Silica and nitrogen modulate physical defense against chewing insect herbivores in bioenergy crops Miscanthus x Giganteus and Panicum virgatum (Poaceae).

Feedstock crops selected for bioenergy production to date are almost exclusively perennial grasses because of favorable physiological traits that enhance growth, water use, and nutrient assimilation efficiency. Grasses, however, tend to rely primarily on physical defenses, such as silica, to deter herbivores. Silica impedes processing of feedstocks and introduces a trade-off between managing for cost efficiency (i.e., yield) and plant defenses. To test how silica modulates herbivory in two of the most preferred feedstock crops for production across the central United States, miscanthus (Miscanthus x giganteus Greef and Deuter ex Hodkinson and Renvoize) and switchgrass (Panicum virgatum L.), we examined the performance of two immature generalist insect herbivores, fall armyworm (Spodoptera frugiperda (J.E. Smith) and the American grasshopper [Schistocerca americana (Drury)], on grasses grown under silica and nitrogen amendment. Both miscanthus and switchgrass assimilated nitrogen and silica when grown in amended soil that altered the consumption and conversion efficiency of herbivores consuming leaf tissue. The magnitude of nutrient assimilation, however, depended on intrinsic plant traits. Nitrogen increased conversion efficiency for both fall armyworm and American grasshopper but increased consumption rate only for fall armyworm. Silica reduced conversion efficiency and increased consumption rate only for the American grasshopper. Because of this variability, management strategies that reduce silica or increase nitrogen content in feedstock crops to enhance yields may directly influence the ability of bioenergy grasses to deter certain generalist herbivores.

Hydraulic limitation not declining nitrogen availability causes the age-related photosynthetic decline in loblolly pine (Pinus taeda L.).

Declining net primary production (NPP) with forest age is often attributed to a corresponding decline in gross primary production (GPP). We tested two hypotheses explaining the decline of GPP in ageing stands (14-115 years old) of Pinus taeda L.: (1) increasing N limitation limits photosynthetic capacity and thus decreases GPP with increasing age; and (2) hydraulic limitations increasingly induce stomatal closure, reducing GPP with increasing age. We tested these hypotheses using measurements of foliar nitrogen, photosynthesis, sap-flow and dendroclimatological techniques. Hypothesis (1) was not supported; foliar N retranslocation did not increase and declines were not observed in foliar N, leaf area per tree or photosynthetic capacity. Hypothesis (2) was supported; declines were observed in light-saturated photosynthesis, leaf- and canopy-level stomatal conductance, concentration of CO(2) inside leaf air-spaces (corroborated by an increase in wood δ(13) C) and specific leaf area (SLA), while stomatal limitation and the ratio of sapwood area (SA) to leaf area increased. The sensitivity of radial growth to inter-annual variation in temperature and drought decreased with age, suggesting that tree water use becomes increasingly conservative with age. We conclude that hydraulic limitation increasingly limits the photosynthetic rates of ageing loblolly pine trees, possibly explaining the observed reduction of NPP.

Fine-root respiration in a loblolly pine and sweetgum forest growing in elevated CO2.

• The loss of carbon below-ground through respiration of fine roots may be modified by global change. Here we tested the hypothesis that a reduction in N concentration of tree fine-roots grown in an elevated atmospheric CO2 concentration would reduce maintenance respiration and that more energy would be used for root growth and N uptake. We partitioned total fine-root respiration (RT ) between maintenance (RM ), growth (RG ), and N uptake respiration (RN ) for loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) forests exposed to elevated CO2 . • A substantial increase in fine-root production contributed to a 151% increase in RG for loblolly pine in elevated CO2 . Root specific RM for pine was 24% lower under elevated CO2 but when extrapolated to the entire forest, no treatment effect could be detected. • R G (< 10%) and RN (< 3%) were small components of RM in both forests. Maintenance respiration was the vast majority of RT , and contributed 92% and 86% of these totals at the pine and sweetgum forests, respectively. • The hypothesis was rejected because the majority of fine-root respiration was used for maintenance and was not reduced by changes in root N concentration in elevated CO2 . Because of its large contribution to RT and total soil CO2 efflux, changes in RM caused by warming may greatly alter carbon losses from forests to the atmosphere.

Interactive effects of elevated CO2 and temperature on water transport inponderosa pine.

Many studies report that water flux through trees declines in response to elevated CO(2), but this response may be modified by exposure to increased temperatures. To determine whether elevated CO(2) and temperature interact to affect hydraulic conductivity, we grew ponderosa pine seedlings for 24 wk in growth chambers with one of four atmospheric CO(2) concentrations (350, 550, 750, and 1100 ppm) and either a low (15°C nights, 25°C days) or high (20°C nights, 30°C days) temperature treatment. Vapor pressure deficits were also higher in the elevated temperature treatment. Seedling biomass increased with CO(2) concentration but was not affected by temperature. Root : shoot ratio was unaffected by CO(2) and temperature. Leaf : sapwood area ratio (A(L)/A(S)) declined in response to elevated temperature but was not influenced by CO(2). Larger tracheid diameters at elevated temperature caused an increase in xylem-specific hydraulic conductivity (K(S)). The increase in K(S) and decrease in A(L)/A(S) led to higher leaf-specific hydraulic conductivity (K(L)) at elevated temperature. Stomatal conductance (g(S)) was correlated with K(L) across all treatments. Neither K(S), K(L), nor g(S) were affected by elevated CO(2) concentrations. High K(L) in response to elevated temperature may support increased transpiration or reduce the incidence of xylem cavitation in ponderosa pine in future, warmer climates.

Photosynthetic responses to CO2 enrichment of four hardwood species in a forest understory.

We compared the CO2- and light-dependence of photosynthesis of four tree species (Acer rubrum, Carya glabra, Cercis canadensis, Liquidambar styraciflua) growing in the understory of a loblolly pine plantation under ambient or ambient plus 200 µl l-1 CO2. Naturally-established saplings were fumigated with a free-air CO2 enrichment system. Light-saturated photosynthetic rates were 159-190% greater for Ce. canadensis saplings grown and measured under elevated CO2. This species had the greatest CO2 stimulation of photosynthesis. Photosynthetic rates were only 59% greater for A. rubrum saplings under CO2 enrichment and Ca. glabra and L. styraciflua had intermediate responses. Elevated CO2 stimulated light-saturated photosynthesis more than the apparent quantum yield. The maximum rate of carboxylation of ribulose-1,5-bisphosphate carboxylase, estimated from gas-exchange measurements, was not consistently affected by growth in elevated CO2. However, the maximum electron transport rate estimated from gas- exchange measurements and from chlorophyll fluorescence, when averaged across species and dates, was approximately 10% higher for saplings in elevated CO2. The proportionately greater stimulation of light-saturated photosynthesis than the apparent quantum yield and elevated rates of maximum electron transport suggests that saplings growing under elevated CO2 make more efficient use of sunflecks. The stimulation of light-saturated photosynthesis by CO2 did not appear to correlate with shade-tolerance ranking of the individual species. However, the species with the greatest enhancement of photosynthesis, Ce. canadensis and L. styraciflua, also invested the greatest proportion of soluble protein in Rubisco. Environmental and endogenous factors affecting N partitioning may partially explain interspecific variation in the photosynthetic response to elevated CO2.

Xylem conductivity and vulnerability to cavitation of ponderosa pine growing in contrasting climates.

We examined the effects of increased transpiration demand on xylem hydraulic conductivity and vulnerability to cavitation of mature ponderosa pine (Pinus ponderosa Laws.) by comparing trees growing in contrasting climates. Previous studies determined that trees growing in warm and dry sites (desert) had half the leaf/sapwood area ratio (A(L)/A(S)) and more than twice the transpiration rate of trees growing in cool and moist sites (montane). We predicted that high transpiration rates would be associated with increased specific hydraulic conductivity (K(S)) and increased resistance to xylem cavitation. Desert trees had 19% higher K(S) than montane trees, primarily because of larger tracheid lumen diameters. Predawn water potential and water potential differences between the soil and the shoot were similar for desert and montane trees, suggesting that differences in tracheid anatomy, and therefore K(S), were caused primarily by temperature and evaporative demand, rather than soil drought. Vulnerability to xylem cavitation did not differ between desert and montane populations. A 50% loss in hydraulic conductivity occurred at water potentials between -2.61 and -2.65 MPa, and vulnerability to xylem cavitation did not vary with stem size. Minimum xylem tensions of desert and montane trees did not drop below -2.05 MPa. Foliage turgor loss point did not differ between climate groups and corresponded to mean minimum xylem tensions in the field. In addition to low A(L)/A(S), high K(S) in desert trees may provide a way to increase tree hydraulic conductivity in response to high evaporative demand and prevent xylem tensions from reaching values that cause catastrophic cavitation. In ponderosa pine, the flexible responses of A(L)/A(S) and K(S) to climate may preclude the existence of significant intraspecific variation in the vulnerability of xylem to cavitation.

Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps.

We hypothesized that photoinhibition of shade-developed leaves of deciduous hardwood saplings would limit their ability to acclimate photosynthetically to increased irradiance, and we predicted that shade-tolerant sugar maple (Acer saccharum Marsh.) would be more susceptible to photoinhibition than intermediately shade-tolerant red oak (Quercus rubra L.). After four weeks in a canopy gap, photosynthetic rates of shade-developed leaves of both species had increased in response to the increase in irradiance, although final acclimation was more complete in red oak. However, photoinhibition occurred in both species, as indicated by short-term reductions in maximum rates of net photosynthesis and the quantum yield of oxygen evolution, and longer-term reductions in the efficiency of excitation energy capture by open photosystem II (PSII) reaction centers (dark-adapted F(v)/F(m)) and the quantum yield of PSII in the light (phi(PSII)). The magnitude and duration of this decrease were greater in sugar maple than in red oak, suggesting greater susceptibility to photoinhibition in sugar maple. Photoinhibition may have resulted from photodamage, but it may also have involved sustained rates of photoprotective energy dissipation (especially in red oak). Photosynthetic acclimation also appeared to be linked to an ability to increase leaf nitrogen content. Limited photosynthetic acclimation in shade-developed sugar maple leaves may reflect a trade-off between shade-tolerance and rapid acclimation to a canopy gap.

Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth rate of ponderosa pine in different CO2 and temperature regimes.

Increases in the concentration of atmospheric carbon dioxide may have a fertilizing effect on plant growth by increasing photosynthetic rates and therefore may offset potential growth decreases caused by the stress associated with higher temperatures and lower precipitation. However, plant growth is determined both by rates of net photosynthesis and by proportional allocation of fixed carbon to autotrophic tissue and heterotrophic tissue. Although CO2 fertilization may enhance growth by increasing leaf-level assimilation rates, reallocation of biomass from leaves to stems and roots in response to higher concentrations of CO2 and higher temperatures may reduce whole-plant assimilation and offset photosynthetic gains. We measured growth parameters, photosynthesis, respiration, and biomass allocation of Pinus ponderosa seedlings grown for 2 months in 2×2 factorial treatments of 350 or 650µ bar CO2 and 10/25° C or 15/30° C night/day temperatures. After 1 month in treatment conditions, total seedling biomass was higher in elevated CO2, and temperature significantly enhanced the positive CO2 effect. However, after 2 months the effect of CO2 on total biomass decreased and relative growth rates did not differ among CO2 and temperature treatments over the 2-month growth period even though photosynthetic rates increased ≈7% in high CO2 treatments and decreased ≈10% in high temperature treatments. Additionally, CO2 enhancement decreased root respiration and high temperatures increased shoot respiration. Based on CO2 exchange rates, CO2 fertilization should have increased relative growth rates (RGR) and high temperatures should have decreased RGR. Higher photosynthetic rates caused by CO2 fertilization appear to have been mitigated during the second month of exposure to treatment conditions by a ≈3% decrease in allocation of biomass to leaves and a ≈9% increase in root:shoot ratio. It was not clear why diminished photosynthetic rates and increased respiration rates at high temperatures did not result in lower RGR. Significant diametrical and potentially compensatory responses of CO2 exchange and biomass allocation and the lack of differences in RGR of ponderosa pine after 2 months of exposure of high CO2 indicate that the effects of CO2 fertilization and temperature on whole-plant growth are determined by complex shifts in biomass allocation and gas exchange that may, for some species, maintain constant growth rates as climate and atmospheric CO2 concentrations change. These complex responses must be considered together to predict plant growth reactions to global atmospheric change, and the potential of forest ecosystems to sequester larger amounts of carbon in the future.

The effects of ultraviolet-B radiation on photosynthesis of different aged needles in field-grown loblolly pine.

We examined the effect of supplemental UV-B radiation (290-320 nm) on photosynthetic characteristics of different aged needles of 3-year-old, field-grown loblolly pine (Pinus taeda L.). Needles in four age classes were examined: I, most recently fully expanded, year 3; II, first flush, year 3; III, final flush, year 2; and IV, oldest needles still present, year 2. Enhanced UV-B radiation caused a statistically significant decrease (6%) in the ratio of variable to maximum fluorescence (F(v)/F(m)) following dark adaptation only in needles from the youngest age class, suggesting transient damage to photosynthesis. However, no effects of enhanced UV-B radiation on other instantaneous measures of photosynthesis, including maximum photosynthesis, apparent quantum yield and dark respiration, were seen for needles of any age. Foliar nitrogen concentration was unaffected by UV-B treatment. However, the (13)C/(12)C carbon isotope ratios (delta(13)C-a time integrated measure of photosynthetic function) of needles in age classes II and IV were 3% (P < 0.01) and 2% (P < 0.05) more negative, respectively, in treated plants than in control plants. Exposure to enhanced UV-B radiation caused a 20% decrease in total biomass and a 4% (P < 0.05), 25% (P < 0.01), and 9% (P < 0.01) decrease in needle length of needles in age classes I, II, and IV, respectively. The observed decreases in delta(13)C, and F(v)/F(m) of the needles in the youngest needle age class suggest subtle damage to photosynthesis, although overall growth reductions were probably a result of decreased total leaf surface rather than decreased photosynthetic capacity. Needles of age class IV had lower light- and CO(2)-saturated maximum photosynthetic rates (39%), lower dark respiration (34%), lower light saturation points (37%), lower foliar nitrogen concentration (28%), and lower delta(13)C (14%) values than needles of age class I. Apparent quantum yield and F(v)/F(m) did not change with needle age. The observed changes in photosynthesis and foliage chemical composition with needle age are consistent with previous studies of coniferous trees and may represent adaptations of older needles to shaded conditions within the canopy.

Are some plant life forms more effective than others in screening out ultraviolet-B radiation?

The unprecedented rate of depletion of the stratospheric ozone layer will likely lead to appreciable increases in the amount of ultraviolet-B radiation (UV-B, 280-320 nm) reaching the earth's surface. In plants, photosynthetic reactions and nucleic acids in the mesophyll of leaves are deleteriously affected by UV-B. We used a fiber-optic microprobe to make direct measurements of the amount of UV-B reaching these potential targets in the mesophyll of intact foliage. A comparison of foliage from a diverse group of Rocky Mountain plants enabled us to assess whether the foliage of some plant life forms appeared more effective at screening UV-B radiation. The leaf epidermis of herbaceous dicots was particularly ineffective at attenuating UV-B; epidermal transmittance ranged from 18-41% and UV-B reached 40-145 µm into the mesophyll or photosynthetic tissue. In contrast to herbaceous dicots, the epidermis of 1-year old conifer needles attenuated essentially all incident UV-B and virtually none of this radiation reached the mesophyll. Although the epidermal layer was appreciably thinner in older needles (7 y) at high elevations (Krumholtz), essentially all incident UV-B was attenuated by the epidermis in these needles. The same epidermal screening effectiveness was observed after removal of epicuticular waxes with chloroform. Leaves of woody dicots and grasses appeared intermediate between herbaceous dicots and conifers in their UV-B screening abilities with 3-12% of the incident UV-B reaching the mesophyll. These large differences in UV-B screening effectiveness suggest that certain plant life forms may be more predisposed than others to meet the challenge of higher UV-B levels resulting from stratospheric ozone depletion.

Limitations of Photosynthesis in Pinus taeda L. (Loblolly Pine) at Low Soil Temperatures.

The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7 degrees C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO(2) concentrations (c(i)). This conclusion was supported by A versus c(i) analysis and measurements of O(2) evolution at saturating CO(2), which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus c(i) analysis appear valid. Bulk shoot water potential (psi) declined as soil temperature dropped below 16 degrees C. When half the root system of seedlings was chilled, shoot psi and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle psi, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7 degrees C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated c(i) and A versus c(i) analysis. In contrast, measurements of O(2) evolution at saturating CO(2) concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of c(i) are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in c(i) are real and the results from O(2)-evolution measurements are in error. The high CO(2) concentration used in O(2)-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7 degrees C.

Effect of soil temperature on stem sap flow, shoot gas exchange and water potential of Picea engelmannii (Parry) during snowmelt.

The effect of cold soils on stem sap flow, shoot gas exchange and water potential of Picea engelmannii (Parry) was investigated during the snowmelt period in the Medicine Bow Mountains, Wyoming, USA. Shoot net photosynthetic rates were higher in young trees (1.5-1.8 m in height) growing in cold soils (<3.5° C) associated with snowpack, than trees in warm soils until about 1500 h. Higher shoot photosynthetic rates of trees in cold soils continued after snow was removed and could not be completely explained by higher visible irradiance over highly reflective snow. Following soil warming higher photosynthetic rates were evident in these trees for five days. High nutrient availability associated with snowmelt may improve shoot nutrient status leading to higher gas-exchange rates during snowmelt. Shoot conductance to water vapor was higher in trees in cold soil until midday, when declining shoot conductance led to lower intercellular CO2 concentrations. Midday through afternoon shoot water potentials of trees in cold soils were similar or higher than those of trees in warm soils and the lower afternoon shoot conductances in cold soils were not the result of lower bulk shoot water potentials. Decline in net photosynthesis of trees in cold soils at 1500 h paralleled increases in intercellular CO2 concentrations, implying a nonstomatal limitation of photosynthesis. This scenario occurred consistently in mid-afternoon following higher morning and midday photosynthesis in cold soils, suggesting a carbohydrate feedback inhibition of photosynthesis. Diurnal patterns in stem sap flow of all trees (cold and warm soils) reflected patterns of shoot conductance, although changes in stem sap flow lagged 1-3 h behind shoot conductance apparently due to stem water storage. Total daily stem sap flow was similar in trees in cold and warm soils, although diel patterns differed. The morning surge and night-time drop in sap flow commenced 1-2 h earlier in trees in cold soils. Overnight stem sap flow was lower in trees in cold soils, possibly due to higher resistance to root water uptake in cold soils, which may explain lower predawn shoot water potentials. However, midday shoot water potentials of trees in cold soils equalled or exceeded those of trees in warm soils. Higher resistance to root water uptake in P. engelmannii in cold soils was apparently overshadowed by transpirational forces and significant shoot water deficits did not develop.

Influence of cold soil and snowcover on photosynthesis and leaf conductance in two Rocky Mountain conifers.

The influence of cold soil and snowcover on photosynthesis and conductance of Picea engelmannii and Pinus contorta was investigated early in the growing season in the Medicine Bow Mountains, Wyoming, USA. Trees of both species growing in cold soil (<1°C) associated with snowpack had 25-40% lower leaf photosynthesis than trees in warm soils (>10°C). In cold soils leaf conductance of both species was lower, but more so in Pinus, leading to lower intercellular CO2 concentrations and greater stomatal limitation of photosynthesis. Soil temperature had no effect on predawn and midday shoot water potentials of Pinus and Picea and lower photosynthesis and conductance did not appear to be a result of lower bulk shoot water potential. Predawn, as well as midday, water potentials of Pinus were consistently higher than Picea suggesting that Pinus may have deeper roots, although trenching experiments indicated young Picea trees have more extensive lateral root systems than similar sized Pinus trees. Young Picea trees (<2 m in height) in snowbanks were capable of utilizing warmer soil 4 m from their base. Under similar conditions Pinus in snowbanks had lower photosynthesis and conductance than controls and Pinus did not appear capable of utilizing warmer soils nearby. Under full sunlight, PPFD reflected from the snow surface was 400-1400 µmol m-2 s-1 higher than from snow-free surfaces. This reflected light resulted in a 10%-20% increase in photosynthesis of Picea. The beneficial effect of reflected light was apparent whether or not photosynthesis was reduced by low soil temperatures.

Reversibility of Photosynthetic Inhibition in Cotton after Long-Term Exposure to Elevated CO(2) Concentrations.

Cotton (Gossypium hirsutum L. cv Stoneville 213) was grown at 350 and 1000 microliters per liter CO(2). The plants grown at elevated CO(2) concentrations contained large starch pools and showed initial symptoms of visible physical damage. Photosynthetic rates were lower than expected based on instantaneous exposure to high CO(2).A group of plants grown at 1000 microliters per liter CO(2) was switched to 350 microliters per liter CO(2). Starch pools and photosynthetic rates were monitored in the switched plants and in the two unswitched control groups. Photosynthetic rates per unit leaf area recovered to the level of the 350 microliters per liter CO(2) grown control group within four to five days. To assess only nonstomatal limitations to photosynthesis, a measure of photosynthetic efficiencies was calculated (moles CO(2) fixed per square meter per second per mole intercellular CO(2)). Photosynthetic efficiency also recovered to the levels of the 350 microliters per liter CO(2) grown controls within three to four days.Recovery was correlated to a rapid depletion of the starch pool, indicating that the inhibition of photosynthesis is primarily a result of feedback inhibition. However, complete recovery may involve the repair of damage to the chloroplasts caused by excessive starch accumulation. The rapid and complete reversal of photosynthetic inhibition suggests that the appearance of large, strong sinks at certain developmental stages could result in reduction of the large starch accumulations and that photosynthetic rates could recover to near the theoretical capacity during periods of high photosynthate demand.

Photosynthetic inhibition after long-term exposure to elevated levels of atmospheric carbon dioxide.

The effect of long-term exposure to elevated levels of CO2 on biomass partitioning, net photosynthesis and starch metabolism was examined in cotton. Plants were grown under controlled conditions at 350, 675 and 1000 µl l(-1) CO2. Plants grown at 675 and 1000 µl l(-1) had 72% and 115% more dry weight respectively than plants grown at 350 µl l(-1). Increases in weight were partially due to corresponding increases in leaf starch. CO2 enrichment also caused a decrease in chlorophyll concentration and a change in the chlorophyll a/b ratio. High CO2 grown plants had lower photosynthetic capacity than 350 µl l(-1) grown plants when measured at each CO2 concentration. Reduced photosynthetic rates were correlated with high internal (non-stomatal) resistances and higher starch levels. It is suggested that carbohydrate accumulation causes a decline in photosynthesis by feedback inhibition and/or physical damage at the chloroplast level.