compared with the UV-inclusive treatment. Effects of UV treatment on the severity of two plant pathogens, Bremia lactucae and Botrytis cinerea, were complex as both the UV-inclusive and zero UV-B filters reduced the severity of pathogen persistence. These results are discussed with reference to known spectral responses of plants, insects and microorganisms, and contrasted with established fundamental responses of plants and other organisms to solar UV radiation, with particular emphasis on the need for future integration between different experimental approaches when investigating the effects of solar UV radiation.

Dynamics of nonpersistent aphid-borne viruses in lettuce crops covered with UV-absorbing nets

Aphid-transmitted viruses frequently cause severe epidemics in lettuce grown under Mediterranean climates. Spatio-temporal dynamics of aphid-transmitted viruses and its vector were studied on lettuce (Lactuca sativa L.) grown under tunnels covered by two types of nets: a commercial UV-absorbing net (Bionet) and a Standard net. A group of plants infected by Cucumber mosaic virus (CMV, family Bromoviridae, genus Cucumovirus) and Lettuce mosaic virus (LMV, family Potyviridae, genus Potyvirus) was transplanted in each plot. The same virus-infected source plants were artificially infested by the aphid Macrosiphum euphorbiae (Thomas). Secondary spread of insects was weekly monitored and plants were sampled for the detection of viruses every two weeks. In 2008, the infection rate of both CMV and LMV were lower under the Bionet than under the Standard cover, probably due to the lower population density and lower dispersal rate achieved by M. euphorbiae. However, during spring of 2009, significant differences in the rate of infection between the two covers were only found for LMV six weeks after transplant. The spatial distribution of the viruses analysed by SADIE methodology was "at random", and it was not associated to the spatial pattern of the vector. The results obtained are discussed analyzing the wide range of interactions that occurred among UV-radiation, host plant, viruses, insect vector and environmental conditions. Our results show that UV-absorbing nets can be recommended as a component of an integrated disease management program to reduce secondary spread of lettuce viruses, although not as a control measure on its own.

The protective shell: sclereids and their mechanical function in corollas of some species of Camellia (Theaceae)

Studies of rain-wash effects on pollen have shown that flower structures can protect susceptible pollen from rain. It remains unclear, however, how a thin corolla can withstand external force and perform its protective function. The sclereids in petals of several species of Camellia (Theaceae) were anatomically investigated to determine their mechanical properties. To examine the effects of changing physical environment on the occurrence of sclereids in petals, sclereid density in petals of six species, including wild samples from different rainfall zones and samples from a greenhouse under mild conditions without wind and rain, were examined and statistically analysed. The results showed that the occurrence of sclereids in petals varied with physical environment. The number of sclereids in the same species increased with the increasing rainfall. There were abundant sclereids in petals of the wild species, but few or no sclereids in species cultivated in the greenhouse. Moreover, the anatomical features of sclereids, especially the unique distribution pattern that has not hitherto been described, were correlated with external environmental pressures. Our observations reveal a novel mechanical system in the corolla and provide further evidence for the hypothesis that flower structures may protect rain-susceptible pollen.

Variation in natural plant products and the attraction of bodyguards involved in indirect plant defenseThe present review is one in the special series of reviews on animal–plant interactions

Plants can respond to feeding or egg deposition by herbivorous arthropods by changing the volatile blend that they emit. These herbivore-induced plant volatiles (HIPVs) can attract carnivorous natural enemies of the herbivores, such as parasitoids and predators, a phenomenon that is called indirect plant defense. The volatile blends of infested plants can be very complex, sometimes consisting of hundreds of compounds. Most HIPVs can be classified as terpenoids (e.g., (E)-β-ocimene, (E,E)-α-farnesene, (E)-4,8-dimethyl-1,3,7-nonatriene), green leaf volatiles (e.g., hexanal, (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate), phenylpropanoids (e.g., methyl salicylate, indole), and sulphur- or nitrogen-containing compounds (e.g., isothiocyanates or nitriles, respectively). One highly intriguing question has been which volatiles out of the complex blend are the most important ones for the carnivorous natural enemies to locate "suitable host plants. Here, we review the methods and techniques that have been used to elucidate the carnivore-attracting compounds. Electrophysiological methods such as electroantennography have been used with parasitoids to elucidate which compounds can be perceived by the antennae. Different types of elicitors and inhibitors have widely been applied to manipulate plant volatile blends. Furthermore, transgenic plants that were genetically modified in specific steps in one of the signal transduction pathways or biosynthetic routes have been used to find steps in HIPV emission crucial for indirect plant defense. Furthermore, we provide an overview on biotic and abiotic factors that influence the emission of HIPVs and how this can affect the interactions between members of different trophic levels. Consequently, we review the progress that has been made in this exciting research field during the past 30 years since the first studies on HIPVs emerged and we highlight important issues to be addressed in the future.

Deleterious effects of UV-B radiation on herbivorous spider mites: they can avoid it by remaining on lower leaf surfaces

The herbivorous spider mite Tetranychus urticae usually remains on the lower leaf surfaces of its host plants. Although terrestrial animals are generally thought to be well protected from damage because of UV radiation, insect herbivory frequently increases when solar UV-B (280-315 nm) radiation is attenuated. As UV transmission through leaves is generally low because of the accumulation of compounds that act as selective sunscreens (e.g., phenolics), we hypothesized that T. urticae avoids solar UV-B radiation by staying on lower leaf surfaces. We examined whether artificial UV irradiation and solar UV affected the survival and reproduction of T. urticae and whether staying on lower leaf surfaces was beneficial to their performance under ambient UV radiation. We found that T. urticae was not well protected from UV-B radiation, because artificial UV-B irradiation strongly decreased survivorship and egg production. More importantly; compulsory solar UV irradiation treatments also had lethal effects on T. urticae, whereas the mites could avoid them if they remained on the lower leaf surfaces of their host plants. These results showed that access to habitats protected from sunlight, such as lower leaf surfaces, is likely essential for T. urticae survival under ambient UV-B radiation. The lethal effects of solar UV radiation may also affect the population dynamics of spider mites, and habitat (resource) limitation may increase the probability of interspecific interactions, such as competition and predation. In turn, the occurrence of these interactions in sheltered areas may be associated with observed increases in herbivory under conditions of solar UV-B-attenuation.

A proteomic analysis of the aphid Macrosiphum euphorbiae under heat and radiation stress

Temperature and solar radiation can be important sources of abiotic stress for small herbivorous insects living in close association with plants. We examined the effects of daily fluctuations of heat and UV radiation on the proteome and performance of winged and wingless morphs of the aphid Macrosiphum euphorbiae. A daily regime of 4h of heat stress at 35 degrees C had more negative effects on the aphid's fitness than a similar period of UV-B stress (11.6kJm(-2) per day), and these effects were most pronounced on wingless aphids. Aphid proteomes as detected on 2-D gels revealed approximately 470 protein spots, with the fluctuating heat stress leading to many more changes than exposure to UV-B. The reduced performance of aphids under heat stress correlated with lower abundance of several enzymes in central pathways of energy metabolism, including the TCA cycle and the respiratory chain. Several exoskeletal proteins were induced or their abundance was increased under high temperature stress, suggesting that cuticle barrier enhancement at molting in response to heat stress is an aphid adaptation to stressful thermal conditions. The proteome of winged aphids was more broadly modulated under stress than that of wingless aphids. Greater homeostatic capabilities as revealed at the proteomic level could explain the higher tolerance of the alate aphid morph to environmental stress and its more stable performance and fitness.

Global change effects on plant chemical defenses against insect herbivores

This review focuses on individual effects of major global change factors, such as elevated CO2, O3, UV light and temperature, on plant secondary chemistry. These secondary metabolites are well-known for their role in plant defense against insect herbivory. Global change effects on secondary chemicals appear to be plant species-specific and dependent on the chemical type. Even though plant chemical responses induced by these factors are highly variable, there seems to be some specificity in the response to different environmental stressors. For example, even though the production of phenolic compounds is enhanced by both elevated CO2 and UV light levels, the latter appears to primarily increase the concentrations of flavonoids. Likewise, specific phenolic metabolites seem to be induced by O3 but not by other factors, and an increase in volatile organic compounds has been particularly detected under elevated temperature. More information is needed regarding how global change factors influence inducibility of plant chemical defenses as well as how their indirect and direct effects impact insect performance and behavior, herbivory rates and pathogen attack. This knowledge is crucial to better understand how plants and their associated natural enemies will be affected in future changing environments.

Terrestrial ecosystems, increased solar ultraviolet radiation, and interactions with other climate change factors

There have been significant advances in our understanding of the effects of UV-B radiation on terrestrial ecosystems, especially in the description of mechanisms of plant response. A further area of highly interesting research emphasizes the importance of indirect UV radiation effects on plants, pathogens, herbivores, soil microbes and ecosystem processes below the surface. Although photosynthesis of higher plants and mosses is seldom affected by enhanced or reduced UV-B radiation in most field studies, effects on growth and morphology (form) of higher plants and mosses are often manifested. This can lead to small reductions in shoot production and changes in the competitive balance of different species. Fungi and bacteria are generally more sensitive to damage by UV-B radiation than are higher plants. However, the species differ in their UV-B radiation sensitivity to damage, some being affected while others may be very tolerant. This can lead to changes in species composition of microbial communities with subsequent influences on processes such as litter decomposition. Changes in plant chemical composition are commonly reported due to UV-B manipulations (either enhancement or attenuation of UV-B in sunlight) and may lead to substantial reductions in consumption of plant tissues by insects. Although sunlight does not penetrate significantly into soils, the biomass and morphology of plant root systems of plants can be modified to a much greater degree than plant shoots. Root mass can exhibit sizeable declines with more UV-B. Also, UV-B-induced changes in soil microbial communities and biomass, as well as altered populations of small invertebrates have been reported and these changes have important implications for mineral nutrient cycling in the soil. Many new developments in understanding the underlying mechanisms mediating plant response to UV-B radiation have emerged. This new information is helpful in understanding common responses of plants to UV-B radiation, such as diminished growth, acclimation responses of plants to UV-B radiation and interactions of plants with consumer organisms such as insects and plant pathogens. The response to UV-B radiation involves both the initial stimulus by solar radiation and transmission of signals within the plants. Resulting changes in gene expression induced by these signals may have elements in common with those elicited by other environmental factors, and generate overlapping functional (including acclimation) responses. Concurrent responses of terrestrial systems to the combination of enhanced UV-B radiation and other global change factors (increased temperature, CO2, available nitrogen and altered precipitation) are less well understood. Studies of individual plant responses to combinations of factors indicate that plant growth can be augmented by higher CO2 levels, yet many of the effects of UV-B radiation are usually not ameliorated by the elevated CO2. UV-B radiation often increases both plant frost tolerance and survival under extreme high temperature conditions. Conversely, extreme temperatures sometimes influence the UV-B radiation sensitivity of plants directly. Plants that endure water deficit stress effectively are also likely to be tolerant of high UV-B flux. Biologically available nitrogen is exceeding historical levels in many regions due to human activities. Studies show that plants well supplied with nitrogen are generally more sensitive to UV-B radiation. Technical issues concerning the use of biological spectral weighting functions (BSWFs) have been further elucidated. The BSWFs, which are multiplication factors assigned to different wavelengths giving an indication of their relative biological effectiveness, are critical to the proper conduct and interpretation of experiments in which organisms are exposed to UV radiation, both in the field and in controlled environment facilities. The characteristics of BSWFs vary considerably among different plant processes, such as growth, DNA damage, oxidative damage and induction of changes in secondary chemicals. Thus, use of a single BSWF for plant or ecosystem response is not appropriate. This brief review emphasizes progress since the previous report toward the understanding of solar ultraviolet radiation effects on terrestrial systems as it relates to ozone column reduction and the interaction of climate change factors.

Protecting crops from non-persistently aphid-transmitted viruses: A review on the use of barrier plants as a management tool

Barrier plants are a management tool based on secondary plants used within or bordering a primary crop for the purpose of disease control. Aphid-transmitted viruses account for approximately 50% of the 600 known viruses with an invertebrate vector. Barrier plants may act as real natural sinks for non-persistent aphid-transmitted viruses and have proved in the past to be an effective crop management strategy to protect against virus infection. Increasing the knowledge on aphid host seeking and flying behaviour, and on how barrier plants may affect the behaviour of aphids and their natural enemies will allow further development of this environmentally-friendly habitat manipulation strategy. An ideal plant barrier should be a non-host for the virus and the vector, but appealing to aphid landing and attractive to their natural enemies and should allow sufficient residence time to allow aphid probing before taking-off occurs. In this review, we have addressed why aphids are manageable by barrier cropping, the mechanisms by which barrier plants affect the occurrence of non-persistently aphid-transmitted viruses and the limitations of using barrier plants as a virus control strategy. Finally, we have pointed out future directions of research that should be conducted to integrate barrier cropping with other disease management strategies, and optimise and extend the use of barrier plants as a strategy for managing aphid-transmitted virus diseases.

The use of wavelength-selective plastic cladding materials in horticulture: understanding of crop and fungal responses through the assessment of biological spectral weighting functions

Plant responses to light spectral quality can be exploited to deliver a range of agronomically desirable end points in protected crops. This can be achieved using plastics with specific spectral properties as crop covers. We have studied the responses of a range of crops to plastics that have either (a) increased transmission of UV compared with standard horticultural covers, (b) decreased transmission of UV or (c) increased the ratio of red (R) : far-red (FR) radiation. Both the UV-transparent and R : FR increasing films reduced leaf area and biomass, offering potential alternatives to chemical growth regulators. The UV-opaque film increased growth, but while this may be useful in some crops, there were trade-offs with elements of quality, such as pigmentation and taste. UV manipulation may also influence disease control. Increasing UV inhibited not only the pathogenic fungus Botrytis cinerea but also the disease biocontrol agent Trichoderma harzianum. Unlike B. cinerea, T. harzianum was highly sensitive to UV-A radiation. These fungal responses and those for plant growth in the growth room and the field under different plastics are analyzed in terms of alternative biological spectral weighting functions (BSWF). The role of BSWF in assessing general patterns of response to UV modification in horticulture is also discussed.

Spectral Properties of Selected UV-blocking and UV-transmitting Covering Materials with Application for Production of High-value Crops in High Tunnels†

The spectral properties of selected UV-blocking and UV-transmitting covering materials were characterized by means of a UV-VIS spectroradiometer or a UV-VIS spectrometer to provide researchers and growers with guidelines for selecting suitable materials for use in studying the effects of ambient solar UV radiation on the production of tomatoes and other high-value crops in high tunnels. A survey was made of a wide range of plastic covering materials to identify commercially available products that had the desired characteristics of transmitting high levels of photosynthetically active radiation and of being stable under ambient solar UV radiation. The study was focused on evaluating films that either blocked or transmitted UV wavelengths below 380 nm to determine comparative growth, yield and market quality and to provide a tool for integrated pest management. Based on this survey, two contrasting covering materials of similar thickness (0.152 mm) and durability (4-year polyethylene), one a UV-blocking film and the other a UV-transmitting film, were selected and used to cover two high tunnels at Beltsville, MD. Spectroradiometric measurements were made to determine comparative spectral irradiance in these two high tunnels covered with these materials and under ambient solar UV radiation. Comparative measurements were also made of selected glass and plastic materials that have been used in UV exclusion studies.

Effects of Crude and Partially Purified Extracts from UV-B–irradiated Rice Leaves on Helicoverpa armigera (Hübner)‡

The effect of crude and partially purified extracts from ultraviolet-B (UV-B)-irradiated rice (Oryza sativa L.) leaves on the growth and development of corn earworm, Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), was investigated. Fifty muL droplets of a liquid diet containing different concentrations of the crude and partially purified extracts were fed to H. armigera neonates to determine possible short-term toxicity effects. A choice test using a solid artificial diet was also performed to determine larval feeding preferences and antifeedant effects. To study effects on the life history of the insect, different concentrations of the crude and partially purified extracts were also incorporated in the artificial diet and fed to individually confined neonates of H. armigera. The neonates were reared up to the adult stage. Results showed that crude and partially purified extracts of UV-B-irradiated rice leaves demonstrated antifeedant, growth-inhibitory and antibiotic properties against H. armigera. At high concentrations, the extract initially stimulated larval feeding; however, there were subsequent negative effects on pupal and adult traits, thereby reducing the reproductive potential of adults. These partially purified extracts appeared to have an antifertility effect because adults laid fewer eggs and, of those eggs laid, viability was lower. These results suggest that the accumulated flavonoids or other phenolics in UV-B-irradiated leaves, extracted from UV-B-resistant rice cultivar 'M202,' affected the growth, development and reproduction of H. armigera, a polyphagous insect pest.