Predator-mediated interactions among the seeds of desert plants.

In theory, seed predators are capable of inducing indirect interactions among the seeds they consume. However, empirical evidence of predator-mediated interactions among seeds is rare. Rodents in the Heteromyidae are highly granivorous and therefore likely to induce indirect interactions among the seeds of desert plants. The indirect interactions may be in the form of apparent competition and apparent mutualism between seeds within a patch. Apparent competition exists when the survival of seeds of a focal species is lessened because of the presence of additional seeds of other species in the patch. Apparent mutualism exists when the presence of the other seeds results in an increase in survival of seeds of the focal species. By measuring seed removal from trays placed in the field, apparent competition between the seeds of several plant species was detected. Apparent mutualism might also exist, but there was no strong evidence of it. Apparent competition appeared most likely to occur among the species whose seeds were the most heavily predated. For instance, predation on seeds of Astragalus cicer, Oryzopsis hymenoides, and Sphaeralcea coccinea was substantial with more than 50% of the seeds removed from the trays, on average. The intensity of apparent competition (measured by the indirect effect, IS) between these species and two others was significant; IS ranged from -0.02 to -0.39 on a scale of 0 to -1. This indicates that, in some communities, indirect effects are most likely to exist when direct effects are strong.

Reproduction of Four Races of Meloidogyne incognita on Hibiscus cannabinus.

The feasibility of cultivation of kenaf (Hibiscus cannabinus) in the United States is receiving a multifaceted evaluation. Among the factors being evaluated is kenafs susceptibility to nematodes. In this investigation, four races of Meloidogyne incognita reproduced extensively on each of the several kenaf genotypes examined in greenhouse tests. Some genotypes of kenaf, however, demonstrated limited resistance to certain races of M. incognita.

Counting nematodes with a microplate reader.

The feasibility of counting plant-parasitic nematodes in aqueous suspensions by measuring light transmittance through aqueous suspensions with an ELISA microplate reader was explored. Absorbance readings for eggs or vermiform stages of three species were linearly related (R(2) > 0.99) to concentrations between 0 and 10,000 nematodes/ml. Coefficients of variation ranged from 12-23%, depending on the species and developmental stage used. The method, therefore, was at least as accurate as direct counts of nematodes in aliquots on a microscope and more than 100 times as fast. The method should have direct application in research programs on plant resistance to nematodes, nematode population dynamics, and nematode behavior.

Collapse of intercellular spaces of canine tracheal mucosa by epinephrine.

Variations in the volume and width of lateral intercellular spaces (LICS) of dog tracheal mucosa in vitro were investigated by use of stereological and linear measurements of electron micrographs. Alterations in the volume or width of LICS were then correlated with physiological conditions and electrical parameters. LICS were quite narrow between the ciliated cells compared with those around the nonciliated dark cells (goblet, brush, and basal cells). LICS comprised 6.8 +/- 2.9% of tissue volume in preparations that were mounted in an Ussing chamber and short-circuited, whereas in unmounted and open-circuited tissues it occupied only 1 +/- 0.2% of the volume of the preparations (P less than 0.016, n = 5). The effects of stimulation of Cl secretion by 1 microM epinephrine were tested. In seven epinephrine-treated tissues LICS volume was 2.9 +/- 0.9% of total epithelial volume compared with 8.7 +/- 2.9% in control tissues (P less than 0.015). The width of LICS around dark cells in epinephrine-treated tissues was 0.42 +/- 0.06 micron compared with 0.98 +/- 0.13 micron in control tissues (P less than 0.001). The data suggest that LICS act as pliable fluid reservoirs that empty and collapse on stimulation of Cl secretion.

Production and Partial Characterization of Stylet Exudate from Adult Females of Meloidogyne incognita.

Adult females of Meloidogyne incognita were excised from tomato roots and incubated in 0.04 M phosphate buffered saline, pH 7.4 for 18-72 hours to allow accumulation of stylet exudate. Twenty-four percent of the females produced exudate during the initial 18-hour incubation period; 70% of those females producing exudate initially produced additional exudate during the subsequent 54-hour incubation period. Analysis of exudate by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of at least nine major protein bands. Differential staining with silver and Coomassie Brilliant Blue G-250 stains indicated that three of the bands were glycoproteins. Upon acid hydrolysis, 14 amino acids were detected in the stylet exudate. The basic amino acids lysine, histidine, and arginine comprised 21.8% of the total amino acids detected. No peroxidase activity was detected in the stylet exudates. Data presented extend and generally confirm prior work on the chemical composition of stylet exudate.

Phytoalexins and their Role in the Resistance of Plants to Nematodes.

Phytoalexins are antibiotic compounds synthesized in an infected plant in response to infection. Nematodes are capable of eliciting phytoalexins in resistant plants. Resistant lima bean (Phaseolus lunatus) infected by Pratylenchus penetrans produces the phytoalexin coumestrol; soybean (Glycine max) infected by Meloidogyne incognita produces glyceollin; cotton (Gossypium hirsuturn) infected by M. incognita produces terpenoid aldehydes.

Histochemical localization and nematoxicity of terpenoid aldehydes in cotton.

In healthy cotton, except for random occasional occurrence in cortical cells, terpenoid aldehydes (TA) are localized in the epidermis and, even there, are absent from the tip 2-4 cm of the root. Since constitutive TA do not occur in the endodermis and stele of the root, they cannot be effective agents against the development of the sedentary stage of the root-knot nematode, Meloidogyne incognita. Within 4 days after inoculation with the root-knot nematode, infection-induced TA accumulated in the endodermis and outer stele. These induced TA were thus localized where they could be effective against the sedentary stage of the nematode. Infection-induced TA accumulation was more rapid and occurred in more stele cells in a resistant cotton cultivar than in two susceptible cultivars.TA extracts from cotton were inhibitory to nematode movement. All second-stage larvae exposed to 1,000 ppm TA for 3 h became rigid, made no movement, and appeared dead. Washing these larvae to remove the TA and incubating them for an additional 24 h did not change their appearance. Shorter exposure times or lower TA concentrations allowed some larvae to recover. Exposing larvae to 10 ppm of TA for 24 h had little effect on them. TA extracted from G. arboreum, a cotton that does not methylate TA, were slightly less inhibitory to the root-knot nematode than TA extracted front G. hirsutum which partially methylates TA.

Terpenoid aldehydes in cotton roots susceptible and resistant to the root-knot nematode, Meloidogyne incognita.

We investigated the role of terpenoid aldehydes in the resistance of cotton (Gossypium hirsutum) to the root-knot nematode (Meloidogyne incognita). Three-day-old, root-knot-resistant ('Auburn 623') and -susceptible ('Deltapine 16') seedlings were inoculated with M. incognita. Comparable portions of inoculated and noninoculated roots were harvested 2, 4, 7, and 10 days later. Terpenoid aldehydes were extracted, separated by thin-layer chromatography, eluted as their phloroglucinol derivatives, and measured colorimetrically. In noninoculated seedlings of each age, the susceptible cultivar contained more total and more of each of five specific terpenoid aldehydes (hemigossypol, methoxyhemigossypol, gossypol, lnethoxygossypol, dimethoxygossypol) than did the resistant cultivar. In both cultivars, the concentration of terpenoid aldehydes increased as seedlings aged. After inoculation, the concentration of terpenoid aldehydes was usually highest in the noninoculated, followed by the infected susceptible, infected resistant, and noninfected resistant seedlings in that order. The changes in concentration that occurred in response to infection, particularly at 7 and 10 days after inoculation, did correlate with host resistance, i.e., there was a net loss of total and each specific terpenoid aldelhyde in tlae susceptible cultivar, and a net gain in the resistant. Our data do not exclude the possibility that localized synthesis of terpenoid aldehydes is involved in resistance to root-knot nematodes.

The Histochemical Localization of Several Enzymes of Soybeans Infected with the Root-knot Nematode Meloidogyne incognita acrita.

The sites of activity of alkaline phosphatase, acid phosphatase, esterase, peroxidase, adenosine triphosphatase, and cytochrome oxidase were demonstrated histochemically in fresh sections of 'Lee' soybeans infected by the root-knot nematode Meloidogyne incognita acrita. Each of the six enzymes was more active at the sites of infection than in adjacent non-infected tissue. During the early stages of infection, an increase in enzyme activity was observed in several cells in the proximity of the lip region of the nematode. However, when definite syncytia were observed, increased enzyme activity was confined primarily within the limits of the syncytium. Increased activity paralleled syncytial development and nematode maturation.