Comparative responses of the Savanna grasses Cenchrus ciliaris and Themeda triandra to defoliation

Summary Two perennial tussock grasses of savannas were compared in a glasshouse study to determine why they differed in their ability to withstand frequent, heavy grazing; Cenchrus ciliaris is tolerant and Themeda triandra is intolerant of heavy grazing. Frequent defoliation at weekly intervals for six weeks reduced shoot biomass production over a subsequent 42 day regrowth period compared with previously undefoliated plants (infrequent) in T. triandra, but not in C. ciliaris. Leaf area of T. triandra expanded rapidly following defoliation but high initial relative growth rates of shoots were not sustained after 14 days of regrowth because of reducing light utilising efficiency of leaves. Frequently defoliated plants were slower in rate of leaf area expansion and this was associated with reduced photosynthetic capacity of newly formed leaves, lower allocation of photosynthate to leaves but not lower tiller numbers. T. triandra appears well adapted to a regime where defoliation is sufficiently infrequent to allow carbon to be fixed to replace that used in initial leaf area expansion. In contrast, C. ciliaris is better adapted to frequent defoliation than is T. triandra, because horizontally orientated nodal tillers are produced below the defoliation level. This morphological adaptation resulted in a 10-fold higher leaf area remaining after defoliation compared with similarly defoliated T. triandra, which together with the maintenance of moderate levels of light utilising efficiency, contributed to the higher leaf area and shoot weight throughout the regrowth period.     

Effect of stress and time for recovery on the amount of compensatory growth after grazing

Summary We tested the hypothesis that the amount of compensatory growth after defoliation is affected by the level of stress at which plants grow when defoliated and by the length of time for recovery. Growth response to defoliation went from partial compensation when plants were growing at high relative growth rates (RGR) to overcompensation when plants were more stressed and growing at low RGR. Defoliation released plants from the limitation imposed by the accumulation of old and dead tissue and this release overrode the negative effect of biomass loss. Compensatory growth resulted from a higher RGR aboveground that was not associated with a reduction in RGR belowground. Time available for recovery had a major impact on the outcome of defoliation. With a short time for recovery, RGR was decreased by defoliation because an immediate increase in net assimilation rate was overridden by a reduction in the ratio of leaf area to plant weight. After defoliation, this ratio increased quickly due to a larger allocation to leaf growth and lower leaf specific weights, resulting in higher RGR. We conclude that the compensatory response to grazing depends on the type and level of stress limiting growth. Allocation and physiological responses to stress may positively or negatively affect the response to grazing and, simultaneously, grazing may alleviate or aggravate the effects of different types of stress.     

Effect of defoliation intensity on aboveground and belowground relative growth rates

According to a simple growth model, grazed and ungrazed plants may have equal absolute growth rates provided that the relative growth rate (RGR) of grazed plants increases exponentially with grazing intensity (proportion of biomass removed). This paper reports results from an experiment designed to determine whether plants of two grass species subjected to a gradient of defoliation intensities, from 0 to 100% aboveground biomass removal, showed such a response. The relationship between aboveground RGR and defoliation intensity was exponential and closely matched the theoretical relationship of equal absolute growth rate. Thus, plants showed the same aboveground growth regardless of defoliation intensity thanks to an exponential stimulation of RGR by defoliation. Belowground RGR was depressed by defoliation of more than 20% of the above-ground biomass. In spite of the drastic modification imposed by the treatments on the relative proportions of different plant parts, after a 42-day recovery period basic allometric relationships, such as root:shoot and leafarea: weight ratios, were not affected by defoliation intensity. Exponential aboveground compensatory responses represent a key feedback process resulting in constant aboveground growth regardless of defoliation intensity and appear to be a simple consequence of strong commitments to certain allometric relationships.     

Plant biomass partitioning and chemical defense: Response to defoliation and nitrate limitation

Summary Plant growth and allocation to root, shoot and carbon-based leaf chemical defense were measured in response to defoliation and nitrate limitation inHeterotheca subaxillaris. Field and greenhouse experiments demonstrated that, following defoliation, increased allocation to the shoot results in an equal root/shoot ratio between moderately defoliated (9% shoot mass removed) and non-defoliated plants. High defoliation (28% shoot mass or >25% leaf area removed) resulted in greater proportional shoot growth, reducing the root/shot ratio relative to moderate or non-defoliated plants. However, this latter effect was dependent on nutritional status. Despite the change in distribution of biomass, defoliation and nitrate limitation slowed the growth and development ofH. subaxillaris. Chronic defoliation decreased the growth of nitrate-rich plants more than that of nitrate-limited plants. The concentration of leaf mono- and sesqui-terpenes increased with nitrate-limitation and increasing defoliation. Nutrient stress resulting from reduced allocation to root growth with defoliation may explain the greater allocation to carbon-based leaf defenses, as well as the defoliation-related greater growth reduction of nitrate-rich plants.     

Lack of intraspecific variation in resistance to defoliation in a grass that evolved under light grazing pressure

Grass species frequently show marked intraspecific variation inmorphology and tolerance to defoliation. Remarkably, most of this evidencecomesfrom grasslands with long evolutionary history of grazing. Here, we explore theintraspecific variation in grazing tolerance and morphometric traits associatedwith grazing avoidance of Paspalum dilatatum (Poir.), agrass from the Flooding Pampa (Argentina), where grazing is a novel disturbancein evolutionary time. We performed a clipping experiment in a greenhouse withtwo populations from sites with contrasting short term grazing regime:continuous grazing and 20 year-old grazing exclosure. The populations did notdiffer in their tolerance to clipping, and showed minor differences in the wayclipping affected plant height, a trait associated with grazing avoidance. Ourresults indicate that there are exceptions to the generalized findings of highlevels of intraspecific variation in grazing resistance among populations ofgrasses. These exceptions may be associated to evolutionary history of grazing.     

Differences in response to defoliation between males and females of Silene dioica

Summary The response by male and female plants to herbivory was studied by experimental defoliation of the dioecious perennial herb Silene dioica in a green-house. Male and female plants were defoliated prior to and during the early flowering phase at two intensities (50% and 100% of leaf-area removed) in two consecutive years. Defoliation resulted in a decrease in the number of flowers initiated in both sexes, while a larger delay of peak flowering and a higher mortality was observed in males compared to females. In female plants, severe defoliation resulted in a reduction in seed number per capsule and in seed size compared to control. Females showed a negative correlation between the production of flowers in the first and second season in all treatments, while flowering in males the first season was not correlated with flowering in the second season. Females also showed a lower frequency of flowering than males during the two seasons studied. However, during the flowering period, males allocated significantly more biomass to flowers than did females. This outcome supports the idea that females may have a higher total reproductive expenditure than males, but males have a higher reproductive effort during flowering. Male rosette leaves were significantly preferred by the generalist herbivore Arianta arbustorum in experiments. This preference was most pronounced in trials with leaves from fertilized plants compared to nonfertilized plants. A greater storage of resources in aboveground leaves during winter by males compared to females may explain the higher preference for male leaves and the higher male mortality following early defoliation. Furthermore, males are smaller than females and may have a lower ability than females to replace lost resources needed for reproduction when defoliated early in the season.     

The effects of host plant defoliation and fertilizer application on larval growth and oviposition behaviour in cinnabar moth

Summary Defoliated ragwort plants produced regrowth foliage that was higher in alkaloid, but lower in amino acid concentrations than primary foliage. Total N was not affected. 2) Plants fertilized with nitrogen (as ammonium sulphate) had lower amino acid concentrations than unfertilized control plants, slightly increased alkaloid levels but similar total N concentrations. 3) Ovipositing females laid eggs upon plants with equal probability for controls, regrowth and fertilized foliage (one rosette in 5 received an egg batch). However, the probability of receiving eggs was significantly lower on the primary leaves of cut-back plants that had had their lower leaves removed a few days before egg laying (only one rosette in 13 was selected). 4) Egg batch size was higher on fertilized control foliage than on other treatments. 5) Larvae attained greater final weights when fed a diet of regrowth foliage, despite the higher levels of alkaloid they contained. Larval development rate was not affected by experimental treatment of the foliage. 6) Larval growth was lowest on the leaves of fertilized plants. This was associated with significant reductions in the concentrations of three amino acids (methionine down 29%, tyrosine 33% and lysine 25%).     

Sectorial root growth in cuttings of Coleus rehneltianus in response to localized aerial defoliation

Asymmetries in root growth in response to localized aerial defoliation were examined in Coleus rehneltianus (Lamiaceae). We confirmed that assimilate transport was sectorial by examining the distribution of ¹⁴C-labeled carbohydrates following a 24-h chase period. Integrated physiological units (IPUs), or sectors, extended from the leaves into the roots, and this was reflected in the differential growth of roots following artificial defoliation of part of the leaf canopy. When defoliation was localized within leaves or leaf halves within sectors, roots grew asymmetrically, with decreased root growth in defoliated sectors. Three root populations were identified by their location and growth responses: stem side, stem corner, and bottom side roots, and asymmetric growth was observed in all three populations. Only the growth of stem corner roots, which made up 35–90% of dry mass of the total root population, was influenced by the pattern of aerial defoliation. In contrast, asymmetries in the growth of the other two root populations appeared to reflect the distribution of leaf biomass prior to defoliation.     

Influence of partial defoliation of green pepper on the senescence,growth, and nitrate reductase of the remaining leaf

Summary Removal of all but one leaf from pepper plants prevented the senescence of the remaining leaf and caused increases of approximately 140, 200, and 200%, respectively in leaf area, weight, and nitrate reductase activity. Development of the fruit (fresh and dry weight increases) was only approximately 65% of that of fruit on control plants.     

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation

Summary The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.     

Intraspecific variation in resistance of frog eggs to fungal infection

Documenting sources of variation in host viability at pathogen exposure within and among populations is an important task in order to predict host-pathogen evolutionary dynamics. In the present study, we investigated family and population variation in the degree of embryonic infection of the pathogenic fungus Saprolegnia spp., by infecting moor frog (Rana arvalis) eggs from six populations and exposing them to two different temperatures. We found a significant family effect on the degree of Saprolegnia-infection of eggs and embryos, suggesting that there is genetic variation in resistance among embryos, or variation among females in some aspect of maternally induced resistance. Furthermore, infection level differed significantly between temperatures, with most families having more infected eggs in the relatively colder temperature. However, eggs and embryos from the different populations showed different degrees of Saprolegnia-infection in the two temperatures, i.e., there was a significant population × temperature interaction on the proportion of infected eggs. Thus, the degree of Saprolegnia-infection is sensitive to variation at the level of the family, population and environmental conditions, suggesting that responses to fungal outbreaks will vary geographically and will be difficult to predict.     

The effect of manual defoliation and Macaria pallidata (Geometridae) herbivory on Mimosa pigra: Implications for biological control

Trials were conducted to test the effects of artificial defoliation and defoliation by Macaria pallidata (Warren) (Geometridae) larvae on the invasive weed Mimosa pigra L. Herbivory is generally thought to be detrimental to plant fitness but it is well documented that many plants can increase growth rates or reproduction to compensate for damage. The compensatory ability of an invasive plant has implications for the potential success of defoliating biocontrol agents. Mimosa compensated for 25% manual defoliation, but at 50% and 100% defoliation levels plants suffered a significant reduction in growth rate, height, stem diameter, and biomass. Defoliation by one cohort of macaria larvae, at densities of eight larvae per plant, significantly reduced growth rates and plant height after 1 week. There were no differences between the effects of macaria larvae and manually simulated defoliation. These results suggest that defoliating biocontrol agents can have a valuable role in mimosa control programs.     

Plasmodesmatal distribution and frequency in vascular bundles and contiguous tissues of the leaf ofThemeda triandra

Small and intermediate vascular bundles and contiguous tissues of the leaf blade ofThemeda triandra var.imberbis (Retz.) A. Camus were examined with transmission and scanning electron microscopes to determine the distribution and frequency of plasmodesmata between various cell types. Plasmodesmata are most abundant at the mesophyll/bundle-sheath cell and bundle-sheath/vascular parenchyma cell interfaces, and their numbers decrease with increasing proximity to both thick- and thin-walled sieve tubes. Among cells of the vascular bundles, the greatest frequency of plasmodesmata occurs between vascular parenchyma cells, followed by that of plasmodesmata between vascular parenchyma cells and companion cells, and then by the pore-plasmodesmata connections between companion cells and thin-walled sieve tubes (sieve tube-companion cell complexes). The sieve tube-companion cell complexes of theT. triandra leaf are not isolated symplastically from the rest of the leaf and, in this respect, differ from their counterparts in theZea mays leaf. However, the thick-walled sieve tubes, like their counterparts inZea mays, lack companion cells and are symplastically connected with vascular parenchyma cells that about the xylem.Abbreviations SEM scanning electron microscope - TEM transmission electron microscope     

Physiological integration in Cassia fasciculata Michx.: inflorescence removal and defoliation experiments

Summary In the annual herb Cassia fasciculata virtually every leaf subtends an axillary inflorescence. We examined the degree to which these leaf-inflorescence units (reproductive nodes) were physiologically independent of each other in the production of flowers, fruits, and seeds. Removal of up to 4 of every 5 inflorescences resulted in substantial increases in fruit and seed production by remaining, intact reproductive nodes. These increases nearly compensated for and manipulated reproductive nodes were associated with different vascular strands. When 2 of every 3 leaves were removed, fruit and seed production were reduced at both intact and defoliated reproductive nodes. Taken together, these results suggest that neighboring reproductive nodes in C. fasciculata are not physiologically independent of one another, and that competition among fruits and seeds for parental resources occurs over several reproductive nodes.Scientific contribution no. 1595 from the New Hampshire Agricultural Experiment Station     

Patterns in grass silicification: response to grazing history and defoliation

Summary Morphologically distinct populations of a North American perennial grass, Agropyron smithii, collected from a heavily grazed prairie dog (Cynomys ludovicianus) colony (PDC) and a grazing exclosure (EX), were grown in an environmental chamber to determine whether: (1) leaf silicon (Si) concentrations are greater in plant populations which differentiated under heavy grazing pressure, and (2) leaf silicification is inducible by defoliation. Mean shoot Si concentration of nondefoliated plants was greater in the PDC population (2.2%) than the EX population (1.9%) over the 18 wk experiment, largely as a result of differences in Si concentrations in leaf blades. However, leaf Si concentration was lower in defoliated plants of each population than in nondefoliated plants, indicating that leaf silicification was not an inducible herbivore defense mechanism in A. smithii. The higher leaf Si concentrations from the heavily grazed population may be associated with grazingrelated environmental stresses such as a warmer, drier microclimate or with morphological characteristics related to grazing tolerance or avoidance.     

Intraspecific variation in resistance to desiccation in the land snailTheba pisana

Theba pisana is a bush-dwelling snail with a circum-Mediterranean distribution. In Israel it is limited to a narrow coastal strip, along which there is a north-to-south gradient in mean annual rainfall (700–300 mm per year). In this study we found significant intraspecific differences among populations ofT. pisana in resistance to desiccation along this gradient, and in a population from Greece, which may be grossly correlated with the climatic gradient. The Greek population was the least resistant to desiccation with an extremely high rate of water loss. The differences in total mass loss were mainly determined by the rate of water loss during the first 4 days of desiccation. A general phenomenon during desiccation was the close regulation of the soft body water content at the expense of the extra-pallial fluid compartment.     

Effects of Festuca paniculata on the compensatory growth response of Centaurea uniflora in the French Alps

The independent effects of herbivores and neighbors on plants are generally negative, and therefore the combined effects of these interactions are generally assumed to have additive or multiplicative negative effects on plant growth. However, because herbivores can stimulate the growth of plants (compensation). and neighbors can facilitate each other, the combined effects of herbivory and plant-plant interactions can be highly variable and poorly predicted by current competition and plant-herbivore theory. In some cases in North America, Festuca species appear to facilitate invasive Centaurea species and enhance their compensatory responses in controlled greenhouse conditions. We explored the interactions between herbivory and neighbor effects in the French Alps by testing the effect of the neighbor, Festuca paniculata L., on the compensatory growth response of defoliated Centaurea uniflora L. over two growing seasons. Seventy percent of aboveground C. uniflora biomass was clipped at each of seven times throughout two growing seasons in the presence or absence of F. paniculata. Centaurea uniflora compensated for severe damage in the first year, but was negatively affected by defoliation in the second year. Defoliating C. uniflora reduced final aboveground biomass by 44% and flower number by 64%, but did not affect survival. Unlike observations for other Centaurea and Festuca species, F. paniculata had significant competitive effects on C. uniflora. Festuca paniculata neither enhanced compensatory responses of C. uniflora nor increased the negative effects of defoliation. Our results show that compensatory responses can weaken over time, but that neighboring plants do not necessarily increase the negative effects of defoliation.     

Temperature effects on summer growth rates in the Antarctic scallop, Adamussium colbecki

Annual growth rates of Antarctic marine organisms are low compared to their relatives from warmer waters. Previous studies hypothesise that high food availability during austral spring–summer may enable Antarctic invertebrates to attain comparatively high short-term growth rates despite the low temperature. Neither a temperature-growth experiment with juvenile Adamussium colbecki (Smith 1902) nor the comparison of A. colbecki summer growth rates with an empirical scallop specific growth-to-temperature relationship could confirm this hypothesis. Hence, summer growth rates of young, immature A. colbecki are strongly affected by temperature, i.e. no uncoupling from temperature.     

Intraspecific variability in Karlodinium veneficum: Growth rates, mixotrophy, and lipid composition

We isolated eleven strains of the harmful algal bloom (HAB)-forming dinoflagellate Karlodinium veneficum during a bloom event in the NW Mediterranean coastal waters and we studied the inter-strain variability in several of their physiological and biochemical traits. These included autotrophic growth parameters, feeding capabilities (mixotrophy), lipid composition, and, in some cases, their responses to biotic and abiotic factors. The strains were found to differ in their growth rates (0.27–0.53 d⁻¹) and in the maximum cell concentrations achieved during stationary phase (6.1 × 10⁴–8.6 × 10⁴ cells mL⁻¹). Their ingestion performance, when offered Rhodomonas salina as prey, was also diverse (0.22–1.3 cells per K. veneficum per day; 8–52% of their daily ration). At least two strains survived for several months under strict heterotrophic conditions (no light, low inorganic nutrients availability, and R. salina as food source). These strains also showed very distinct fatty acid compositions, with very low contents of monounsaturated and polyunsaturated fatty acids. According to a Bray Curtis similarity analysis, three or four strain groups able to perform different roles in bloom development were identified. We further analyzed one strain from each of the two most distinct groups with respect to prey concentration, light intensity, nutrient availability, and we determined the functional responses (growth and feeding rates) to food concentration. Taken together, the results served to highlight the role of mixotrophy and clone variability in the formation of HABs.     

Isolation and growth rates of methanol utilizing rhodospirillaceae

38 pure culture strains belonging to seven species of the Rhodospirillaceae were isolated from 39 methanol enrichment cultures inoculated with water and mud samples of different habitats. None of the strains exhibited doubling times shorter than 10 h in methanol-bicarbonate media.