Tulio B. Macedo

Wheat Stem Sawfly, Cephus cinctus Norton, Impact on Wheat Primary Metabolism: An Ecophysiological Approach

Abstract The impact of wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), on the photosynthetic capacity and primary metabolism of wheat, Triticum aestivum L., was evaluated in three different environments: environmental growth chamber, greenhouse, and field. C. cinctus elicited different photosynthetic responses in different environments. Wheat gas exchange parameters, such as photosynthesis, stomatal conductance, intercellular CO2, and transpiration in the growth chamber environment were negatively affected by C. cinctus feeding. Conversely, the same gas exchange responses were not observed under greenhouse and field conditions. This study shows the important role of environmental variables, such as ambient CO2 concentrations and light intensity, on plant responses to herbivores.

Use of spatial structure analysis of hyperspectral data cubes for detection of insect-induced stress in wheat plants

Wheat plants were experimentally infested with wheat stem sawflies, and hyperspectral images (reflectance range from 402.8–838.7 nm) were collected from leaves of infested and non‐infested plants. Mean and variance reflectance per leaf were calculated in five of 213 spectral bands (452, 553, 657, 725, and 760 nm) and compared with vegetation indices (NDVI, SI and PRI), and standard variogram parameters (nugget, sill and range values). Mean reflectance values and their variance values and vegetation indices showed significant effects of sawfly infestation in one dataset but not in another. Based on directional variogram analyses, we showed that: (1) better separation of leaf type and infested/non‐infested wheat plants was seen in variograms in longitudinal direction compared to transverse; (2) mainly spectral bands in the red edge and NIR showed consistent effect of sawfly infestation; (3) range values were not affected significantly by either sawfly infestation or leaf type; and (4) sawfly‐induced stress was most likely to be detected about three weeks after infestation. Variogram analysis is one of the key standards in quantitative spatial ecology, and this study supports further research into its use in remote sensing with particular emphasis on detection of biotic stress.

Photosynthesis in Wheat at the Grain Filling Stage Is Altered by Larval Wheat Stem Sawfly (Hymenoptera: Cephidae) Injury and Reduced Water Availability

The impact of larval feeding by wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), and reduced water availability on the photosynthesis and primary metabolism of wheat, Triticum aest...

Impact of Diuraphis noxia and Rhopalosiphum padi (Hemiptera: Aphididae) on Primary Physiology of Four Near-Isogenic Wheat Lines

ABSTRACT The impact of feeding injury by the Russian wheat aphid, Diuraphis noxia (Mordvilko) (Hemiptera: Aphididae), and bird cherry-oat aphid, Rhopalosiphum padi (L.) (Hemiptera: Aphidi-dae) on susceptible and resistant wheat, Triticum aestivum L., near-isogenic lines ‘Tugela’ (susceptible), Tugela-Dn1 (antibiotic), Tugela-Dn2 (tolerant), and Tugela-Dn5 (antixenotic) was evaluated by assessing photosynthetic parameters. Photosynthesis and closely related parameters, pigment composition, and nonstructural carbohydrates were measured at 1, 3, and 9 d after aphids were introduced on plants maintained under greenhouse conditions. Overall, R. padi had a higher reproductive capacity within a period of 9 d compared with D. noxia on all lines except Tugela-Dn2. Although the visible injury symptoms associated with aphid injury can be highly species specific, the data indicate that photosynthetic reduction is a common physiological pattern of wheat response to aphid feeding, irrespective of chlorosis elicitation. Although both aphids negatively affected net photosynthesis, D. noxia had a greater impact than R. padi, even when aphid numbers were considerably fewer for D. noxia (100–150 aphids per plant) compared with R. padi (>200 aphids per plant). The photosynthetic pigment and carbohydrate data suggest that the initial net photosynthesis reduction elicited by aphid feeding may not be directly related to the light reaction portion of the photosynthetic pathway via pigment losses. It is also unlikely that source-sink manipulation is the primary cause for the observed short-term inhibition of photosynthesis.

Photosynthetic Responses of Wheat, Triticum aestivum L., to Defoliation Patterns on Individual Leaves

Abstract The impact of defoliation by fall armyworm, Spodoptera frugiperda (J. E. Smith), on the photosynthetic rates of injured, individual wheat, Triticum aestivum L., leaves and the impact of different spatial patterns of artificial insect defoliation on photosynthesis of remaining leaf tissue of injured, individual wheat leaves were evaluated in this study. Photosynthesis, stomatal conductance, transpiration, and chlorophyll a fluorescence were recorded in the flag-leaves of wheat plants 1 and 24 h after defoliation in 2003 and at 1 h, 24 h, 7 d, and 14 d after defoliation in 2004. Photosynthesis of injured leaves was not significantly affected by any defoliation treatment (i.e., control, natural, and artificial). Similarly, we did not observe interactions between defoliation treatments and time after defoliation. Stomatal conductance was significantly affected by time after defoliation and by the interaction between defoliation treatment and time after defoliation. However, in general, our results showed that wheat responded similarly to insect defoliation and artificial defoliation, which, therefore, may be used to simulate leaf mass consumption. Spatial defoliation patterns had a significant effect on photosynthetic parameters of injured leaves, but responses were dependent on plant developmental stages. The chlorophyll a fluorescence data revealed no significant effects from any defoliation pattern on the photochemical efficiency of the injured leaf. No significant interactions between defoliation patterns and time after defoliation were observed. Our findings reveal that the spatial pattern of defoliation in wheat affects photosynthetic and other gas exchange responses, which suggests that when simulating insect defoliation in wheat, researchers need to be cognizant of the defoliation pattern to adequately simulate insect defoliation.

Photosynthetic Responses of Wheat, Triticum aestivum L., Plants to Simulated Insect Defoliation During Vegetative Growth and at Grain Fill

Abstract The impact of different levels of whole plant partial defoliation (WPPD) on the photosynthesis and primary metabolism of wheat, Triticum aestivum L., was evaluated at the vegetative and reproductive (grain-filling) developmental stages. Photosynthetic parameters such as photosynthesis, stomatal conductance, and transpiration, chlorophyll a fluorescence, and plant morphological parameters, such as main stem height, flag-leaf and undefoliated leaf areas, and number of tillers, were recorded 1 h and 1, 9, and 12 d after defoliation in 2004 and 1 h, 3 d, and 6 d after defoliation in 2005. Plants with high defoliation levels (i.e., defoliation > 75%) had ≈21 and 20% greater photosynthesis rates compared with control and low defoliation level treatments, respectively. Our data show that stomatal conductance for flag leaves was not significantly affected by WPPD. In addition, we did not observe a significant effect of defoliation on intercellular CO2 concentrations or on transpiration rates remaining flag leaf tissue. Similar responses were observed for the overall photosynthesis of defoliated plants during vegetative stages. Whole plant source-sink manipulation of wheat by WPPD during the major plant developmental stages (i.e., vegetative and reproductive) did not elicit any significant long-term modifications to growth, morphological, or primary physiological characteristics of wheat plants.

Characterization of the Impact of Wheat Stem Sawfly, Cephus cinctus Norton, on Pigment Composition and Photosystem II Photochemistry of Wheat Heads

Abstract Impact of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), feeding injury on chlorophyll content and photosystem II (PSII) photochemistry in heads of wheat, Triticum aestivum L., at the grain-filling developmental stage was evaluated by biochemically assessing the total chlorophyll, chlorophyll a (Chla), chlorophyll b (Chlb), chlorophyll a/b ratio (Chla/b), and carotenoid concentrations in the glumes in combination with a chlorophyll a fluorescence test. C. cinctus–infested stems had altered head glume pigment composition and photochemistry. Chlorophyll content, Chla, Chlb, Chla/b, and total chlorophyll, and the photochemical efficiency of PSII were greater for glumes of heads developing on infested stems. Chlorophyll a fluorescence was also affected by C. cinctus. In this study, wheat plants in a controlled environment were able to compensate for injury imposed by C. cinctus. The mechanism underlying the compensatory processes seems to involve the alteration of wheat head physiology. Based on our results, either the photochemical efficiency of heads on infested stems was greatly improved or their senescence was delayed.

Parasitism and the demography of wheat stem sawfly larvae, Cephus cinctus

Previous research has suggested that insect herbivores in protected environments have higher mortalities from biotic factors such as parasitism compared to herbivores in less protected situations, although overall mortalities in protected environments are often lower. However, this has not been examined using life table approaches. In this study, we used demographic methods to characterize the mortality dynamics of parasitism for pre-diapause wheat stem sawfly larvae, Cephus cintus Norton (Hymenoptera: Cephidae), a stem-mining wheat pest. Larvae were intensively sampled from commercial wheat fields at two locations in Montana, USA from 2005 through 2008. The feeding larvae present in the summer succumbed to only two mortality categories: parasitism and unknown factors. In Conrad, a location with relatively high densities of the parasitoids, mortality by parasitism was largely irreplaceable. In contrast, in Amsterdam, where parasitism did not occur to any appreciable degree, overall mortality remained relatively low.