Responses of leaf photosynthetic traits to elevated growth [CO2] vary among species, but there is limited understanding of underlying trait trade-offs, especially for tropical species with continuous leaf formation. Persea americana and Annona muricata with significant investments in defense structures (idioblasts) were used to study the impacts of growth [CO2] (400 vs 800 μmol mol-1) on leaf structural, chemical, and photosynthetic characteristics at different leaf developmental stages. Growth at elevated [CO2] increased whole plant leaf area (ST) and whole plant average leaf dry mass per unit area (MAv) in P. americana, whereas both ST and MAv were reduced in A. muricata. Elevated [CO2] moderately reduced foliage N and P contents per dry mass in P. americana but increased in A. muricata. In P. americana, elevated [CO2] increased anthocyanin content in young leaves and decreased in mature leaves, and increased the share of idioblast tissue fraction (fI) with moderate downregulation of photosynthesis (A). In A. muricata, elevated [CO2] reduced anthocyanin content in young leaves and fI was unaffected, whereas a major downregulation in A was observed. In this species, photosynthetic downregulation was not associated with nutrient starvation, but occurred due to direct inhibition of stomatal conductance by elevated [CO2], ultimately limiting leaf development and growth and curbing Vcmax and Jmax in mature leaves. These results demonstrate a limited impact of primary/secondary metabolism trade-off on photosynthetic response to growth [CO2], underscore major species differences in response to elevated [CO2] and emphasize the impact of leaf age in determining whole plant growth response.

Climate change is associated with higher atmospheric [CO2] and more frequent temperature extremes, with the strongest impact expected in the tropics where plants often operate close to their heat stress limit. How the resistance of foliage photosynthetic traits to heat stress varies with [CO2] elevation remains largely unknown, particularly in tropical species with continuously expanding canopies, where the heat resistance of leaves can vary with age. We studied the impact of heat shock stress resembling heatflecks due to fluctuating light (48 °C for 10 min) on foliage physiological traits and chemical contents in young-mature and old-mature foliage of the tropical species Persea americana Mill. plants grown under ambient (400 μmol mol-1) and elevated (800 μmol mol-1) [CO2]. Leaf characteristics were studied through a 48 h recovery period. Light-saturated net assimilation rate (A) decreased with leaf age in both ambient and elevated [CO2]. In young-mature leaves, A in plants grown under elevated [CO2] was greater than A in plants grown under ambient [CO2]. In old-mature leaves, A was similar under both [CO2] and this was associated with increased nutrient limitation under elevated [CO2]. Upon heat stress application, A decreased in all cases due to both reduction in stomatal conductance and inhibition of biochemical photosynthetic capacity (maximum Rubisco carboxylase activity). During recovery, A increased to pre-stress level in all but in young-mature plants grown under ambient [CO2] where A remained much lower (78% reduction) than in control plants. As young leaves have a longer remaining lifespan and higher future potential contribution to plant carbon gain, preservation of photosynthetic capacity in young leaves under elevated [CO2] suggests that elevated [CO2] can enhance long-term photosynthetic production in P. americana exposed to heat episodes.

Climate change enhances the frequency of heatwaves that negatively affect photosynthesis and can alter constitutive volatile emissions and elicit emissions of stress volatiles, but how pre-exposure to mildly warmer temperatures affects plant physiological responses to subsequent severe heat episodes remains unclear, especially for aromatic plants with high and complex volatile defenses. We studied the impact of heat shock (45 °C/5 min) applied alone and after exposure to moderate heat stress (35 °C/1 h, priming) on foliage photosynthesis and volatile emissions in the aromatic plant Origanum vulgare through 72 h recovery period. Heat stress decreased photosynthesis rates and stomatal conductance, whereas the reductions in photosynthesis were primarily due to non-stomatal factors. In non-primed plants, heat shock-induced reductions in photosynthetic activity were the greatest, but photosynthetic activity completely recovered by the end of the experiment. In primed plants, a certain inhibition of photosynthetic activity remained, suggesting a sustained priming effect. Heat shock enhanced the emissions of volatiles including lipoxygenase pathway volatiles, long-chained fatty acid-derived compounds, mono- and sesquiterpenes, geranylgeranyl diphosphate pathway volatiles, and benzenoids, whereas different heat treatments resulted in unique emission blends. In non-primed plants, stress-elicited emissions recovered at 72 h. In primed plants, volatile emissions were multiphasic, the first phase, between 0.5 and 10 h, reflected the primary stress response, whereas the secondary rise, between 24 and 72 h, indicated activations of different defense metabolic pathways. Our results demonstrate that exposure to mild heat leads to a sustained physiological stress memory that enhances plant resistance to subsequent severe heat stress episodes.

Rust infection results in decreases in photosynthesis and stress volatile emissions, but how these changes vary among host species has not been studied. We demonstrated that the impact of the obligate biotrophic fungus, Puccinia coronata f. sp. avenae, on foliage physiological processes is stronger in the primary host, Avena sativa (cultivated oat), than in the alternate host, Rhamnus frangula (alder buckthorn). Photosynthesis decreased with increasing percentage of damaged leaf area (DA) in both species, but reductions were greater in A. sativa. In A. sativa, photosynthetic reductions resulted from reductions in stomatal conductance and photosynthetic capacity; in R. frangula, reductions were due to reduced capacity. Infection reduced photosynthetic biomass and key nutrients in A. sativa, but not in R. frangula. In A. sativa, stress-elicited emissions (methyl jasmonate, green leaf volatiles, long-chain saturated aldehydes, mono- and sesquiterpenes, benzenoids, and carotenoid breakdown products) increased with increasing DA from 0% to 40%, but decreased with further increases in DA. In R. frangula, volatile emissions were slightly elicited but, surprisingly, constitutive isoprene emissions were enhanced. Different hosts had characteristic volatile fingerprints, indicating differential activation of biochemical pathways. Fungal-elicited reductions in photosynthesis scale uniformly with stress severity. In the sensitive host, biphasic scaling of volatiles indicates that heavy spread of chlorosis/necrosis leads to an overall cessation of physiological functioning.

Methyl jasmonate (MeJA) induces various defence responses in seed plants, but for early plant lineages, information on the potential of jasmonates to elicit stress signalling and trigger physiological modifications is limited. The spikemoss Selaginella martensii was exposed to a range of MeJA concentrations (0, 10, 25, and 50 mM), and biogenic volatile organic compound (BVOC) emissions, photosynthetic rate (A), and stomatal conductance (gs) were continuously measured. In addition, changes in phytohormone concentrations and gene expression were studied. Enhancement of methanol, lipoxygenase pathway volatiles and linalool emissions, and reductions in A and gs, were MeJA dose-dependent. Before MeJA treatment, the concentration of 12-oxo-phytodienoic acid (OPDA) was 7-fold higher than jasmonic acid (JA). MeJA treatment rapidly increased OPDA and JA concentrations (within 30 min), with the latter more responsive. Some genes involved in BVOC biosynthesis and OPDA-specific response were up-regulated at 30 min after MeJA spraying, whereas those in the JA signalling pathway were not affected. Although JA was synthesized in S. martensii, OPDA was prioritized as a signalling molecule upon MeJA application. MeJA inhibited primary and enhanced secondary metabolism; we propose that fast-emitted linalool could serve as a marker of elicitation of stress-induced metabolism in lycophytes.

Heat stress is one of the most important abiotic stresses confronted by plants under global climate change. Plant exposure to abiotic or biotic stress can improve its tolerance to subsequent severe episodes of the same or different stress (stress priming), but so far there is limited comparative information about how pre-exposures to different abiotic and biotic elicitors alter plant resistance to severe heat stress. We exposed the perennial herb Melilotus albus Medik., a species rich in secondary metabolites, to moderate heat stress (35 °C) and greenhouse whitefly (Trialeurodes vaporariorum West.) infestation to comparatively determine whether both pre-treatments could enhance plant tolerance to the subsequent heat shock (45 °C) stress. Plant physiological responses to stress were characterized by photosynthetic traits and volatile organic compound emissions through 72 h recovery. Heat shock treatment reduced net assimilation rate (A) and stomatal conductance in all plants, but heat-primed plants had significantly faster rates of recovery of A than other plants. By the end of the recovery period, A in none of the three heat shock-stressed groups recovered to the control level, but in whitefly-infested plants it reached the pre-heat shock level. In heat-primed plants, the heat shock treatment was associated with a fast rise of monoterpene emissions, and in whitefly-infested plants with benzenoid emissions and an increase in total phenolic content.

Tropical plants are considered to be highly heat stress resistant, but heat waves of different frequency and duration induced by global climate change can exceed the plant heat stress tolerance in the tropics. To gain insight into the overall heat resistance, leaves of papaya were exposed to mild (37 and 41 degrees C), moderate (46 degrees C) and severe (49 degrees C) heat stress (applied for 5 min), and plant recovery was monitored for 48 h upon return to prestress conditions. Leaf photosynthetic characteristics were reduced immediately after stress in all cases, but photosynthesis rate almost fully recovered after mild heat treatment, and partly recovered in 48 h after moderate stress, while no recovery was observed under severe stress. Constitutive BVOC emissions dominated by long chained aldehydes, and glucosinolate breakdown products, attained the emission maxima at 46 degrees C. Acute heat stress led only to a minor induction of BVOC emissions, and the greatest share of induced emissions was observed in leaves exposed to 46 degrees C. Induced emissions were dominated by oxygenated monoterpenes, indicating an increase in leaf oxidative status. Our results demonstrate that papaya could withstand moderately severe heat stress of up to 46 degrees C, but this level of stress will have negative impacts on plant carbon gain due to delayed photosynthetic recovery after heat stress. Although volatile profiles sensitively responded to heat stress, the magnitude and degree of elicitation were surprisingly small throughout the recovery, suggesting that papaya is relatively heat resistant and possess a suite of heat-protective traits.

Heatwaves are expected to become more frequent and directly exert major stress on plants. Warmer weather can also increase the frequency of biotic infestations. However, how biotic stress alters heat resistance and how interacting heat and biotic stresses alter volatile organic compound (VOC) emissions remain unclear. We studied how heat shock (45 celcius for 5 min) and Trialeurodes vaporariorum infestation alone and in combination affect foliage photosynthetic characteristics and VOC emissions in Origanum vulgare, right after heat stress through 48 h recovery. Heat stress alone decreased photosynthesis rate (A) but increased stomatal conductance (gs), emissions of lipoxygenase pathway volatiles (LOX), benzenoids and terpenoids. Neither A nor VOC emissions recovered to pre-stress values at 48 h after stress application. Whitefly infestation reduced A and increased gs, and resulted in a moderate increase in terpene emissions, but inhibited constitutive LOX and benzenoid emissions. Heat stress applied on whitefly infestation reduced A and increased gs, and resulted in a much lower enhancement of LOX and terpene emissions. Photosynthetic characteristics fully recovered at 48 h after stress treatment. Our results suggest that under phloem-feeding insect herbivory, VOC emission responses to extreme temperature are highly desensitized and photosynthetic thermal tolerance is improved.

Severe heat stress leads to a major reduction in foliage photosynthetic characteristics and the initial reduction and degree of recovery depend on species heat resistance. However, how the emissions of biogenic volatile organic compounds (BVOC) from different biochemical pathways are coordinated with photosynthetic modifications from the initial stress response through recovery are poorly understood, and the data are especially limited for short-living tropical crop species. We exposed leaves of five tropical crop species (A. cruentus, A. hybridus, S. aethiopicum, T. occidentalis and V. unguiculata) to severe heat treatment of 49 degrees C and a control treatment of 25 degrees C for 5 min and studied the modifications in foliage photosynthetic characteristics and constitutive BVOC emissions and elicitation of stress-induced volatiles through a 48 h recovery period upon return to non-stressed conditions. Overall, heat shock application resulted in a major inhibition of net assimilation rate similarly in all species indicating cellular damage and photosynthetic decay. However, species strongly varied in photosynthetic recovery with A. hybridus recovering the most and S. aethiopicum the least. Heat shock stress led to enhancement of emissions characterizing increased leaf oxidative status, in particular to enhanced emissions of lipoxygenase pathway volatiles (LOX). Surprisingly, LOX emissions were not associated with species heat resistance, but were greater in physiologically more active species with greater photosynthetic capacity. Species with greater constitutive isoprenoid emission capacity were generally more heat resistant, but terpenoid emissions were induced to a greater degree in less heat resistant species that likely suffered the most from the applied level of stress. Heat stress affected different groups of terpenoids - isoprene, monoterpenes and sesquiterpenes - to a different degree in different species, resulting in unique species-specific emission blends at different times of recovery. Heat stress had relatively minor effects on benzenoid emissions with moderately enhanced emissions primarily in V. unguiculata that differed from the other species by lower constitutive terpenoid emissions correspondent to its relatively high heat resistance. The results collectively demonstrate that species heat resistance is associated with the quantitative and qualitative characteristics of heat stress-dependent volatile emissions in different tropical crop species. We argue that these species-specific patterns need consideration in simulating volatile emissions under heat stress.

Ozone (O3) entry into plant leaves depends on atmospheric O3 concentration, exposure time and openness of stomata. O3 negatively impacts photosynthesis rate (A) and might induce the release of reactive volatile organic compounds (VOCs) that can quench O3, and thereby partly ameliorate O3 stress. Water stress reduces stomatal conductance (gs) and O3 uptake and can affect VOC release and O3 quenching by VOC, but the interactive effects of O3 exposure and water stress, as possibly mediated by VOC, are poorly understood. Well-watered (WW) and water-stressed (WS) Brassica nigra plants were exposed to 250 and 550 ppb O3 for 1 h, and O3 uptake rates, photosynthetic characteristics and VOC emissions were measured through 22 h recovery. The highest O3 uptake was observed in WW plants exposed to 550 ppb O3 with the greatest reduction and poorest recovery of gs and A, and elicitation of lipoxygenase (LOX) pathway volatiles 10 min–1.5 h after exposure indicating cellular damage. Ozone uptake was similar in 250 ppb WW and 550 ppb WS plants and, in both treatments, O3-dependent reduction in photosynthetic characteristics was moderate and fully reversible, and VOC emissions were little affected. Water stress alone did not affect the total amount and composition of VOC emissions. The results indicate that drought ameliorated O3 stress by reducing O3 uptake through stomatal closure and the two stresses operated in an antagonistic manner in B. nigra.

Leaf mechanical wounding triggers a rapid release—within minutes—of a blend of volatile organic compounds. A wounding-induced VOC blend is mainly composed of oxygenated ubiquitous stress volatiles such as methanol and volatile products of lipoxygenase (LOX) pathway (mainly C5 and C6 alcohols and aldehydes and their derivatives), but also includes multiple minor VOCs that collectively act as infochemicals, inducing defences in non-damaged plant leaves and neighbouring plants and attracting herbivore enemies. At present, the interspecific variability of the rate of induction and magnitude of wounding-induced emissions and the extent to which plant structural traits and physiological activity alter these emissions are poorly known. Particularly scarce is information on the induced emissions in tropical agricultural plant species, despite their economic importance and large area of cultivation at regional and global scales. We chose five tropical crops with varying photosynthetic activity and leaf structural characteristics—Abelmoschus esculentus, Amaranthus cruentus, Amaranthus hybridus, Solanum aethiopicum, and Telfairia occidentalis—to characterize the kinetics and magnitude of wounding-induced emissions, hypothesizing that the induced emission response is greater and faster in physiologically more active species with greater photosynthetic activity than in less active species. Rapid highly repeatable leaf wounds (12 mm cuts) were generated by a within-leaf-chamber cutting knife. Wounding-induced VOC emissions were measured continuously with a proton-transfer reaction time-of-flight mass spectrometer and gas-chromatography mass spectrometry was used to separate isomers. Twenty-three ion VOCs and twelve terpenoid molecule structures were identified, whereas ubiquitous stress volatiles methanol (on average 40% of total emissions), hexenal (24%), and acetaldehyde (11%) were the main compounds across the species. Emissions of low-weight oxygenated compounds (LOC, 70% of total) and LOX products (29%) were positively correlated across species, but minor VOC components, monoterpenoids and benzenoids, were negatively correlated with LOC and LOX, indicating a reverse relationship between signal specificity and strength. There was a large interspecific variability in the rate of induction and emission magnitude, but the hypothesis of a stronger emission response in physiologically more active species was only partly supported. In addition, the overall emission levels were somewhat lower with different emission blend compared to the data reported for wild species, as well as different shares for the VOCs in the blend. The study demonstrates that wounding-dependent emissions from tropical agricultural crops can significantly contribute to atmospheric volatiles, and these emissions cannot be predicted based on current evidence of wild plant model systems.

Diseases constitute an important economic problem in oilseed rape (Brassica napus) cultivation. Although downy mildew has been counted so far as a minor disease, under intensive cultivation system and short rotation interval, the impact of diseases could increase in the future, especially under predicted more humid northern climatic conditions. This research study is the first report about the severity of downy mildew infection on cruciferous crops in Estonia. During two years (2010 − 2011), downy mildew infection severity was assessed in six different cruciferous crops (B. napus, B. juncea, B. nigra, Sinapis alba, Raphanus sativus, Eruca sativa) in field trials located in Eerika, Tartu County, Estonia. On both study years, four disease assessments were done. Downy mildew infection started a week earlier in 2010, but the overall disease pressure was lower compared to 2011. Based on our field trial results, S. alba, E. sativa and R. sativus plants were significantly more resistant to downy mildew compared to other tested crops. Furthermore, in 2010 downy mildew symptoms were not recorded on two cruciferous species E. sativa and R. sativus. Brassica nigra and B. napus plants were the most susceptible to downy mildew, with especially severe infection in 2011. Based on our two years data, downy mildew can damage foliage on spring oilseed rape (B. napus) and black mustard (B. nigra) in a notable extent. We conclude that downy mildew is an important pathogen, which should be monitored on different oilseed cruciferous crops in Estonia.

The mechanism of heat priming, triggering alteration of secondary metabolite pathway fluxes and pools to enhance heat tolerance is not well understood. Achillea millefolium is an important medicinal herbal plant, rich in terpenoids and phenolics. In this study, the potential of heat priming treatment (35°C for 1 hr) to enhance tolerance of Achillea plants upon subsequent heat shock (45°C for 5 min) stress was investigated through recovery (0.5-72 hr). The priming treatment itself had minor impacts on photosynthesis, led to moderate increases in the emission of lipoxygenase (LOX) pathway volatiles and isoprene, and to major elicitation of monoterpene and benzaldehyde emissions in late stages of recovery. Upon subsequent heat shock, in primed plants, the rise in LOX and reduction in photosynthetic rate (A) was much less, stomatal conductance (gs ) was initially enhanced, terpene emissions were greater and recovery of A occurred faster, indicating enhanced heat tolerance. Additionally, primed plants accumulated higher contents of total phenolics and condensed tannins at the end of the recovery. These results collectively indicate that heat priming improved photosynthesis upon subsequent heat shock by enhancing gs and synthesis of volatile and non-volatile secondary compounds with antioxidative characteristics, thereby maintaining the integrity of leaf membranes under stress.

Plant essential oils (EOs) represent an area of interest for managing agricultural pests. We examined the insecticidal efficacy of seven plant EOs on the survival and mobility of the pollen beetle Brassicogethes aeneus, a primary pest of oilseed rape (Brassica napus) in Europe. Topical dosing tests showed the EO of the inner bark of Cinnamomum verum (cinnamon) to be the most effective EO examined in our study. Subsequent bioassays, exposing B. aeneus to oilseed rape plant surfaces sprayed with various concentrations of C. verum EO, additionally suggested a concentration threshold at which this EO may significantly control B. aeneus populations. We suggest that further studies on B. aeneus examine the effect of certain pure compounds associated with the most promising EOs (individually as well as in binary combinations) in order to reveal optimal chemical composition and ratios to exploit within a B. aeneus management framework.

In this study, the effects of a winter cover crop (CC) and its combination with composted cattle manure (CC-M) on the development of potato late blight (Phytophthora infestans) and crop yield were investigated and compared with a system with no cover crop (NC). In the CC and CC-M systems, winter oilseed rape (Brassica napus L) was used as the winter cover crop before potatoes. The hypothesis that the cover crop could be used as a possible control strategy against late blight in organic potato cultivation was not confirmed. Although the progression of foliar late blight damage in CC treatment was significantly slower in the early stages of disease development relative to NC, CC did not significantly reduce late blight at the later stages, or over the full season, and it did not increase tuber yield. Adding cattle manure to the CC treatment favoured late blight development and the damage was significantly more severe than in the other systems and tuber yield was nominally lower (though not significantly). The late blight pressure differed between treatments, with early disease development significantly slower on CC plots compared to CC-M in both growing seasons, but still not different to that in the plants grown in the NC system. Based on the results, in regions where oospores are a primary infection source, making P. infestans a soilborne pathogen, the use of cover crops may provide a sustainable strategy for reducing late blight damage in an organic system, but further investigation is needed.

Runno-Paurson, E., Hansen, M., Tamela, L., Kaurilind, E., Einola, A., Einola P. 2018. Evaluation of potato tuber diseases on different potato cultivars in Einola Farm. – Agronomy 2018. Potato growers are interested to produce healthy and high-quality potato tubers. However for Estonian potato growers common scab (Streptomyces spp) and wireworms (Agriotes spp) are the main important factors that lower directly the marketable potato tuber quality for processing and during storage period. Estonian potato growers mostly grow cultivars bred by Western European breeding companies and regularly import new cultivars. In spite of cultivar descriptions from breeding company, there is a lack of knowledge on suitability and susceptibility to diseases and pests in local Estonian conditions. Therefore the main aim of this research was to evaluate the extent of tuber damage extent caused by common scab, black scurf, silver scurf, dry rot, black leg and wireworms on 13 different potato cultivars from breeding companies Agrico (‘Arielle’, ‘Esmee’, Ranomi’, ‘Champion’, ‘Madeleine’, ‘Mariska’, ‘Laudine’, ‘Bellefleur’, ‘Rosagold’, ‘Manitou’), Norika (‘Merlot’) and EuroPlant (‘Viviana’, ‘Antonia’). Field trial was carried out in 2014 at Einola Farm in Uhti. Wireworm damage was recorded on all 13 cultivars tested. Significant potato tuber damage was found from early cultivar ‘Viviana’, early medium cultivar ‘Laudine’, medium late cultivars ‘Manitou’ and ‘Bellefleur’ where in average 47–51% of tubers were injured by wireworm. Less infection was found on early medium cultivar ‘Champion’, medium late cultivar ‘Rosagold’ and early cultivar ‘Arielle’ that is already known cultivar and grown on large field areas over Estonia. Growing season 2014 became unfavourable for common scab assessments. Common scab infection was not recorded on tubers of the cultivar ‘Bellefleur’. Quite low infection (1–10% of the tuber surface covered with scab) was recorded on early medium cultivars ‘Madeleine’, ‘Champion’, ‘Antonia’ and on early cultivar ’Viviana’. However, rest of the cultivars had very low rate of scab. Cultivars ‘Viviana’, ’Manitou’ and ’Rosagold’ were quite susceptible to dry rot. All 13 cultivars were infected by silver scurf, but heaviest infection was recorded on ‘Mariska’, ‘Madeleine’, ‘Antonia’, ‘Champion’ and ‘Viviana’.

Methyl salicylate (MeSA) is a long-distance signal transduction chemical that plays an important role in plant responses to abiotic stress and herbivore and pathogen attacks. However, it is unclear how photosynthesis and elicitation of plant volatile organic compounds (VOC) from different metabolic pathways respond to the dose of MeSA. We applied different MeSA concentrations (0-50 mM) to study how exogenous MeSA alters VOC profiles of silver birch (Betula pendula Roth) leaves from application through recovery (0.5-23 h). Methyl salicylate application significantly reduced net assimilation rate in 10 mM and 20 mM MeSA-treated plants. No significant effects of MeSA were observed on the stomatal conductance at any MeSA concentration. Methyl salicylate elicited emissions of benzenoids (BZ), monoterpenes (MT) and fatty acid derived compounds (LOX products). Emission rates of BZ were positively, but emission rates of MT were negatively correlated with MeSA concentration. Total emission of LOX products was not influenced by MeSA concentration. Emission rate of MT was negatively correlated with BZ and the share of MT in the total emission blend decreased and the share of BZ increased with increasing MeSA concentration. Although the share of LOX products was similar across MeSA treatments, some LOX products responded differently to MeSA concentration, ultimately resulting in unique VOC blends. Overall, this study demonstrates inverse responses of MT and BZ to different MeSA doses such that plant defense mechanisms induced by lower MeSA doses mainly lead to enhanced MT synthesis, whereas greater MeSA doses trigger BZ-related defense mechanisms. Our results will contribute to improving the understanding of birch defenses induced upon regular herbivore attacks and pathogen infections in boreal forests.

Plants are exposed to abiotic and biotic stress conditions throughout their lifespans that activates various defense programs. Programmed cell death (PCD) is an extreme defense strategy the plant uses to manage unfavorable environments as well as during developmentally induced senescence. Here we investigated the role of leaf age on the regulation of defense gene expression in Arabidopsis thaliana. Two lesion mimic mutants with misregulated cell death, catalase2 (cat2) and defense no death1 (dnd1) were used together with several double mutants to dissect signaling pathways regulating defense gene expression associated with cell death and leaf age. PCD marker genes showed leaf age dependent expression, with the highest expression in old leaves. The salicylic acid (SA) biosynthesis mutant salicylic acid induction deficient2 (sid2) had reduced expression of PCD marker genes in the cat2 sid2 double mutant demonstrating the importance of SA biosynthesis in regulation of defense gene expression. While the auxin-and jasmonic acid (JA)-insensitive auxin resistant1 (axr1) double mutant cat2 axr1 also led to decreased expression of PCD markers; the expression of several marker genes for SA signaling (ISOCHORISMATE SYNTHASE 1, PR1 and PR2) were additionally decreased in cat2 axr1 compared to cat2. The reduced expression of these SA markers genes in cat2 axr1 implicates AXR1 as a regulator of SA signaling in addition to its known role in auxin and JA signaling. Overall, the current study reinforces the important role of SA signaling in regulation of leaf age-related transcript signatures.

BACKGROUND:To survive in a changing environment plants constantly monitor their surroundings. In response to several stresses and during photorespiration plants use reactive oxygen species as signaling molecules. The Arabidopsis thaliana catalase2 (cat2) mutant lacks a peroxisomal catalase and under photorespiratory conditions accumulates H2O2, which leads to activation of cell death.METHODS:A cat2 double mutant collection was generated through crossing and scored for cell death in different assays. Selected double mutants were further analyzed for photosynthetic performance and H2O2 accumulation.RESULTS:We used a targeted mutant analysis with more than 50 cat2 double mutants to investigate the role of stress hormones and other defense regulators in H2O2-mediated cell death. Several transcription factors (AS1, MYB30, MYC2, WRKY70), cell death regulators (RCD1, DND1) and hormone regulators (AXR1, ERA1, SID2, EDS1, SGT1b) were essential for execution of cell death in cat2. Genetic loci required for cell death in cat2 was compared with regulators of cell death in spontaneous lesion mimic mutants and led to the identification of a core set of plant cell death regulators. Analysis of gene expression data from cat2 and plants undergoing cell death revealed similar gene expression profiles, further supporting the existence of a common program for regulation of plant cell death.CONCLUSIONS:Our results provide a genetic framework for further study on the role of H2O2 in regulation of cell death. The hormones salicylic acid, jasmonic acid and auxin, as well as their interaction, are crucial determinants of cell death regulation.

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K S), leaf blades (K lb), petioles (K P) and branches (i.e. leafless stem; K B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m−2 s−1, using different light sources. K lb depended significantly (P < 0.001) on light quality, canopy position and leaf blade area (A L). K lb increased from crown base to tree top, in parallel with vertical patterns of A L. However, the analysis of data on shade and sun leaves separately revealed an opposite trend: the bigger the A L the higher K lb. Leaf anatomical study of birch saplings revealed that this trend is attributable to enhanced vascular development with increasing leaf area. Hydraulic traits (K S, K B, K lb) of sun shoots were well co-ordinated and more strongly correlated with characteristics of shoot size than those of shade shoots, reflecting their greater evaporative load and need for stricter adjustment of hydraulic capacity with shoot size. K S increased with increasing xylem cross-sectional area to leaf area ratio (Huber value; P < 0.01), suggesting a preferential investment in water-conducting tissue (sapwood) relative to transpiring tissue (leaves), and most likely contributing to the functional stability of the hydraulic system, essential for fast-growing pioneer species.