Fikru J. Haile

Changes in Soybean Gas-Exchange After Moisture Stress and Spider Mite Injury

Abstract Field experiments were conducted in 1997 and 1998 to understand the physiological responses of soybean, Glycine max (L.) Merrill, to injury from two-spotted spider mite, Tetranychus urticae Koch, and to examine the contribution of soil moisture toward soybean tolerance to spider mite injury. A split-plot treatment design was used consisting of moisture stress as the main plot and spider mite injury as the sub-plot treatments in a randomized complete block design with eight replications. Soybeans were moisture-stressed beginning approximately at V10 growth stage, and spider mites were maintained on soybeans for 10 d, and then physiological responses of soybean were determined. Moisture-stressed soybeans had lower leaf water potentials, photosynthetic rates, stomatal conductances, and transpiration rates. Spider mite injury also caused a significant reduction in photosynthesis, stomatal conductance, transpiration, and chlorophyll content. The lack of a significant impact of spider mite injury on chlorophyll fluorescence and similar light curves, at low light intensities, of soybean leaves with and without spider mite injury, suggest that spider mite injury does not interfere with the light reaction center at the initial stage of photosynthesis. Despite measurable reductions in total chlorophyll content from mite injury, fluorescence data and light curves strongly indicate photosynthetic rate reductions from mite injury were not immediately associated with chlorophyll loss or effects on photosynthetic electron transport. There were significant interactions between moisture stress and spider mite injury for some gas-exchange parameters. Photosynthetic rate reductions by spider mites were greater in moisture-unstressed than stressed soybeans. Superimposing spider mite injury did not reduce photosynthetic rates greatly in moisture-stressed soybeans because of an initially low photosynthetic rate from moisture stress. However, comparison of absolute photosynthetic rates of spider mite-injured soybean leaves with and without moisture stress suggested that soil moisture improved soybean tolerance to spider mite injury.

Influence of Western Corn Rootworm (Coleoptera: Chrysomelidae) Larval Injury on Photosynthetic Rate and Vegetative Growth of Different Types of Maize

Abstract Two field experiments were conducted in 1995–1996 to determine if there are common photosynthetic and vegetative growth responses among genotypes of maize, Zea mays L., to larval western corn rootworm, Diabrotica virgifera virgifera LeConte injury. Specific variables measured from rootworm infested and noninfested plants were midday leaf photosynthetic rate, plant growth stage, and plant height during 9–18 leaf growth stages. Three yellow-dent hybrids, five white food-grade dent hybrids, and a popcorn hybrid were included in the study. Results suggest that there may be a common negative photosynthetic response within maize to larval injury during vegetative growth stages. Transient reductions in photosynthetic rate occurred in rootworm infested maize at both low to moderate levels of root injury, which after a lag period, led to significant reductions in plant height. This trend was consistent across hybrids in both experiments during each year. Plant growth stage was not significantly affected by rootworm injury during vegetative periods.

Impact of Insecticides and Surfactant on Lettuce Physiology and Yield

Abstract Insecticides are used extensively on lettuce, Lactuca sativa L., grown in southwestern Arizona because of heavy insect pressure that can potentially reduce lettuce productivity. Multiple sprays are made per season to manage these insects in lettuce. One of the major concerns related to extensive insecticide applications in lettuce is the potential subtle impact of insecticides that may reduce lettuce photosynthesis and yield. We conducted field and greenhouse experiments to examine the impact of multiple insecticides and surfactant spray applications on lettuce photosynthesis and yield. Lettuce was planted in the field in 1998, insecticides and surfactant were applied, and lettuce gas-exchange and dry weights were determined. Treatments were arranged in a split-plot consisting of insecticides as main plot and surfactant as subplot treatments in a randomized complete block design with four replications. Photosynthetic rates of lettuce were significantly reduced by endosulfan, methomyl, acephate, and surfactant at seedling stage 4 h and 2 d after the spray application was made. However, the reduction in lettuce photosynthesis by these insecticides and surfactant was only transient, and lettuce photosynthesis recovered 5 d after the spray application was made. Photosynthetic rates were not altered by zeta-cypermethrin, emamectin benzoate, and spinosad at the seedling stage. Insecticides or surfactant (Kinetic, a nonionic surfactant) did not significantly affect lettuce photosynthesis after rosette formation. In addition, lettuce dry weight was not significantly altered. These studies suggest that lettuce photosynthesis may be susceptible to some insecticides at the seedling stage. Consequently, we found that biorational insecticides, introduced to manage insect pests in lettuce, have no influence on lettuce physiology at the seedling stage, unlike the chlorinated hydrocarbons, organophosphates, or carbamates tested in this study. In a greenhouse study, we found that lettuce photosynthesis and yield were not altered by Bacillus thuringiensis application. Our results indicate that B. thuringiensis and the newer insecticides, particularly biorationals, can be used to manage lettuce insect pests without significantly altering lettuce gas-exchange and yield.

Seasonal patterns of leaf photosynthesis after insect herbivory on common milkweed, Asclepias syriaca: reflection of a physiological cost of reproduction, not defense?

Abstract After defoliation occurs leaf photosynthetic rate (Pn) can increase, decrease or remain unchanged relative to control uninjured leaves depending on the plant species. With common milkweed Asclepias syriaca (Asclepiadaceae) we conducted 27 experiments with insect herbivory or mechanical tissue removal to examine whether A. syriaca leaf Pn reductions were correlated with the occurrence of gross cardenolide induction and/or with reproductive phenology. Using spectrophotometry, positive cardenolide induction was detected in only one study when Pn impairment was detected with gas exchange data from injured A. syriaca leaves, while negative or no cardenolide induction was detected in the other five A. syriaca studies with Pn impairment. The occurrence of Pn impairment after partial leaf defoliation did have a seasonal pattern which correlated with A. syriaca reproductive phenology: little or no Pn impairment occurred on leaves of pre-flowering or maturing seed pod plants, while moderate to severe leaf Pn impairment occurred on leaves of flowering and early seed pod formation plants. Our results fail to support either constitutive cardenolide levels or gross cardenolide induction trade-offs to be reflected in injured leaf Pn impairment; however, our results could be explained by a resource or hormone trade-off between investment into reproduction with maintaining leaf photosynthesis after herbivory. Specifically, we suggest that a physiological ‘cost of reproduction’ is increased susceptibility to Pn impairment after herbivory injury on a leaf. Future studies will need to examine whether resource or hormonal regulation trade-offs cause this proposed physiological trade-off between reproduction and photosynthesis.