Invasive annual grasses indirectly increase virus incidence in California native perennial bunchgrasses

In California valley grasslands, Avena fatua L. and other exotic annual grasses have largely displaced native perennial bunchgrasses such as Elymus glaucus Buckley and Nassella pulchra (A. Hitchc.) Barkworth. The invasion success and continued dominance of the exotics has been generally attributed to changes in disturbance regimes and the outcome of direct competition between species. Here, we report that exotic grasses can also indirectly increase disease incidence in nearby native grasses. We found that the presence of A. fatua more than doubled incidence of infection by barley and cereal yellow dwarf viruses (B/CYDVs) in E. glaucus. Because B/CYDV infection can stunt E. glaucus and other native bunchgrasses, the indirect effects of A. fatua on virus incidence in natives suggests that apparent competition may be an additional mechanism influencing interactions among exotic and native grasses in California. A. fatua's influence on virus incidence is likely mediated by its effects on populations of aphids that vector B/CYDVs. In our study, aphids consistently preferred exotic annuals as hosts and experienced higher fecundity on them, suggesting that the exotics can attract and amplify vector populations. To the best of our knowledge, these findings are the first demonstration that exotic plants can indirectly influence virus incidence in natives. We suggest that invasion success may be influenced by the capacity of exotic plant species to increase the pathogen loads of native species with which they compete.     

A fluorescent compound in oat root plasma membrane

Homogenates of 7-day-old oat (Avena sativa L. cv. Brighton) roots were highly fluorescent (excitation and emission maxima around 360 and 440 nm, respectively). Less than ¹/₁₀ as much fluorescence per g fresh weight was found in oat shoots or in wheat (Triticum aestivum L. cv. Drabant) roots or shoots. Most of the fluorescence of oat roots was found in the soluble fraction (150 000g supernatant). However, some could be detected in the plasma membrane fraction (excitation and emission maxima 365 and 417 nm, respectively), which contained a 3-fold higher fluorescence per mg protein than the homogenate. Growth of oat or wheat in a medium containing, 10-^?5M scopoletin (6-methoxy-7-hy-droxy coumarin), a fluorescent compound previously reported to be present in both wheat and oat roots, caused the disappearance of scopoletin from the medium (proportional to the amount of roots) and the appearance of increased fluorescence in the root homogenates but not in the shoot homogenates. In both oat and wheat roots ail of the extra fluorescence was recovered in the soluble fraction and at least in wheat it consisted of unconverted scopoletin. The concentration of scopoletin in wheat roots grown in 10-^?5M scopoletin was around 50 nmol (g fresh weight)^?1, or about five times the concentration in the growth medium. Scopoletin in the growth medium (10-^?5M) or in the assays (up to 10-^?4M) did not affect Mg²⁺-, Mg²⁺+K⁺- or Ca²⁺-ATPase activities in wheat or oat roots. The fluorescence properties of the oat plasma membrane were different from those of authentic scopoletin. Either the surroundings modify the fluorescence of membrane-associated scopoletin or the endogenous fluorescent compound is not scopoletin but a glycoside-derivative of scopoletin or some completely unrelated compound.     

Changes in distribution of nucleoids in developing and dividing chloroplasts and etioplasts ofArena sativa

Summary Nucleoid distribution in chloroplasts and etioplasts at the different developmental stages was examined with the first leaves ofAvena sativa by using a DNA-specific fluorescent probe, 46-diamidino-2-phenylindole (DAPI). In light-grown first leaves, three types of plastid nucleoid distribution were recognized. 1. Peripheral distribution in undeveloped chloroplasts which contain only a few thylakoids in the middle region of the leaf sheath. 2. Ring-like arrangement along the rim of developing and dividing young chloroplasts, of which grana were composed of four to eight layers of thylakoids, at the base of the leaf blade. The plane of the nucleoids' ring is in parallel with the face of the thylakoids. 3. Scattered distribution of 10 to 20 discrete spherular nucleoids in the stroma of fully developed chloroplasts, of which grana were composed of up to 20 thylakoids, in the regions of the middle and the tip of the leaf blade. In dark-grown first leaves two types were recognized. 1. Peripheral distribution in developing and dividing young etioplasts in the leaf sheath and the base of the leaf blade. 2. Scattered distribution of 10 or more discrete spherular nucleoids in fully developed etioplasts, containing extended prothylakoids, in the regions of the middle and the tip of the leaf blade. Ring-like arrangement of nucleoids was not observed in any etioplasts. The results indicates that spatial arrangement of plastid nucleoids dynamically changes in close relationship with the development of the inner membrane systems of plastids.     

Quantitative estimation of the surface carbohydrates on the infection structures of rust fungi with enzymes and lectins

Lectins of Triticum vulgaris (WGA), Concanavalia ensiformis (ConA), Phaseolus vulgaris (PHA), Lotus tetragonolobus (LTA), Arachis hypogaea (PNA), Ricinus communis (RCA I), Griffonia simplicifolia (GSA II) and the enzymes endo-(13)--D-glucanase, exo-(13)--D-glucanase and laminarinase were tested for binding to the infection structures of Puccinia coronata and Uromyces appendiculatus. The enzymes and lectins were labeled with fluorescein and the fluorescence was measured with a microscope photometer. GSA II and ConA bound to all parts of the two rust fungi to a certain extent. The germ tubes of P. coronata bound at least two times more WGA than did the germ tubes of U. appendiculatus. The appressoria of both rust fungi additionally bound exo-(13)--glucanase, endo-(13)--glucanase and laminarinase. The substomatal vesicle and the infection hypha of both rust fungi mainly bound the glucanases. Furthermore, the substomatal vesicle of U. appendiculatus bound PHA. No obvious binding with LTA, RCA I and PNA was observed. Binding generally could be inhibited by appropriate haptens. Binding to uredospores generally appeared unspecific. The results indicate that the germ tubes have chitin on their outer surfaces, the appressoria chitin and glucans and the substomatal vesicles and infection hyphae mainly glucans. Compared to P. coronata, U. appendiculatus has more terminal linked glucose residues or the glucan has more (13)--linkages. Also, U. appendiculatus has N-acetylgalactosamine or a similar sugar on the surface of the substomatal vesicle.Abbreviations ConA Concanavalia ensiformis agglutinin - FITC fluorescein isothiocyanate - GSA II Griffonia simplicifolic agglutimin II - LTA Lotus tetragonolobus agglutinin - PBS phosphate buffered saline - PNA Peanut agglutinin - RCA I Ricinus communis agglutinin I - PHA Phaseolus vulgaris agglutinin - WGA Wheat germ agglutinin     

Irradiance-dependent senescence of isolated leaves

Isolated leaves from pea ( Pisum sativum L. cv. Alaska or the genotype L-1107), oat ( Avena sativa L. cv. Victory), and fuchsia ( Fuchsia triphylla L. cv. Koralle) were retained at irradiances between 0 and 130 μmol m^-2 s^-1 PAR (photosynthetic active radiation). Irradiance-dependent CO₂ fixation was measured when the leaves were excised, and time-dependent changes in light compensation point were determined. If the irradiance was below the light compensation point for CO₂ fixation, the respiratory quotient was low, indicating that lipids were respired. The isolated leaves senesced at these low light levels. At higher light intensities the decrease in chlorophyll level was not accompainied by diminished protein level, and the respiratory quotient was close to unity. Only an irradiance equal to or slightly above the light compensation point maintained a stable chlorophyll level for a long time. This suggested that depletion of reserves in darkness or at low levels of irradiance is important for the initiation of the senescence syndrome. At high levels of irradiance, the decrement in chlorophyll level may be caused by photooxidation. Only in leaves placed under an irradiance close to the compensation point, was CO₂ fixation able to prevent aging of the leaves.     

Effects of the growth retardant tetcyclacis on the sterol composition of oat (Avena sativa)

The norbornenodiazetine plant growth regulator tetcyclacis, when applied to roots of Avena sativa, caused a substantial increase in the cholesterol content of the shoots. Amounts of the C-24 alkylated sterols campesterol, stigmasterol and sitosterol all declined. A similar alteration in the sterol profile was observed for a plasma membrane preparation from the shoots. Changes in the sterol composition of root tissue were much less pronounced.     

Sugar Transport and Sulfite Action in Etioprotoplasts,Semi-Etioprotoplasts and Green Protoplasts of Avena sativa L.

The mechanism of glucose and sucrose transport and the influence of various concentrations of sulfite on its activity was studied in mesophyll protoplasts (etioprotoplasts, semi-etioprotoplasts and green protoplasts) isolated from oat (Avena sativa L.) seedlings. Kinetic analysis of [¹⁴C] glucose loading (in darkness) revealed in each kind of protoplasts the presence of two transport components. At low exogenous glucose concentrations a saturable system was the main mode of transport. At concentrations higher than 20 mM the loading of glucose in all types of protoplasts was dominated by a non-saturable, linear diffusion-like component. The rate of glucose uptake was greatest in etioprotoplasts and lowest in green protoplasts. In contrast to the above we have not found saturable components of sucrose transport in any kind of protoplasts. The rate of its uptake was greatest in semi-etioprotoplasts. Sulfite, at a concentration of < 1.0 mM stimulated and at ≥ 1.0 mM inhibited the uptake of glucose to etioprotoplasts and semi-etioprotoplasts and inhibited that to green protoplasts at any concentration. The transport of sucrose underwent a significant inhibition in the various types of protoplasts only under the influence of 10.0 mM of sulfite ions. Inhibition of glucose uptake by sulfite was of the non-competitive type. Sulfite also affected the level of adenylic nucleotides and lowered the energy charge and ATP/ADP ratio. Intensity of sulfite uptake was significantly higher in green protoplasts than in etioprotoplasts.     

Thigmomorphogenesis: XXIII. Promotion of foliar senescence by mechanical perturbation of Avena sativa and four other species

Mechanical perturbation (MP, gentle tubbing) promoted the senescence of detached oat ( Avena sativa L. cv. Victory) leaf segments in the dark. The promotion of senescence increased with increase in the number of rubbings and could be seen after 24 h of dark incubation; the maximum effect was reached on day 3. The effect (% of control) of MP on the loss of protein was greater than the effect on chlorophyll (Chl) loss on day 1. However, on day 3 the effect of MP on the loss of Chl became greater than the effect on the loss of protein. Ethephon and 1-aminocyclopropane-1-carboxylic acid (ACC) marginally promoted the loss of Chl by both control and rubbed oat leaf segments, and the effect was additive with MP. Chloramphenicol (CAP), spermine, aminoethoxyvinylglycine (AVG) and Ca²⁺ marginally delayed the loss of Chl and protein in both control and rubbed segments. Kinetin greatly retarded the senescence of all segments. Even in the presence of these substances, the amounts of Chl and protein in the rubbed segments were always less than in their respective controls, thus retaining the effect of the MP. However, abscisic acid (ABA) and cycloheximide (CHI) caused the rubbed oat leaf segments to retain more Chl and protein than their respective control segments. The effect of CHI was actually enhanced by MP. Rubbing promoted the senescence of attached leaves of oats ( Avena sativa L. cv. Victory), maize ( Zea mays L. cv. Early Belle) and pumpkin ( Cucurbita pepo L. cv. Jack-o-lantern) cotyledons in the dark. Rubbing promoted the senescence of oat leaf segments even in light, although to a lesser extent compared to the effect in the dark. The senescence of leaves of pumpkin and cocklebur ( Xanthium strumarium Wallr. var. Pennsylvanicum ) in situ was also enhanced by MP.     

Improving cultivated oats (A vena sativa L.) with alleles for vegetative growth index from A. sterilis L.

Summary Ten Avena sterilis L. lines of Mediterranean origin were crossed with six A. sativa L. cultivars from the North Central USA. Additionally, six intervarietal crosses were made among the A. sativa cultivars. F₂- derived lines from each cross type (interspecific and intraspecific) were evaluated for transgressive segregation for grain yield and several vigor traits. Mean percentages of transgressive segregates one LSD_0.05 above the high parent for vegetative growth index and biomass were 9.0% and 9.8%, respectively, from interspecific crosses, but only 4.5% and 2.9%, respectively, from intraspecific crosses. However, there were two and a half times more high transgressive segregates for grain yield from intra than from interspecific crosses. The maximum vegetative growth index among segregates from interspecific crosses was 0.2 q/day/ha greater than the highest segregate from intraspecific crosses. However, mean harvest index was reduced materially by the introgression of A. sterilis germplasm. Because there was no genetic association between vegetative growth index and harvest index, however, it should be possible to improve both harvest index and vegetative growth index and, thus, the grain yield of cultivated oats.Journal Paper No. J-11228 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA 50011. Project 2447     

Inhibition of gravitropism in oat coleoptiles by the calcium chelator, ethyleneglycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid

A treatment period as brief as 8 h in 10⁻³ M EGTA completely blocks gravitropism in 70–80% of the treated coleoptiles of oats ( Avena sativa , L. cv. Garry) without inhibiting growth. Only about 10% of the plants perfused in water failed to exhibit gravitropism. Subsequent perfusion of EGTA-treated plants with calcium completely restores gravitropism; post-perfusion with water does not. After perfusion in water for 10 h, gravistimulated oat coleoptile segments show the same asmmetry of ⁴⁵Ca distribution as reported earlier for non-perfused coleoptiles and sunflower hypocotyls. The degree of this asymmetry is reduced in those coleoptiles partially completely inhibited by perfusion in EGTA and is essentially absent in those coleoptiles completely inhibited by EGTA. The fact that calcium reverses the inhibitory effects of EGTA on gravitropism indicates that the inhibition was probably due to a reduction in the availability of free calcium required for one or more of the transduction steps of gravitropism.