Age-dependent resistance of a perennial herb, Aristolochia contorta against specialist and generalist leaf-chewing herbivores

Plants need to balance investments in growth and defense throughout their life to increase their fitness. To optimize fitness, levels of defense against herbivores in perennial plants may vary according to plant age and season. However, secondary plant metabolites often have a detrimental effect on generalist herbivores, while many specialists have developed resistance to them. Therefore, varying levels of defensive secondary metabolites depending on plant age and season may have different effects on the performance of specialist and generalist herbivores colonizing the same host plants. In this study, we analyzed concentrations of defensive secondary metabolites (aristolochic acids) and the nutritional value (C/N ratios) of 1^st-, 2^nd- and 3^rd-year Aristolochia contorta in July (the middle of growing season) and September (the end of growing season). We further assessed their effects on the performances of the specialist herbivore Sericinus montela (Lepidoptera: Papilionidae) and the generalist herbivore Spodoptera exigua (Lepidoptera: Noctuidae). Leaves of 1^st-year A. contorta contained significantly higher concentrations of aristolochic acids than those of older plants, with concentrations tending to decrease over the first-year season. Therefore, when first year leaves were fed in July, all larvae of S. exigua died and S. montela showed the lowest growth rate compared to older leaves fed in July. However, the nutritional quality of A. contorta leaves was lower in September than July irrespective of plant age, which was reflected in lower larval performance of both herbivores in September. These results suggest that A. contorta invests in the chemical defenses of leaves especially at a young age, while the low nutritional value of leaves seems to limit the performance of leaf-chewing herbivores at the end of the season, regardless of plant age.

Estimation of Cold Stress, Plant Age, and Number of Leaves in Watermelon Plants Using Image Analysis

Watermelon (Citrullus lanatus) is a widely consumed, nutritious fruit, rich in water and sugars. In most crops, abiotic stresses caused by changes in temperature, moisture, etc., are a significant challenge during production. Due to the temperature sensitivity of watermelon plants, temperatures must be closely monitored and controlled when the crop is cultivated in controlled environments. Studies have found direct responses to these stresses include reductions in leaf size, number of leaves, and plant size. Stress diagnosis based on plant morphological features (e.g., shape, color, and texture) is important for phenomics studies. The purpose of this study is to classify watermelon plants exposed to low-temperature stress conditions from the normal ones using features extracted using image analysis. In addition, an attempt was made to develop a model for estimating the number of leaves and plant age (in weeks) using the extracted features. A model was developed that can classify normal and low-temperature stress watermelon plants with 100% accuracy. The R², RMSE, and mean absolute difference (MAD) of the predictive model for the number of leaves were 0.94, 0.87, and 0.88, respectively, and the R² and RMSE of the model for estimating the plant age were 0.92 and 0.29 weeks, respectively. The models developed in this study can be utilized in high-throughput phenotyping systems for growth monitoring and analysis of phenotypic traits during watermelon cultivation.

[Mineral nitrogen accumulation and its spatial distribution in soils in dense planting dwarf rootstock apple orchard on the Weibei dry plateau, Northwest China]

With the rapid development of dense apple tree plantings with the dwarf rootstock cultivation method, determining accumulation and distribution characteristics of soil mineral nitrogen in densely planted orchards with dwarf rootstock is important to enable scientific fertilization of apple orchards. We investigated densely planted apple orchards with dwarf rootstocks and different plant ages (6 a, 9 a, and 12 a). We collected soil samples under trees, between trees, between rows, and at the midpoints between the trees and rows, and examined the accumulation and distribution characteristics of nitrate, ammonium, and mineral nitrogen. The cumulative amount of nitrate in the 0-300 cm soil layer increased with plant age. The difference between orchards with different plant ages was significant and showed the trend 6 a<9 a<12 a. The cumulative amount of nitrate increased from 1729 kg·hm^-2 to 3771 kg·hm^-2 with increasing plant age. The ammonium content was low for orchards of all plant ages and had little effect on the accumulation and spatial distribution of mineral nitrogen. There were two accumulation peaks of nitrate nitrogen in the vertical direction. The depth of soil layer where the second accumulation peak was located decreased from 180 cm to 220 cm with increasing plant age. In the horizontal direction, soil nitrate nitrogen content between rows increased from 27 mg·kg^-1 to 138 mg·kg^-1 with increasing plant age, representing a more than 400% increase. The difference between orchards with different plant ages was significant. In summary, excessive usage of nitrogen fertilizer and serious leaching of nitrate were problematic in all orchards with different ages. Less nitrogen fertilizer should be applied, and anti-seepage measures should be used at the fertilization location to prevent the leaching of nitrate to deep layers.

Host plant adaptability and proteomic differences of diverse Rhopalosiphum maidis (Fitch) lineages

Corn leaf aphid Rhopalosiphum maidis (Fitch) can feed on various cereal crops and transmit viruses that may cause serious economic losses. To test the impact of both host plant species and age on R. maidis, as well as the proteomic difference of diverse populations, we first investigated the survival and reproduction of six R. maidis populations (i.e., LF, HF, GZ, DY, BJ, and MS) via a direct observation method in the laboratory on 10 and 50 cm high maize seedlings, and 10 cm high barley seedlings. Then a proteomic approach was implemented to identify the differentially expressed proteins from both aphids and endosymbionts of BJ and MS populations. Results indicated that the BJ population performed significantly better than the others on both barley and 50 cm high maize seedlings, while no population could survive on 10 cm high maize seedlings. The proteomic results demonstrated that the expression levels of myosin heavy chain (muscle isoform X12) (spot 781) and peroxidase (spot 1383) were upregulated, while ATP-dependent protease Hsp 100 (spot 2137) from Hamiltonella defensa and protein SYMBAF (spot 2703) from Serratia symbiotica were downregulated in the BJ population when compared to expression levels of the MS population. We hypothesize that the fatalness observed on 10 cm high maize seedlings may be caused by secondary metabolites that are synthesized by the seedlings and the MS population of R. maidis should be more stress-resistant than the BJ population. Our results also provide insights for understanding the interaction between host plants and aphids.

Benzoxazinoids selectively affect maize root-associated nematode taxa

Although the effects of plant secondary metabolites on plant defence have been studied for decades, the exact roles of secondary metabolites in shaping plant-associated microbial and nematode communities remain elusive. We evaluated the effects of benzoxazinoids, a group of secondary metabolites present in several cereals, on root-associated nematodes. We employed 18S rRNA metabarcoding to compare maize root-associated nematode communities in a bx1 knockout maize line impaired in benzoxazinoid synthesis and in its parental wild type. Both genotype and plant age affected the composition of the nematode community in the roots, and the effects of benzoxazinoids on nematode communities were stronger in the roots than in the rhizosphere. Differential abundance analysis and quantitative PCR showed that the root lesion nematode Pratylenchus neglectus was enriched in the bx1 mutant line, while another root lesion nematode, Pratylenchus crenatus, was reduced. Correlation analysis showed that benzoxazinoid concentrations in maize roots mostly correlated negatively with the relative abundance of nematode sequence reads. However, positive correlations between benzoxazinoids and nematode taxa, including several plant-parasitic nematodes, were also identified. Our detailed nematode community analysis suggests differential and selective effects of benzoxazinoids on soil nematodes depending on both the nematode species and the benzoxazinoid compound.

Leaf traits and performance vary with plant age and water availability in Artemisia californica

BACKGROUND AND AIMS

Leaf functional traits are strongly tied to growth strategies and ecological processes across species, but few efforts have linked intraspecific trait variation to performance across ontogenetic and environmental gradients. Plants are believed to shift towards more resource-conservative traits in stressful environments and as they age. However, uncertainty as to how intraspecific trait variation aligns with plant age and performance in the context of environmental variation may limit our ability to use traits to infer ecological processes at larger scales.

METHODS

We measured leaf physiological and morphological traits, canopy volume and flowering effort for Artemisia californica (California sagebrush), a dominant shrub species in the coastal sage scrub community, under conditions of 50, 100 and 150 % ambient precipitation for 3 years.

KEY RESULTS

Plant age was a stronger driver of variation in traits and performance than water availability. Older plants demonstrated trait values consistent with a more conservative resource-use strategy, and trait values were less sensitive to drought. Several trait correlations were consistent across years and treatments; for example, plants with high photosynthetic rates tended to have high stomatal conductance, leaf nitrogen concentration and light-use efficiency. However, the trade-off between leaf construction and leaf nitrogen evident in older plants was absent for first-year plants. While few traits correlated with plant growth and flowering effort, we observed a positive correlation between leaf mass per area and performance in some groups of older plants.

CONCLUSIONS

Overall, our results suggest that trait sensitivity to the environment is most visible during earlier stages of development, after which intraspecific trait variation and relationships may stabilize. While plant age plays a major role in intraspecific trait variation and sensitivity (and thus trait-based inferences), the direct influence of environment on growth and fecundity is just as critical to predicting plant performance in a changing environment.

Production of Defense Phenolics in Tomato Leaves of Different Age

Phenolics play an essential role in the defense reaction of crop plants against pathogens. However, the intensity of their production induced by infection may differ during the life of a plant. Here, we identified age-related differences in phenolic biosynthesis in the pathosystem Solanum lycopersicum cv. Amateur and Pseudomonas syringae pv. tomato DC3000. We analyzed concentrations of total phenolics, phenolic profiles, and concentrations of selected phenolic acids. The influence of bacterial infection, together with leaf and plant age, was assessed. The changes in concentrations of caffeic acid, 4-hydroxybenzoic acid, and salicylic acid glucoside caused by infection were found to be influenced by age. In concrete, the increases in the concentrations of these metabolites were all evident only in young plants.

Variation in Morphological and Quality Parameters in Garlic (Allium sativum L.) Bulb Influenced by Different Photoperiod, Temperature, Sowing and Harvesting Time

Photoperiod (light) and temperature as abiotic factors having significant impact on the garlic bulb morphology and quality. In various bulb plants including garlic, bulbing is affected by photoperiod, temperature, sowing date and the plant age. In this backdrop experiments were performed to understand the effect of different photoperiods (10 h/14 h, 12 h/12 h and 14 h/10 h (light/dark)), temperatures (25 °C/18 °C and 30 °C/20 °C (light/dark)), sowing dates (D₀₈₀₁: 1^st August, D₀₉₀₁: 1^st September and D₁₀₀₁: 1^st October) and plant ages (A₈₀, A₆₀ and A₄₀: 80, 60 and 40 days after planting) on garlic cultivars viz; G103, G024 and G2011-5. Parameters including morphological (plant height, fresh weight and pseudostem diameter), bulb attributes (diameter, weight, height and bulbing index), growth period and bulb quality related traits (total soluble solid (TSS), contents of soluble protein, soluble sugar, total sugar, glucose, sucrose, fructose, starch, total phenol and total flavonoid) were assayed. Longer photoperiod (14 h), higher temperature (30 °C), early sowing (D₀₈₀₁) and maximum plant age (A₈₀) had maximum morphological and bulb quality related traits for cv. G103. These results showed that early sowing, maximum plant age, longer photoperiod and higher temperature are important for garlic bulb formation and quality. Moreover, the regulation of garlic bulb morphology and quality is achievable over the switch of sowing date, plant age, light and growth temperature.

Response of plasmodesmata formation in leaves of C4 grasses to growth irradiance

Rapid metabolite diffusion across the mesophyll (M) and bundle sheath (BS) cell interface in C₄ leaves is a key requirement for C₄ photosynthesis and occurs via plasmodesmata (PD). Here, we investigated how growth irradiance affects PD density between M and BS cells and between M cells in two C₄ species using our PD quantification method, which combines three-dimensional laser confocal fluorescence microscopy and scanning electron microscopy. The response of leaf anatomy and physiology of NADP-ME species, Setaria viridis and Zea mays to growth under different irradiances, low light (100 μmol m^-2  s^-1 ), and high light (1,000 μmol m^-2  s^-1 ), was observed both at seedling and established growth stages. We found that the effect of growth irradiance on C₄ leaf PD density depended on plant age and species. The high light treatment resulted in two to four-fold greater PD density per unit leaf area than at low light, due to greater area of PD clusters and greater PD size in high light plants. These results along with our finding that the effect of light on M-BS PD density was not tightly linked to photosynthetic capacity suggest a complex mechanism underlying the dynamic response of C₄ leaf PD formation to growth irradiance.

Flavonoid Biosynthesis Is Likely More Susceptible to Elevation and Tree Age Than Other Branch Pathways Involved in Phenylpropanoid Biosynthesis in Ginkgo Leaves

Ginkgo leaves are always resources for flavonoids pharmaceutical industry. However, the effect of the elevation and tree age changes on flavonoid biosynthesis have not been detailly explored in Ginkgo leaves. In addition, whether these environmental pressures have similar effects on the biosynthesis of other non-flavonoids polyphenolics in phenylpropanoid biosynthesis is not known at present. In this research, de novo transcriptome sequencing of Ginkgo leaves was performed coupled with ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry analyses to obtain a comprehensive understanding of the influence of elevation and tree age on phenylpropanoid biosynthesis. A total of 557,659,530 clean reads were assembled into 188,155 unigenes, of which 135,102 (71.80%) were successfully annotated in seven public databases. The putative DFRs, LARs, and ANRs were significantly up-regulated with the increase of elevation in young Ginkgo tree leaves. The relative concentration of flavonoid derivatives with high parent ion intensity was likely to imply that the elevation increase promoted the biosynthesis of flavonoids. Complex gene variations involved in flavonoid biosynthesis were observed with the tree age increase. However, flavonoid derivatives analysis predicted that the rise of tree age was more likely to be detrimental to the flavonoids manufacture. Otherwise, multiple genes implicated in the synthesis of hydroxycinnamates, lignin, and lignan exhibited fluctuations with the elevation increase. Significantly up-regulated CADs and down-regulated PRDs potentially led to the accumulation of p-Coumaryl alcohol, one of the lignin monomers, and might inhibit further lignification. Overall, the putative DFRs seemed to show more considerable variability toward these stress, and appeared to be the main regulatory point in the flavonoid biosynthesis. Light enhancement caused by elevation increase may be the main reason for flavonoids accumulation. Flavonoid biosynthesis exhibited a greater degree of perturbation than that of hydroxycinnamates, lignins and lignans, potentially suggesting that flavonoid biosynthesis might be more susceptible than other branch pathways involved in phenylpropanoid biosynthesis. This research effectively expanded the functional genomic library and provide new insights into phenylpropanoid biosynthesis in Ginkgo.

Plant Stage, Not Drought Stress, Determines the Effect of Cultivars on Bacterial Community Diversity in the Rhizosphere of Broomcorn Millet (Panicum miliaceum L.)

Broomcorn millet (Panicum miliaceum L.) is one of the oldest domesticated crops and has been grown in arid and semiarid areas in China since 10,000 cal. BP. However, limited information is available about how bacterial communities within the rhizosphere of different broomcorn millet cultivars respond to drought stress. Here, we characterized the changes in the rhizobacterial assemblages of two broomcorn millet cultivars, namely, P. miliaceum cv. HeQu Red (HQR) and P. miliaceum YanLi 10 (YL10), from the jointing stage to the grain filling stage after they were exposed to a short-term drought stress treatment at the seedling stage. Drought significantly inhibited the growth of both cultivars, but the effect on YL10 was higher than that on HQR, indicating that the drought tolerance of HQR was greater than that of YL10. Proteobacteria (33.8%), Actinobacteria (21.0%), Acidobacteria (10.7%), Bacteroidetes (8.2%), Chloroflexi (6.3%), Gemmatimonadetes (5.9%), Firmicutes (3.5%), Verrucomicrobia (2.9%), and Planctomycetes (2.7%) were the core bacterial components of broomcorn millet rhizosphere as suggested by 16S rDNA sequencing results. The diversity and composition of bacterial rhizosphere communities substantially varied at different developmental stages of broomcorn millet. As the plants matured, the richness and evenness of the rhizobacterial community significantly decreased. Principal coordinate analysis showed that the structure of the bacterial rhizosphere community changed notably only at the flowering stage between the two cultivars, suggesting a stage-dependent effect. Although drought stress had no significant effect on the diversity and structure of the bacterial rhizosphere community between the two cultivars, differential responses to drought was found in Actinobacteria and Acinetobacter, Lysobacter, Streptomyces, and Cellvibrio. The relative abundance of Actinobacteria and Lysobacter, Streptomyces, and Cellvibrio in the YL10 rhizosphere was stimulated by the drought treatment compared with that in the HQR rhizosphere, whereas the opposite effect was found in Acinetobacter. Our results suggested that the effects of cultivars on bacterial rhizosphere communities were highly dependent on plant developmental stage, reflecting the genetic variations in the two broomcorn millet cultivars.

Effect of plant development (age and size) on the Mi-1-mediated resistance of tomato to whitefly Bemisia tabaci

Whitefly, Bemisia tabaci, is one of the most important pests of tomato, Solanum lycopersicum L. The Mi-1 gene mediates tomato resistance to the Middle East-Asia Minor 1 (MEAM1) and Mediterranean (MED) species of B. tabaci, three species of root-knot nematodes, Meloidogyne spp., and the potato aphid, Macrosiphum euphorbiae. Tomato seedlings bearing the Mi-1 gene are resistant to nematodes soon after germination but resistance to aphids is developmentally regulated; a reliable conclusion about Mi-1 resistance to B. tabaci was not available to date. In the present work, 3-, 5- and 8-week-old plants of the tomato cultivars Motelle and Moneymaker (bearing and lacking the Mi-1 gene, respectively) were simultaneously tested under free-choice (antixenosis) and no-choice (antibiosis) conditions, to assess the real influence of plant age on the Mi-1-mediated resistance to the MED species of B. tabaci. Subsequently, plants of the same age but with different level of development were compared to check whether the plant size can also affect this tomato resistance. Obtained results demonstrated that Mi-1-mediated resistance to B. tabaci is developmentally regulated, as variations in the age of bearing-Mi-1 plants affects most infestation parameters tested. Differences between cultivars with and without the Mi-1 gene were significant for 8- but not for 3-week-old plants. For 5-week-old plants, differences between cultivars were less pronounced than in older plants, expressing an intermediate level of resistance in Motelle. Plant size also influenced whitefly infestation and reproductive activity on the resistant cultivar. However, plant age has more impact than plant size on the Mi-1-mediated resistance of tomato to B. tabaci.

From sensitivity to resistance - factors affecting the response of Conyza spp. to glyphosate

BACKGROUND

Conyza bonariensis and C. canadensis are troublesome weeds, particularly in fields with minimum tillage, on roadsides and in perennial crops. The distribution of these difficult-to-control species is further increased by the spread of glyphosate-resistant populations. A preliminary investigation has demonstrated the existence of various degrees of glyphosate tolerance/resistance in these populations, underscoring the need to examine the relationship between glyphosate efficacy and plant growth conditions.

RESULTS

In populations exposed to glyphosate at different temperatures, glyphosate tolerance increased linearly as the temperature was increased, whereas when grown under the same temperatures, they largely responded similarly to the herbicide. Furthermore, the sensitivity of plants to glyphosate decreased significantly with plant age and increased following temporal exposure to shading. Dose-response studies confirmed the glyphosate resistance of four C. bonariensis populations that were 8-30 times more resistant to glyphosate than the most glyphosate-sensitive population. These populations retained their characteristic glyphosate resistance even under unfavourable growth conditions.

CONCLUSION

These findings indicate that the effect of glyphosate on both Conyza species is strongly linked to growing conditions. This has great importance for our understanding of glyphosate resistance and for control of these weeds in agricultural systems. © 2015 Society of Chemical Industry.

Bottom-Up Mechanisms Generate the Same Temporal Pattern of Attack by a Specialist and a Generalist Caterpillar on Short-Lived Plants

The local population dynamics of insect herbivores in ephemeral patches of short-lived plants are poorly known. We investigated whether a specialist and a generalist caterpillar exhibit contrasting temporal patterns of attack during plant development and also assessed bottom-up forces related to plant ontogeny that govern such population trends. Immature stages of the polyphagous Trichoplusia ni (Hübner) and the oligophagous Plutella xylostella (L.) were sampled throughout the development of cabbage (Brassica oleracea L. var. capitata L.) crops. We measured protein and glucosinolate contents and insect performance with regard to plant age and leaf strata. The populations of both caterpillar species changed in close parallel throughout plant development, and a nonlinear temporal pattern of egg laying was reproduced in sequential population patterns of the larval stages until pupation. Reduced protein availability and insect performance coincided with a decline in egg laying and subsequent larval abundance in mature plants. By standardizing the plant size, we found that young and nutritious plants support proportionately more insects than large and mature plants. In our models of the population oscillations, the interaction between plant size and quality provided a strong causal explanation for the densities of both oligophagous and polyphagous caterpillars. Patches of fast-growing herbaceous plants are very common worldwide in the form of crop fields, and a generalized temporal pattern of attack may be widespread among caterpillars, regardless of their feeding specialization. Our results highlight the role of bottom-up forces in shaping the population dynamics of caterpillars in such systems.

Bamboo Flowering from the Perspective of Comparative Genomics and Transcriptomics

Bamboos are an important member of the subfamily Bambusoideae, family Poaceae. The plant group exhibits wide variation with respect to the timing (1-120 years) and nature (sporadic vs. gregarious) of flowering among species. Usually flowering in woody bamboos is synchronous across culms growing over a large area, known as gregarious flowering. In many monocarpic bamboos this is followed by mass death and seed setting. While in sporadic flowering an isolated wild clump may flower, set little or no seed and remain alive. Such wide variation in flowering time and extent means that the plant group serves as repositories for genes and expression patterns that are unique to bamboo. Due to the dearth of available genomic and transcriptomic resources, limited studies have been undertaken to identify the potential molecular players in bamboo flowering. The public release of the first bamboo genome sequence Phyllostachys heterocycla, availability of related genomes Brachypodium distachyon and Oryza sativa provide us the opportunity to study this long-standing biological problem in a comparative and functional genomics framework. We identified bamboo genes homologous to those of Oryza and Brachypodium that are involved in established pathways such as vernalization, photoperiod, autonomous, and hormonal regulation of flowering. Additionally, we investigated triggers like stress (drought), physiological maturity and micro RNAs that may play crucial roles in flowering. We also analyzed available transcriptome datasets of different bamboo species to identify genes and their involvement in bamboo flowering. Finally, we summarize potential research hurdles that need to be addressed in future research.

A reevaluation of the use of rhizome scars to age plants of Trillium erectum (Melanthiaceae)1

PREMISE OF STUDY

Herbaceous perennials are important long-lived plants in North American forests. Trillium has been used as a model organism to examine the effects of ecological processes on age structure in herbaceous forest perennials. Here, the methods of aging Trillium rhizomes are critically examined. •

METHODS

Rhizomes of seedlings, single-bract plants, three-bract nonflowering plants, and flowering plants of Trillium erectum were examined. The patterns of cataphyll and scape scars on rhizomes were examined with respect to demographic category. •

KEY RESULTS

Trillium rhizomes produce two cataphyll scars per year on single-bract, three-bract nonflowering, and flowering plants. Scape scars were only evident on rhizomes of three-leaved nonflowering and flowering plants. The percentage of intact rhizomes ranged from 10-67% for three-bract nonflowering plants, and 0-51% for flowering plants. Rhizomes in all demographic categories had evidence of recessing tissues from the oldest portion of the rhizome indicating that accurate age estimates are not possible on many plants. •

CONCLUSIONS

Accepted methods of aging Trillium rhizomes have significant drawbacks. The primary problem is that rhizomes rot from the oldest portions in all demographic categories. A second problem is that plants producing multiple scape scars in a given year could mistakenly be counted as multiple years. Finally, confusing terminology and misrepresentations in Trillium literature suggests that many previous studies did not correctly determine age. Given the challenges of aging Trillium, we suggest that future studies use rhizome aging to study early demographic stages only and the ecological processes that influence their growth.

The impact of plant and flower age on mating patterns

BACKGROUND

Over a season, plant condition, amount of ongoing reproduction and biotic and abiotic environmental factors vary. As flowers age, flower condition and amount of pollen donated and received also vary. These internal and external changes are significant for fitness if they result in changes in reproduction and mating.

SCOPE

Literature from several fields was reviewed to provide a picture of the changes that occur in plants and flowers that can affect mating over a season. As flowers age, both the entire flower and individual floral whorls show changes in appearance and function. Over a season, changes in mating often appear as alteration in seed production vs. pollen donation. In several species, older, unpollinated flowers are more likely to self. If flowers are receiving pollen, staying open longer may increase the number of mates. In wild radish, for which there is considerable information on seed paternity, older flowers produce fewer seeds and appear to discriminate less among pollen donors. Pollen donor performance can also be linked to maternal plant age. Different pollinators and mates are available across the season. Also in wild radish, maternal plants appear to exert the most control over paternity when they are of intermediate age.

CONCLUSIONS

Although much is known about the characters of plants and flowers that can change over a season, there is less information on the effects of age on mating. Several studies document changes in self-pollination over time, but very few, other than those on wild radish, consider more subtle aspects of differential success of pollen donors over time.

Morphology of nectaries and biology of nectar production in the distylous species Fagopyrum esculentum

BACKGROUND AND AIMS

The mechanisms of floral nectar production in buckwheat (Fagopyrum esculentum, Polygonaceae), a distylous pseudo-cereal, have received relatively little attention, prompting an investigation of the factors that regulate this process. The aim was to perform a refined study of the structures that secrete nectar and of the internal and external parameters influencing nectar volumes and sugar concentrations.

METHODS

In order to control environmental parameters, plants were cultivated in growth rooms under controlled conditions. The structure of nectaries was studied based on histological sections from flowers and flower buds. Nectar was extracted using glass micropipettes and the sugar concentration was measured with a hand refractometer. Sugar concentration in the phloem sap was measured using the anthrone method. To test the influence of photosynthesis on nectar production, different light and defoliation treatments were applied.

KEY RESULTS

Unicellular trichomes were located in the epidermis at the ventral part of eight nectary glands situated on the flower receptacle alternately with stamens. Vascular bundles consisting of both phloem and xylem were identified at the boundary between a multilayered nectary parenchyma and a sub-nectary parenchyma with chloroplasts. A higher volume of nectar in thrum morphs was observed. No other difference was found in morphology or in sugar supply to inflorescences between morphs. Nectar secretion was strongly influenced by plant age and inflorescence position. Nectar volumes were higher in the upper inflorescences and during the flowering peak. Light had a dual role, (1) acting directly on reproductive structures to trigger flower opening, which conditions nectar secretion, and (2) stimulating photosynthetic activity, which regulates nectar accumulation in open flowers.

CONCLUSIONS

In buckwheat, nectar is secreted by trichomes and probably proceeds, at least in part, from phloem sap. Nectar secretion is strongly influenced by floral morph type, plant age, inflorescence position and light.

Impacts of ozone on Plantago major: apoplastic and symplastic antioxidant status

The aim of this work was to examine the correspondence between apoplastic/symplastic antioxidant status and previously reported plant age-related shifts in the ozone (O₃ ) resistance of Plantago major L. Seed-grown plants were fumigated in duplicate controlled environment chambers with charcoal/Purafil?-filtered air (CFA) or CFA plus 70 nmol mol^-1 O₃ for 7 h d^-1 over a 42 d period. Measurements of stomatal conductance and antioxidants were made after 14, 28 and 42 d fumigation, on leaves at an equivalent stage of development (youngest fully expanded leaf, measured c. 9 d after emergence). Ozone exposure resulted in a similar decline in stomatal conductance across plant ages, indicating that increases in O₃ resistance with plant age were mediated through changes in the tolerance of leaf tissue rather than enhanced pollutant exclusion. Leaf apoplastic washing fluid was found to contain 'unspecific' peroxidase, ascorbate peroxidase, superoxide dismutase and ascorbate, but not glutathione and the enzymes required to facilitate the regeneration of ascorbate from its oxidized forms. A weak induction in the activity of certain symplastic antioxidants was found after 14 d O₃ fumigation, despite a lack of visible symptoms of injury, but shifts in symplastic antioxidant enzyme activity were not consistent with previously observed increases in resistance to O₃ with plant age. By contrast, changes in 'unspecific' peroxidase activity and in the small pool of ascorbate in the leaf apoplast were found to accompany age-related shifts in O₃ resistance. It is concluded that constituents of the leaf apoplast may constitute a potentially important front line defence against O₃ .

Planting Date, Fungicide, and Cultivar Effects on Sclerotinia Crown and Stem Rot Severity in Alfalfa

This study was conducted to determine the effect of planting date, fungicide, and cultivar on severity of Sclerotinia crown and stem rot (SCSR) and subsequent productivity of alfalfa. The cultivars Armor and A9109 were seeded no-till in May, early August, mid August, and late August 1993 and 1994 in a sod uniformly infested with sclerotia of Sclerotinia trifoliorum. Four applications of the fungicide vinclozolin effectively controlled the disease. Averaged over years and cultivars, disease severity (percentage of plot area affected) in the no-fungicide treatment was 4, 12, 23, and 41% for the spring, early August, mid August, and late August plantings, respectively. Armor had higher disease severity than A9109 in the 1993 seeding, but not in the 1994 seeding. Forage yield the year after seeding reflected differences in disease severity ratings. The risk of severe SCSR damage in no-till summer seedings of alfalfa can be reduced dramatically if stands become established early enough so plants reach at least 10 weeks of age by the time apothecia emerge in the fall.