Probing behavior of the corn leafhopper Dalbulus maidis on susceptible and resistant maize hybrids

The corn leafhopper Dalbulus maidis is the main vector of the pathogens that cause corn stunt, a major disease of maize in the Americas. As host resistance is an efficient tool to control diseases, the findings of a previous report showed that some corn hybrids are resistant to D . maidis . In this work, we assessed the probing behavior of D . maidis on susceptible and resistant corn hybrids using EPG (Electrical Penetration Graph) technology. Fifteen-day-old females were monitored for 20 hours, with access to hybrids DK390, DK670, DK79-10, and DK72-10. Hybrids DK390 and DK72-10 showed resistance to D . maidis in phloem, since insects feeding on these hybrids presented more salivation events in phloem without subsequent ingestion, which are seen as failed attempts to ingest. A reduction of the total duration of phloem ingestion was observed, and accordingly of the time spent by insects with access to these hybrids on xylem ingestion. The hybrid DK390 also had mesophyll resistance, seen as less probing time and a higher number of probes of short duration. These findings support and are consistent with previous research, providing useful information to characterize maize hybrids resistant to D . maidis , and so to corn stunt.

The rate-limiting step for photosynthetic CO2 utilization under varying atmospheric evaporative demand in Solanum lycopersicum (tomato)

Background: Despite atmospheric vapour pressure deficit (VPD) was demonstrated as significant environmental factors affecting plant photosynthesis and productivity, the regulating mechanism under varying atmospheric evaporative demand was still unclarified. The contribution of stomatal, mesophyll resistance and biochemical limitation imposed on photosynthesis in tomato under varying evaporative demand was highlighted in the present study. Quantitative photosynthetic limitation analysis across a series of VPD was performed in well-watered tomato, by combining gas exchange and chlorophyll fluorescence. Results: Photosynthetic performance in tomato was gradually depressed with increasing in VPD. Under low VPD condition, stomatal and mesophyll conductance were sufficiently high for CO2 transport, which facilitated high chloroplast CO2 concentration for carbon fixation. Stomatal and mesophyll limitation accounted a low fraction, and photosynthetic potential was mostly constrained by biochemical limitation inside chloroplasts under low VPD condition. With increasing in VPD, plant water stress was gradually pronounced and triggered declines in stomatal and mesophyll conductance. Contribution of stomatal and mesophyll limitation on photosynthesis increased gradually with rise in VPD. Consequently, the low CO2 availability inside chloroplast substantially constrained photosynthesis under high VPD condition. Conclusion: Photosynthetic potential in tomato was mostly constrained by biochemical limitation inside chloroplasts under low VPD condition. CO2 diffusion limitation in series of stomatal and mesophyll resistance was the key rate-limiting step for photosynthesis under high VPD condition.

Elevated CO2-induced changes in mesophyll conductance and anatomical traits in wild type and carbohydrate-metabolism mutants of Arabidopsis

Decreases in photosynthetic rate, stomatal conductance (gs), and mesophyll conductance (gm) are often observed under elevated CO2 conditions. However, which anatomical and/or physiological factors contribute to the decrease in gm is not fully understood. Arabidopsis thaliana wild-type and carbon-metabolism mutants (gwd1, pgm1, and cfbp1) with different accumulation patterns of non-structural carbohydrates were grown at ambient (400 ppm) and elevated (800 ppm) CO2. Anatomical and physiological traits of leaves were measured to investigate factors causing the changes in gm and in the mesophyll resistance (expressed as the reciprocal of mesophyll conductance per unit chloroplast surface area facing to intercellular space, Sc/gm). When grown at elevated CO2, all the lines showed increases in cell wall mass, cell wall thickness, and starch content, but not in leaf thickness. gm measured at 800 ppm CO2 was significantly lower than at 400 ppm CO2 in all the lines. Changes in Sc/gm were associated with thicker cell walls rather than with excess starch content. The results indicate that the changes in gm and Sc/gm that occur in response to elevated CO2 are independent of non-structural carbohydrates, and the cell wall represents a greater limitation factor for gm than starch.

Evaluation of Resistance to Asiatic Citrus Canker among Selections of Pêra Sweet Orange (Citrus sinensis)

Asiatic citrus canker (ACC, caused by the bacterium Xanthomonas citri subsp. citri) is a destructive disease of citrus in Brazil and in several other citrus-producing countries. ACC management is problematic, and bactericides such as copper can be reasonably efficacious but do not completely control the disease. Furthermore, injury by citrus leaf miner (CLM) can exacerbate severity of ACC. Host resistance is the most desirable solution for management of ACC; however, evaluations of germplasm indicate that resistance is limited in many popular species and cultivars that are grown commercially. Limited evaluations have been made of sweet orange (Citrus sinensis) selections. We evaluated resistance of 25 Pêra sweet orange selections to X. citri subsp. citri by wound inoculation and measuring lesion diameter under greenhouse conditions (wound inoculation indicates mesophyll resistance which will be valuable in areas where CLM exists). ACC severity was assessed on the same 25 selections at three locations in the field in Brazil, relying on natural inoculum and conditions to cause disease. In the greenhouse experiments, the selections EEL, Bianchi/CC, Ipiguá, Olimpia, IAC 2000/1, and Ovale Siracusa consistently had the smallest diameter lesions, indicating greatest resistance, although differences in lesion diameter were small. Results from the field experiments were less conclusive, although EEL and Ovale Siracusa were consistently numerically least affected by ACC. These results indicate selections of sweet orange that might be preferable to consider in canker-prone areas in Brazil and elsewhere.

Quantification of Mesophyll Resistance and Apoplastic Ascorbic Acid as an Antioxidant for Tropospheric Ozone in Durum Wheat (Triticum durumDesf. cv. Camacho)

The daily variations in cellular and apoplastic ascorbic acid and dehydroascorbic acid levels in a Mediterranean durum wheat cultivar (Triticum durumDesf. cv. Camacho) were analyzed in order to relate them to ambient ozone exposure and to subsequent stomatally absorbed ozone fluxes. The aim of this study is to prove the effectiveness and accuracy of a computer model (SODA) to calculate the mesophyll resistance (rm) to ozone uptake, the percentage of ozone detoxification by apoplastic ascorbic acid, and the ozone flux to the plasmalemma (Fm) in a Mediterranean durum wheat cultivar. These calculated factors were related to apoplastic ascorbic acid levels and to ambient ozone concentrations. These relationships were obtained with a view to explaining the detoxification of ozone by apoplastic ascorbic acid. Ozone detoxifications of up to 52% were found at midday, when maximum ozone concentrations and maximum apoplastic ascorbic acid are seen. Mesophyll resistance was minimum at this time, and ozone flux to the plasmalemma was reduced because of the reaction of ozone with apoplastic ascorbic acid.

Water Requirements and Drought Tolerance of Bedding Plants

Optimal substrate volumetric water content (θ) and drought tolerance of impatiens, petunia, salvia, and vinca were investigated by growing plants under four constant levels of θ (0.09, 0.15, 0.22, and 0.32 m3·m-3). Gas exchange, quantum efficiency (ΦPSII), electron transport rate (ETR), non-photochemical quenching (NPQ), and leaf water potential (ϒ) were measured for all species, and response of photosynthesis (Pn) to internal CO2 concentration (Ci) was studied in petunia and salvia. Leaf photosynthesis (Pmax) was highest at a θ of 0.22 m3·m-3 for all species and did not differ between a θ of 0.15 and 0.22 m3·m-3 for vinca and petunia. The Pn-Ci response curves for petunia were almost identical at a θ of 0.22 and 0.15 m3·m-3. Regardless of species, ETR and ΦPSII were highest and NPQ was lowest at a θ of 0.22 m3·m-3. Based on these results, a θ of 0.22 m3·m-3 for salvia and impatiens and a slightly lower θ of 0.15 m3·m-3 for vinca and petunia, is optimal. Mean osmotic potential in all treatments was lower in vinca and salvia and resulted in higher turgor potential in these species than other species. Analysis of Pn-Ci response curves indicated that Pn at a θ of 0.09 m3·m-3 was limited by both gas phase (stomatal and boundary layer) and non-gas phase (mesophyll) resistance to CO2 transfer in salvia. At the lowest θ level, Pn in petunia was only limited by gas phase resistance, indicating that absence of mesophyll resistance during drought may play a role in the drought tolerance of petunia.