Will increases in atmospheric CO2 affect regrowth following grazing in C4 grasses from tropical grasslands? A test with Sporobolus kentrophyllus

We grew a C₄ grass from the Serengeti ecosystem under ambient (370 ppm) and elevated (700 ppm) CO₂, and under clipped and unclipped conditions to test whether regrowth following grazing would be affected by elevated CO₂. Above-ground productivity was slightly decreased under elevated CO₂, and was similar between clipped and unclipped plants. Regrowth (clipping offtake) following clipping was similar in the two CO₂ treatments, and there was no CO₂ by clipping interaction on biomass, productivity, or leaf nutrient concentrations. Based on this evidence, we suggest that C₄ grasses from the Serengeti will show little direct response to future increases in atmospheric CO₂.     

Elevated CO2 enhances resprouting of a tropical savanna tree

The savannas (cerrado) of south-central Brazil are currently subjected to frequent anthropogenic burning, causing widespread reduction in tree density. Increasing concentrations of atmospheric CO₂ could reduce the impact of such frequent burning by increasing the availability of nonstructural carbohydrate, which is necessary for resprouting. We tested the hypotheses that elevated CO₂ stimulates resprouting and accelerates replenishment of carbohydrate reserves. Using a factorial experiment, seedlings of a common Brazilian savanna tree, Keilmeyera coriacea, were grown at 350 ppm and 700 ppm CO₂ and at two nutrient levels. To simulate burning, the plants were either clipped at 15 weeks or were left unclipped. Among unclipped plants, CO₂ and nutrients both stimulated growth, with no significant interaction between nutrient and CO₂ effects. Among clipped plants, both CO₂ and nutrients stimulated resprouting. However, there was a strong interaction between CO₂ and nutrient effects, with CO₂ having a significant effect only in the presence of high nutrient availability. Under elevated CO₂, carbohydrate reserves remained at higher levels following clipping. Root total nonstructural carbohydrate remained above 36% in all treatments, so carbohydrate reserves did not limit regrowth. These results indicate that under elevated CO₂ this species may be better able to endure the high frequency of anthropogenic burning in the Brazilian savannas. Received: 19 May 1999 / Accepted: 8 November 1999     

Embryo development after vitrification of immature and in vitro-matured equine oocytes

Effects of meiotic stage and cumulus status on development of equine oocytes after vitrification was evaluated. Immature oocytes with corona radiata (IMM); in vitro-matured oocytes with corona radiata (MAT CR+); and in vitro-matured oocytes denuded of cumulus (MAT CR-) were vitrified using the Cryotech® method. Warming medium was equilibrated either in 5% CO₂ or Air. IMM oocytes underwent in vitro maturation after warming. Recovery, survival, and maturation rates, and cleavage and blastocyst rates after ICSI, were evaluated. Recovery was higher for oocytes warmed in CO₂- than Air-equilibrated medium (86 ± 3 vs. 76.9 ± 4%, respectively). Maturation for all vitrified-warmed oocyte treatments (37 ± 6.5 to 45.9 ± 5.8%) was not different from control (50 ± 4.1%), except for MAT CR- CO₂ (20.3 ± 4.6%). Cleavage for MAT CR- CO₂ and Air groups was similar to control (67.7 ± 12.1, 71.4 ± 8.1, and 78 ± 5.3%, respectively). One blastocyst was produced (MAT CR + CO₂), representing the first equine blastocyst reported after vitrification of an in vitro-matured oocyte.     

Temperature response of carbon isotope discrimination and mesophyll conductance in tobacco

The partial pressure of CO₂ at the sites of carboxylation within chloroplasts depends on the conductance to CO₂ diffusion from intercellular airspace to the sites of carboxylation, termed mesophyll conductance (g_m). We investigated the temperature response of g_m in tobacco (Nicotiana tabacum) by combining gas exchange in high light, ambient CO₂ in either 2 or 21% O₂ with carbon isotope measurements using tuneable diode laser spectroscopy. The g_m increased linearly with temperature in 2 or 21% O₂. In 21% O₂, isotope discrimination associated with g_m decreased from 5.0 ± 0.2 to 1.8 ± 0.2‰ as temperature increased from 15 to 40 °C, but the photorespiratory contribution to the isotopic signal is significant. While the fractionation factor for photorespiration (f = 16.2 ± 0.7‰) was independent of temperature between 20 and 35 °C, discrimination associated with photorespiration increased from 1.1 ± 0.01 to 2.7 ± 0.02‰ from 15 to 40 °C. Other mitochondrial respiration contributed around 0.2 ± 0.03‰. The drawdown in CO₂ partial pressure from ambient air to intercellular airspaces was nearly independent of leaf temperature. By contrast, the increase in g_m with increasing leaf temperature resulted in the drawdown in CO₂ partial pressure between intercellular airspaces and the sites of carboxylation decreasing substantially at high temperature.     

Impact of In-Situ CO2 Nano-Bubbles Generation on Freezing Parameters of Selected Liquid Foods

The impact of in-situ CO₂ nano-bubbles generation on the freezing properties of soft serve, milk, and apple juice was investigated. Carbonated (0, 1000, and 2000 ppm) liquid foods contained in a tube were submerged and cooled for 90 min in a pre-set ethylene glycol bath (−15 °C). Before the enclosed liquid reached 0 °C, the vibration was discharged through ultrasound in the bath to create nano-bubbles within the carbonated food samples, and the changes in temperature for 90 min of each food were recorded as a freezing curve. The time for onset of nucleation of control soft serve mix was halved in samples with 2000-ppm CO₂ due to the presence of nano-bubbles. Likewise, the nucleation time for milk with and without nano-bubbles at the same CO₂ concentration of 2000 ppm was 7.9 ± 0.1 and 2.8 ± 0.8 min_, respectively. The generation of CO₂ nano-bubbles from 2000-ppm CO₂ level in 10 ^oBx apple juice displayed −9.3 ± 0.3 °C nucleation temperature while the control one had −11.7 ± 0.9 °C.

     

Effects of hair coat characteristics on radiant surface temperature in horses

Horse owners may lack knowledge about natural thermoregulation mechanisms in horses. Horses are managed intensively; usually stabled at night and turned out during the day. Some are clipped and many wear a blanket, practices which reduce the horse's ability to regulate heat dissipation. The aim of this study was to investigate the relationship between hair coat characteristics, body condition and infrared surface temperatures from different body parts of horses. Under standard conditions, the body surface temperature of 21 adult horses were investigated using infrared thermography. From several readings on the same body part, a mean temperature was calculated for each body part per horse. Detailed information on horse breed, age, management and body condition was collected. Hair coat samples were also taken for analyses. A mixed statistical model was applied. Warmblood horse types (WB) had lower hair coat sample weights and shorter hair length than coldblood horse types (CB). The highest radiant surface temperatures were found at the chest 22.5 ± 0.9 °C and shoulders 20.4 ± 1.1 °C and WB horses had significantly higher surface temperatures than CB horses on the rump (P < 0.05). Horses with a higher hair coat sample weight had a lower surface temperature (P < 0.001) and hind hooves with iron shoes had a significant lower surface temperature than unshod hind hooves (P = 0.03). In conclusion, individual assessment of radiant surface temperature using infrared thermography might be a promising tool to gather data on heat loss from the horses' body. Such data may be important for management advice, as the results showed individual differences in hair coat characteristics and body condition in horses of similar breeds.     

Climate warming increases soil erosion,carbon and nitrogen loss with biofuel feedstock harvest in tallgrass prairie

Anthropogenic soil erosion severely affects land ecosystems by reducing plant productivity and stimulating horizontal carbon and nitrogen movement at the surface. Climate warming may accelerate soil erosion by altering soil temperature, moisture, and vegetation coverage. However, no experiments have been carried out to quantify soil erosion with warming. In a long‐term field experiment, we explored how annual clipping for biofuel feedstock production and warming caused soil erosion and accompanying carbon and nitrogen losses in tallgrass prairie in Oklahoma, USA. We measured relative changes in soil surface elevation between clipped and unclipped plots with or without experimental warming. Our results show that average relative erosion depth caused by clipping was 1.65±0.09 and 0.54±0.08 mm yr^?1, respectively, in warmed and control plots from November 21, 1999 to April 21, 2009. The soil erosion rate was 2148±121 g m^?2 yr^?1 in the warmed plots and 693±113 g m^?2 yr^?1 in the control plots. Soil organic carbon was lost at a rate of 69.6±5.6 g m^?2 yr^?1 in the warmed plots and 22.5±2.7 g m^?2 yr^?1 in the control plots. Total nitrogen was lost at a rate of 4.6±0.4 g m^?2 yr^?1 in the warmed plots and 1.4±0.1 g m^?2 yr^?2 in the control plots. The amount of carbon and nitrogen loss caused by clipping is equivalent to or even larger than changes caused by global change factors such as warming and rising atmospheric CO₂ concentration. In addition, soil erosion rates were significantly correlated with clipping‐induced changes in soil moisture. Our results suggest that clipping for biofuel harvest results in significant soil erosion and accompanying losses of soil carbon and nitrogen, which is aggravated by warming.     

Temperature sensitivity of biomass‐specific microbial exo‐enzyme activities and CO2 efflux is resistant to change across short‐ and long‐term timescales

Accurate representation of temperature sensitivity (Q₁₀) of soil microbial activity across time is critical for projecting soil CO₂ efflux. As microorganisms mediate soil carbon (C) loss via exo‐enzyme activity and respiration, we explore temperature sensitivities of microbial exo‐enzyme activity and respiratory CO₂ loss across time and assess mechanisms associated with these potential changes in microbial temperature responses. We collected soils along a latitudinal boreal forest transect with different temperature regimes (long‐term timescale) and exposed these soils to laboratory temperature manipulations at 5, 15, and 25°C for 84 days (short‐term timescale). We quantified temperature sensitivity of microbial activity per g soil and per g microbial biomass at days 9, 34, 55, and 84, and determined bacterial and fungal community structure before the incubation and at days 9 and 84. All biomass‐specific rates exhibited temperature sensitivities resistant to change across short‐ and long‐term timescales (mean Q₁₀ = 2.77 ± 0.25, 2.63 ± 0.26, 1.78 ± 0.26, 2.27 ± 0.25, 3.28 ± 0.44, 2.89 ± 0.55 for β‐glucosidase, N‐acetyl‐β‐d ‐glucosaminidase, leucine amino peptidase, acid phosphatase, cellobiohydrolase, and CO₂ efflux, respectively). In contrast, temperature sensitivity of soil mass‐specific rates exhibited either resilience (the Q₁₀ value changed and returned to the original value over time) or resistance to change. Regardless of the microbial flux responses, bacterial and fungal community structure was susceptible to change with temperature, significantly differing with short‐ and long‐term exposure to different temperature regimes. Our results highlight that temperature responses of microbial resource allocation to exo‐enzyme production and associated respiratory CO₂ loss per unit biomass can remain invariant across time, and thus, that vulnerability of soil organic C stocks to rising temperatures may persist in the long term. Furthermore, resistant temperature sensitivities of biomass‐specific rates in spite of different community structures imply decoupling of community constituents and the temperature responses of soil microbial activities.     

Biomass and mineral element responses of a Serengeti short-grass species to nitrogen supply and defoliation: compensation requires a critical [N]

Large mammalian herbivores in grassland ecosystems influence plant growth dynamics in many ways, including the removal of plant biomass and the return of nutrients to the soil. A 10-week growth chamber experiment examined the responses of Sporobolus kentrophyllus from the heavily grazed short-grass plains of Serengeti National Park, Tanzania, to simulated grazing and varying nitrogen nutrition. Plants were subjected to two clipping treatments (clipped and unclipped) and five nitrogen levels (weekly applications at levels equivalent to 0, 1, 5, 10, and 40 g N m⁻²), the highest being equivalent to a urine hit. Tiller and stolon production were measured weekly. Total biomass at harvest was partitioned by plant organ and analyzed for nitrogen and mineral element composition. Tiller and stolon production reached a peak at 3–5 weeks in unclipped plants, then declined drastically, but tiller number increased continually in clipped plants; this differential effect was enhanced at higher N levels. Total plant production increased substantially with N supply, was dominated by aboveground production, and was similar in clipped and unclipped plants, except at high nitrogen levels where clipped plants produced more. Much of the standing biomass of unclipped plants was standing dead and stem; most of the standing biomass of clipped plants was live leaf with clipped plants having significantly more leaf than unclipped plants. However, leaf nitrogen was stimulated by clipping only in plants receiving levels of N application above 1 g N m⁻² which corresponded to a tissue concentration of 2.5% N. Leaf N concentration was lower in unclipped plants and increased with level of N. Aboveground N and mineral concentrations were consistently greater than belowground levels and while clipping commonly promoted aboveground concentrations, it generally diminished those belowground. In general, clipped plants exhibited increased leaf elemental concentrations of K, P, and Mg. Concentrations of B, Ca, K, Mg, and Zn increased with the level of N. No evidence was found that the much greater growth associated with higher N levels diminished the concentration of any other nutrient and that clipping coupled with N fertilization increased the total mineral content available in leaf tissue. The results suggest that plants can (1) compensate for leaf removal, but only when N is above a critical point (tissue [N] 2.8%) and (2) grazing coupled with N fertilization can increase the quality and quantity of tissue available for herbivore removal. Received: 25 August 1997 / Accepted: 14 April 1998     

Characterization of hydrocarbon-degrading bacteria isolated from oil-contaminated sediments in the Sultanate of Oman and evaluation of bioaugmentation and biostimulation approaches in microcosm experiments

Two oil-polluted sediments (PD and KH) were sampled from a coastal region in Oman for the isolation of hydrocarbon-degrading bacteria and for testing different bioremediation approaches. Fourty strains were isolated, eighteen were affiliated to Marinobacter whereas the rest belonged to Pseudomonas, Halomonas, Hahella and Alcanivorax. All strains grew well at 2–7% salinity and between 20 and 60 °C. The strains exhibited a better growth on long chain than on short chain alkanes. Biostimulation and bioaugmentation were compared in both sediments and oil biodegradation was followed by measuring CO₂ evolution and by gas chromatography (GC). The evolved CO₂ reached 0.45 ± 0.02 and 2.23 ± 0.07 mg CO₂ g⁻¹ sediment after 88 days in the untreated PD and KH sediments, respectively. While the addition of inorganic nutrients resulted in 1.2–3.7 fold increase in CO₂ evolution in both sediments, the addition of the bacterial consortium was only effective in the PD sediment. The maximum CO₂ evolution was measured when both nutrients and bacteria were added and this corresponded to a total oil mineralization of 2.6 ± 0.12 and 1.49 ± 0.04% of the initial oil after 88 days in the PD and KH sediments, respectively. GC analysis confirmed the CO₂ data and showed that most of the degraded compounds belonged to alkanes. We conclude that the Omani polluted sediments contain halotolerant and thermotolerant bacteria and biostimulation is more efficient than bioaugmentation for their cleanup.     

Effect of CO2 on uninfected Sf‐9 cell growth and metabolism

A problem in the mass production of recombinant proteins and biopesticides using insect cell culture is CO₂ accumulation. This research investigated the effect of elevated CO₂ concentration on insect cell growth and metabolism. Spodoptera frugiperda Sf‐9 insect cells were grown at 20% air saturation, 27^°C, and a pH of 6.2. The cells were exposed to a constant CO₂ concentration by purging the medium with CO₂ and the headspace with air. The population doubling time (PDT) of Sf‐9 cells increased with increasing CO₂ concentration. Specifically, the PDT for 0‐37, 73, 147, 183, and 220 mm Hg CO₂ concentrations were 23.2 ± 6.7, 32.4 ± 7.2, 38.1 ± 13.3, 42.9 ± 5.4, and 69.3 ± 35.9 h (n = 3 or 4, 95% confidence level), respectively. The viability of cells in all experiments was above 90%, i.e., while increased CO₂ concentrations inhibited cell growth, it did not affect cell viability. The osmolality for all bioreactor experiments was observed to be 300–360 mOsm/kg, a range that is known to have a negligible effect on insect cell culture. Elevated CO₂ concentration did not significantly alter the cell specific glucose consumption rate (2.5–3.2 × 10^?17 mol/cell s), but slightly increased the specific lactate production rate from ?3.0 × 10^?19 to 10.2 × 10^?19 mol/cell s. Oxidative stress did not contribute to CO₂ inhibition in uninfected Sf‐9 cells as no significant increase in the levels of lipid hydroperoxide and protein carbonyl concentrations was discovered at elevated CO₂ concentration. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:465–469, 2016     

Impacts of forest management type and season on soil carbon fluxes in Eastern Mau Forest,Kenya

The value of ecosystems functions performed by forests in the climate change era has prompted increasing attention towards assessment of carbon stocks and fluxes in tropical forests. The aim of this study was to understand how forest management approaches and environmental controls impacted on soil CO₂ efflux in a tropical Eastern Mau forest which is one of the blocks of the greater Mau complex in Kenya. Nested experimental design approach was employed where 32 plots were nested into four blocks (disturbed natural, undisturbed natural, plantation and glades). In 10 m² plots, data were collected on soil CO₂ efflux, soil temperature and soil moisture using soda lime methods, direct measurement and proxy techniques, respectively. There was significant forest management type effect (F_3,127 = 3.01, p = 0.033) and seasonality effect (t test = 3.31, df = 1, p < 0.05) on mean soil CO₂ efflux. The recorded mean soil CO₂ efflux levels were as follows: plantation forest (9.219 ± 3.067 g C M^?2 day^?1), undisturbed natural forest (8.665 ± 4.818 g C M^?2 day^?1), glades (8.592 ± 3.253 g C M^?2 day^?1) and disturbed natural forest (7.198 ± 3.457 g C M^?2 day^?1). The study concludes that managing a forest in plantation form is primarily responsible for forest soil CO₂ efflux levels due to aspects such as increased microbial activity and root respiration. However, further studies are required to understand the role and impact of soil CO₂ efflux on the greater forest carbon budget.     

Bioaccumulation of lead nitrate in tissues and its effects on hematological and biochemical parameters of Clarias gariepinus

Clarias gariepinus, weighing 119.18 ± 5.21 g, were exposed to 0%, 20%, 40%, and 60% of the LC₅₀ lead nitrate, Pb(NO₃)₂, which represents the following treatments 0 (control), 16, 32, and 48 mg/l, respectively, for a period of 10 and 20 days. The results showed that the bioaccumulation of Pb(NO₃)₂ in gills were significantly increased (p < 0.05) after 10 days having 0.17 ± 0.07, 5.05 ± 1.04, 6.01 ± 0.82, and 9.61 ± 1.76 mg/100 g wet weight, respectively, for the treatments. However, after 20 days these values increased ((0.17 ± 0.07, 4.34 ± 1.27, 10.83 ± 0.97, and 19.18 ± 2.40 mg/100 g) for 0%, 20%, 40%, and 60% of LC₅₀ Pb(NO₃)₂ respectively. There was an increase with each increasing concentration level of Pb(NO₃)₂ as compared with that of the control group. The accumulation of Pb(NO₃)₂ in the liver, showed a significant increase (p < 0.05) with the increasing period of exposed and Pb concentration with LC₅₀ values ranging between 3.32 ± 0.91 and 4.42 ± 0.78 after 10 days as compared with that of the control group 0.08 ± 0.02 mg/100 g wet weight. Although white muscles and skin displayed lower values of bioaccumulation than gills and liver after 10 days, after 20 days the results were slightly more increased in the white muscles than in the skin. However, the observed pattern of increase was the same compared to that of the control group. Therefore, hematological parameters, such as red blood cells (RBCs), hemoglobin (Hb), and hematocrit (Hct) showed significant (p < 0.05) concentration-dependent decreases in fish exposed to Pb(NO₃)₂ during both periods. However, the values of white blood cells (WBCs) showed a significant reduction when levels of Pb concentration increased. Hepatic enzyme activities of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) displayed a significant increase with increasing concentrations and exposure time.     

Enhanced abundance of tintinnids under elevated CO<Subscript>2</Subscript> level from coastal Bay of Bengal

The role of microzooplankton (MZP) in the pelagic trophodynamics is highly significant, but the responses of marine MZP to increasing CO₂ levels are rather poorly understood. Hence the present study was undertaken to understand the responses of marine plankton to increasing CO₂ concentrations. Natural water samples from the coastal Bay of Bengal were incubated under the ambient condition and high CO₂ levels (703–711 μatm) for 5 days in May and June 2010. A significant negative correlation was obtained between phytoplankton and MZP abundance which indicated that phytoplankton community structure can considerably be controlled by MZP in this region. The average relative abundances of tintinnids under elevated CO₂ levels were found to be significantly higher (68.65 ± 5.63% in May; 85.46 ± 9.56% in June) than observed in the ambient condition (35.68 ± 6.83% in May; 79 ± 5.36% in June). The observed dominance of small chain forming diatom species probably played a crucial role as they can be potentially grazed by tintinnids. This fact was strengthened by the observed high negative correlations between the relative abundance of major phytoplankton and tintinnids. Moreover, particulate organic carbon and total bacterial counts were also enhanced under elevated CO₂ level and can serve as additional food source for ciliates. The observed responses of tintinnids to increasing CO₂ might have multiple impacts on the energy transfer, nutrient and carbon cycling in the coastal water. The duration of the present study was relatively short and therefore further investigation on longer time scale needs to be done and might give us a better insight about phytoplankton and MZP species succession under elevated CO₂ level.     

Tritrophic Interactions of Cowpea [Vigna unguiculata subsp unguiculata(L.)], Aphids [Aphis craccivora (Koch)] and Coccinellids [Menochilus sexmaculatus (Fab.)] under eCO2 and eTemp

Experiments were conducted to understand the direct and indirect effects of temperature and elevated CO₂ (eCO₂), on tritrophic interactions of cowpea (Vigna unguiculata subsp. unguiculata L.), legume aphid Aphis craccivora Koch and coccinellid predator Menochilus sexmaculatus Fab. Reduction of the leaf nitrogen (6%), amino acid (6%) and protein (7%) of cowpea foliage with increased carbon (13%) and C:N ratio (21%) at eCO₂ over aCO₂ indicated the dilution of biochemical constituents at first trophic level. Shortened development time, DT and increment of reproductive rate, RR at eCO₂ over ambient CO₂(aCO₂)was significant with increase in temperature from 20 to 35?°C. Reduction of the mean degree day, DD requirement of both nymphal (75.79?±?15.163) and adult stages (157.15?±?67.04) at eCO₂ over aCO₂ and same was reflected in the summation DD for both the stages at eCO₂ (232.96?±?80.32)and aCO₂ (247.07?±?64.77) across six temperatures. The ‘r_m’ and ‘R_o’ increased gradually with increase in temperature followed the non-linear trend and reached maximum values at 27?°C with shortened ‘T’ across 20 to 35?°C temperatures at eCO₂ indicating the significant variation of growth and development at the second trophic level. Decreased grub duration (23%) with increased predation capacity (19%) of M. sexmaculatus on A. craccivora at eCO₂ over ambient was noted, indicating the incidence of A. craccivora is likely to be higher with increased predation in the future climate change scenario.     

Synergistic control of CO2 emissions by fish and nutrients in a humic tropical lake

Using experimental mesocosms, we tested the strength of bottom–up controls by nutrients and top–down controls by an omnivorous fish (Hyphessobrycon bifasciatus; family Characidae), and the interaction between them on the CO₂ partial pressure (pCO₂) in the surface waters of a tropical humic lake (Lake Cabiúnas, Brazil). The experiment included the addition of nutrients and fish to the mesocosms in a factorial design. Overall, persistent CO₂ emissions to the atmosphere, supported by an intense net heterotrophy, were observed in all treatments and replicates over the 6-week study period. The CO₂ efflux (average ± standard error) integrated over the experiment was similar among the control mesocosms and those receiving only fish or only nutrients (309 ± 2, 303 ± 16, and 297 ± 17 mmol CO₂ m⁻² day⁻¹, respectively). However, the addition of nutrients in the presence of fish resulted in a high algal biomass and daytime net autotrophy, reducing the CO₂ emissions by 35% (by 193 ± 7 mmol CO₂ m⁻² day⁻¹). These results indicate that high CO₂ emissions persist following the eutrophication of humic waters, but that the magnitude of these emissions might depend on the structure of the food web. In conclusion, fish and nutrients may act in a synergistic manner to modulate persistent CO₂ emissions from tropical humic lakes.     

Plant-herbivore interactions: Examination of potential effects of bison saliva on regrowth of Bouteloua gracilis (H.B.K.) lag

Summary Laboratory experiments were performed to determine whether regrowth of blue grama was affected by potential growth-promoting substances in saliva of North American bison. We observed no statistically significant effects of foliar application of whole bison saliva on net photosynthesis (P_N), root respiration (R_R), allocation patterns of photosynthetically fixed ¹⁴C, or regrowth rates over a 10-day period following clipping to various heights. In a 10-week experiment, there were no significant effects of saliva on leaf, crown or root growth or tiller production in plants clipped to heights of 6, 4 or 2 cm above crowns. Similarly, nitrogen-stressed plants failed to show significant changes in growth rates or tillering in response to saliva over a 3-week period. Clipped blue grama plants did exhibit significant compensatory growth responses, including higher P_N rates from 3–10 days following clipping and allocation of a higher proportion of current photosynthate to synthesis of new leaf tissue with increasing severity of defoliation. Nevertheless, unclipped plants invariably outproduced clipped plants following defoliation.     

Within‐plant variation in defences in response to simulated herbivory in a semi‐arid southern African savannah

Within‐plant spatial variation in herbivore pressure can induce localized antiherbivory defence responses. We tested this hypothesis by studying branch‐specific responses of Acacia robusta, Dichrostachys cinerea and Ziziphus mucronata to simulated mammalian herbivory. Herbivory was simulated by clipping the terminal shoots (3 cm from tip) of tree branchlets, allowing them one year of regrowth and then comparing their spine length and density and condensed tannins with those of adjacent unclipped branchlets. Condensed tannins concentrations were higher in clipped branchlets than in unclipped branchlets in all three woody species (P < 0.05). Spine length was higher in clipped branchlets than in unclipped branchlets in A. robusta (P < 0.05) but was similar in both D. cinerea and Z. mucronata (P > 0.05). Spine density was double in clipped branchlets as compared to the unclipped branchlets in Z. mucronata (P < 0.05) but was similar in both A. robusta and D. cinerea (P > 0.05). We found evidence of within‐plant variation in condensed tannins concentration and spine length and density in response to simulated herbivory in the three woody species.     

Does pollination limit tolerance to browsing in <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis>?

Ungulate browsing of flowering stalks of the semelparous herb Ipomopsis aggregata leads to regrowth of lateral inflorescences, a response that has been reported to yield overcompensation in some cases (browsed plants with higher reproductive success than unbrowsed), but undercompensation in others. Little is known about the mechanisms that cause such variable tolerance to herbivory. We explored one possible mechanism—variation in effects of browsing on pollination—by clipping I. aggregata inflorescences to mimic browsing, observing subsequent visits by pollinators and nectar-robbers, and adding pollen by hand to flowers of some clipped and unclipped plants. Clipping reduced floral display size and increased inflorescence branching, but neither hummingbirds, the primary pollinators, nor nectar-robbing bumblebees showed any preference for unclipped versus clipped plants. Clipping delayed flowering; this shift in phenology caused clipped plants to miss the peak of hummingbird activity and to have lower per-flower visitation rates than unclipped controls in one year, but to have greater overlap with birds and higher visitation rates in the subsequent year. In three sites and 2 years, clipped plants exposed to natural pollination suffered extreme undercompensation, producing on average only 16% as many seeds as unclipped controls. This was not directly attributable to clipping effects on pollination, however, because clipped plants were unable to increase fecundity when provided with supplemental pollen by hand. Taken altogether, our results suggest that compensation was constrained less by indirect effects of browsing on pollination than by its direct impacts on resource availability and hence on the ability of plants to regrow lost inflorescence tissue and to fill seeds. Exploring the physiological and developmental processes involved in regrowth of inflorescences and provisioning of seeds is a promising future direction for research designed to understand variation in browsing tolerance.     

Regulation of Renal Na-HCO3 Cotransporter: VIII. Mechanism of Stimulatory Effect of Respiratory Acidosis

We examined the effect of respiratory acidosis on the Na-HCO₃ cotransporter activity in primary cultures of the proximal tubule of the rabbit exposed to 10% CO₂ for 5 min, 2, 4, 24 and 48 hr. Cells exposed to 10% CO₂ showed a significant increase in Na-HCO₃ cotransporter activity (expressed as % of control levels, 5 min: 142 ± 6, 2 hr: 144 ± 13, 4 hr: 145 ± 11, 24 hr: 150 ± 15, 48 hr: 162 ± 24). The increase in activity was reversible after 48 hr. The role of protein kinase C (PKC) on the stimulatory effect of respiratory acidosis on the cotransporter was examined in presence of PKC inhibitor calphostin C or in presence of PKC depletion. Both calphostin C and PKC depletion prevented the effect of 10% CO₂ for 5 min or 4 hr to increase the activity of the cotransporter. 10% CO₂ for 5 min or 4 hr increased total and particulate fraction PKC activity. To examine the role of phosphotyrosine kinase (PTK) on the increase in cotransporter activity we studied the effect of two different inhibitors, 2-hydroxy-5-(2,5-dihydroxylbenzyl) aminobenzoic acid (HAC) and methyl 2,5-dihydroxycinnamate (DHC) which inhibit phosphotyrosine kinase in basolateral membranes. Cells were pretreated either with vehicle or HAC or DHC and then exposed to 10% CO₂ for 5 min or 4 hr. In cells treated with vehicle, 10% CO₂ significantly increased cotransporter activity as compared to control cells exposed to 5% CO₂. This stimulation by 10% CO₂ was completely prevented by HAC or DHC at 5 min (5% CO₂: 1.8 ± 0.2, 10% CO₂: 2.6 ± 0.2, 10% CO₂+ HAC: 1.6 ± 0.2, 10% CO₂: +DHC: 2.0 ± 0.3 pH unit/min) and also at 4 hr. The protein synthesis inhibitors actinomycin D and cycloheximide appear to prevent the effect of 10% CO₂ for 4 hr on the cotransporter. Our results show that early respiratory acidosis stimulates the Na-HCO₃ cotransporter through PKC and PTK-dependent mechanisms and the late effect appears to be mediated through protein synthesis. Received: 28 March 1997/Revised: 22 December 1997