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Previous work demonstrates conflicting evidence regarding the influence of snowmelt timing on forest net ecosystem exchange (NEE). Based on 15 years of eddy covariance measurements in Colorado, years with earlier snowmelt exhibited less net carbon uptake during the snow ablation period, which is a period of high potential for productivity. Earlier snowmelt aligned with colder periods of the seasonal air temperature cycle relative to later snowmelt. We found that the colder ablation‐period air temperatures during these early snowmelt years lead to reduced rates of daily NEE. Hence, earlier snowmelt associated with climate warming, counterintuitively, leads to colder atmospheric temperatures during the snow ablation period and concomitantly reduced rates of net carbon uptake. Using a multilinear‐regression (R2= 0.79, P< 0.001) relating snow ablation period mean air temperature and peak snow water equivalent (SWE) to ablation‐period NEE, we predict that earlier snowmelt and decreased SWE may cause a 45% reduction in midcentury ablation‐period net carbon uptake. Earlier snowmelt aligns with colder periods of the seasonal air temperature cycleLower air temperatures reduce net carbon uptake during the annual snow ablation periodUnder a warming scenario we expect decreased carbon uptake during the annual snow ablation period

We used co‐located observations of snow depth, soil temperature, and moisture and energy fluxes to monitor variability in snowmelt infiltration and vegetation water use at mixed‐conifer sub‐alpine forest sites in the Valles Caldera, New Mexico (3020 m) and on Niwot Ridge, Colorado (3050 m). At both sites, vegetation structure largely controlled the distribution of snow accumulation with 29% greater accumulation in open versus under‐canopy locations. Snow ablation rates were diminished by 39% in under‐canopy locations, indicating increases in vegetation density act to extend the duration of the snowmelt season. Similarly, differences in climate altered snow‐season duration, snowmelt infiltration and evapotranspiration. Commencement of the growing season was coincident with melt‐water input to the soil and lagged behind springtime increases in air temperature by 12 days on average, ranging from 2 to 33 days under warmer and colder conditions, respectively. Similarly, the timing of peak soil moisture was highly variable, lagging behind springtime increases in air temperature by 42 and 31 days on average at the Colorado and New Mexico sites, respectively. Latent heat flux and associated evaporative loss to the atmosphere was 28% greater for the year with earlier onset of snowmelt infiltration. Given the large and variable fraction of precipitation that was partitioned into water vapour loss, the combined effects of changes in vegetation structure, climate and associated changes to the timing and magnitude of snowmelt may have large effects on the partitioning of snowmelt into evapotranspiration, surface runoff and ground water recharge. Copyright © 2009 John Wiley & Sons, Ltd.

The biosynthesis and emission of volatile plant terpenoids, such as isoprene and methylbutenol (MBO), depend on the chloroplastic production of dimethylallyl diphosphate (DMAPP). To date, it has been difficult to study the relationship of cellular DMAPP levels to emission of these volatiles because of the lack of a sensitive assay for DMAPP in plant tissues. Using a recent DMAPP assay developed in our laboratories, we report that species with the highest potential for isoprene and MBO production also exhibit elevated light-dependent DMAPP production, ranging from 110% to 1,063%. Even species that do not produce significant amounts of volatile terpenoids, however, exhibit some potential for light-dependent production of DMAPP. We used a nonaqueous fractionation technique to determine the intracellular distribution of DMAPP in isoprene-emitting cottonwood (Populus deltoides) leaves; approximately 65% to 70% of the DMAPP recovered at midday occurred in the chloroplasts, indicating that most of the light-dependent production of DMAPP was chloroplastic in origin. The midday concentration of chloroplastic DMAPP in cottonwood leaves is estimated to be 0.13 to 3.0 mM, which is consistent with the relatively high K(m)s that have been reported for isoprene synthases (0.5-8 mM). The results provide support for the hypothesis that the light dependence of isoprene and MBO emissions is in part due to controls over DMAPP production.

Abstract Terrestrial and marine ecosystems function as sources and sinks for reactive trace gases, and in doing so, profoundly influence the oxidative photochemistry in the troposphere. Principal biogenic processes include microbial methane production and oxidation, the emission of volatile organic compounds from forest ecosystems, the emission of nitric oxide from soils, the emission of reactive sulfur compounds and carbon monoxide from marine ecosystems, control over the production of hydroxyl radical concentration by regional hydrologic processes, and deposition of ozone and nitrogen oxides to ecosystems. The combined influence of these processes is to affect the tropospheric concentrations of ozone, hydroxyl radicals, reactive nitrogen oxides, carbon monoxide, and inorganic acids, all of which constitute fundamental components of oxidative photochemistry. In this review we discuss the recent literature related to the primary controls over the biosphere-atmosphere exchange of reactive trace gases, and also to efforts to model the dominant biospheric influences on oxidative dynamics of the troposphere. These studies provide strong support for the paradigm that biospheric processes exert the dominant control over oxidative chemistry in the lower atmosphere. Improvements in our ability to model biospheric influences on tropospheric chemistry, and its susceptibility to global change, will come from inclusion of more explicit information on the processes that control the emission and uptake of reactive trace gases and the impact of changes in ecosystem cover and land-use change.

Abstract: Isoprene (2-methyl-1,3,-butadiene), produced by many woody and a few herbaceous plant species, is the dominant volatile organic compound released from vegetation. It represents a non-trivial carbon loss to the plant (typically 0.5–2%, but much higher as temperatures exceed 30°C), and plays a major role in tropospheric chemistry of forested regions, contributing to ozone formation. This review summarizes current knowledge concerning the occurrence of isoprene production within the plant kingdom, and discusses other aspects of isoprene biology which may be of interest to the ecological community. The ability to produce significant amounts of isoprene may or may not be shared by members of the same plant family or genus, but emitting species have been found among bryophytes, ferns, conifers and Ephedra and in approximately one-third of the 122 angiosperm families examined. No phylogenetic pattern is obvious among the angiosperms, with the trait widely scattered and present (and absent) in both primitive and derived taxa, although confined largely to woody species. Isoprene is not stored within the leaf, and plays no known ecological role as, for example, an anti-herbivore or allelopathic agent. The primary short-term controls over isoprene production are light and temperature. Growth in high light stimulates isoprene production, and growth in cool conditions apparently inhibits isoprene, production of which may be induced upon transfer to warmer temperatures. The stimulation of isoprene production by high irradiance and warm temperatures suggests a possible role in ameliorating stresses associated with warm, high-light environments, a role consistent with physiological evidence indicating a role in thermal protection.

Abstract: Amino acids have been shown to be a potentially significant N source for the alpine sedge, Kobresia myosuroides. We hypothesised that freeze-thaw and dry-rewet events allow this plant species increased access to amino acids by disrupting microbial cells, which decreases the size of competing microbial populations, but increases soil amino acid concentrations. To test this hypothesis, we characterized freeze-thaw and dry-rewet events in the field and simulated them in laboratory experiments on plant-soil microcosms. In one experiment, 15N,13C-[2]-glycine was added to microcosms that had previously been subjected to a freeze-thaw or dry-rewet event, and isotopic concentrations in the plant and microbial fractions were compared to non-stressed controls. Microbial biomass and uptake of the labeled glycine were unaffected by the freezing and drying treatments, but microbial uptake of 15N was lower in the two warmer treatments (dry-rewet and summer control) then in the two colder treatments (freeze-thaw and fall control). Plant uptake of glycine-15N was decreased by climatic disturbance, and uptake in plants that had been frozen appeared to be dependent on the severity of the freeze. The fact that intact glycine was absorbed by the plants was confirmed by near equal enrichment of plant tissues in 13C and 15N. Plants under optimal conditions recovered 3.5% of the added 15N and microbes recovered 5.0%. The majority of the 13C and 15N label remained in a non-extractable fraction in the bulk soil. To better understand the isolated influences of environmental perturbations on soil amino acid pools and population sizes of amino-acid utilizing microbes, separate experiments were performed in which soils, alone, were subjected to drying and rewetting or freezing and thawing. Potential respiration of glycine and glutamate (substrate-induced respiration; SIR) by the soil microbial communities was unaffected by a single freeze-thaw event. Glycine SIR was decreased slightly (∼10%) by the most extreme drying treatment, but glutamate SIR was not significantly affected. Freezing lowered the concentration of water-extractable amino acids while drying increased their concentration. We interpret the surprising former result as either a decrease in proteolytic activity in frozen soils relative to amino acid uptake, or a stimulation in microbial uptake by physical nutrient release from the soil. We conclude that climatic disturbance does not provide opportunities for increased amino acid uptake by K. myosuroides, but that this plant competes well for amino acid N under non-stressed conditions, especially when soils are warm. We also note that this alpine tundra microbial community's high resistance to freeze-thaw and dry-rewet events is novel and contrasts with studies in other ecosystems.

Analyses of carbon-assimilation patterns in response to intercellular CO2 concentrations, and the photosynthetic water-and nitrogen-use efficiencies, were conducted for a C3, a C4, and three C3-C4 species in the genus Flaveria in order to determine some of the advantages and disadvantages of C3-C4 intermediate photosynthesis. Operational intercellular CO2 partial pressures (pi), determined when the atmospheric CO2 partial pressure (pa) was approximately 330 µbar, in the C3-C4 species were generally equal to, or greater than, those observed in the C3 species under well-watered or water-stressed conditions. This reflects equal, or lower, water-use efficiencies (WUEs) in the C3-C4 species. The only case in which higher WUEs were observed in the C3-C4 species, compared to the C3 species, was when photosynthesis rates were limited by available nitrogen and were less than 12.5 µmol CO2 m-2s-1. At higher photosynthesis rates, the C3-C4 species exhibited lower values of photosynthesis rate for equal values of stomatal conductance (lower WUE), compared to the C3 species. Comparing slopes for the linear regions of the relationship between leaf nitrogen content and net photosynthesis rate (taken as an index of photosynthetic nitrogen-use efficiency, NUE), the C4 species exhibited the highest NUE, followed by the C3-C4 species, F. ramosissima, with the other two C3-C4 species and the C3 species being equal and exhibiting the lowest NUEs. The lack of consistent advantages in NUE and WUE in the C3-C4 species F. pubescens and F. floridana suggest that in some C3-C4Flaveria species C4-like anatomy and biochemistry do not provide the same gas exchange advantages that we typically attribute to the CO2-concentrating mechanism of fully-expressed C4 plants.

Summary Field measurements of gas exchange and growth were conducted on a C3 grass,Agropyron smithii, and a C4 grass,Bouteloua gracilis, in order to further establish the adaptive significance of the C4 pathway under natural conditions. Maximum rates of leaf area expansion in tillers and maximum seasonal photosynthesis rates of both species occurred during the cool, early summer month of June. The occurrence of maximum growth and photosynthesis inB. gracilis during this cool period was apparently related to its occupation of warm microenvironments next to the ground surface. As temperatures increased during the midsummer, photosynthesis rates decreased to 47% and 55% of the seasonal maximum inB. gracilis andA. smithii, respectively. Water-use efficiencies in both species were similar or slightly higher forB. gracilis during June, the period of maximum growth. By mid-July, however, leaves of the C3 grass,A. smithii, exhibited water-use efficiencies approximately half as high asB. gracilis. These differences in water-use efficiency were the result of differences in stomatal conductance, rather than differences in daily CO2 uptake rates which were similar in both species. The results demonstrate that in certain environments there are no offset periods of growth and maximum photosynthesis during the growing season in these C3 and C4 species. The greater amounts of daily water use inA. smithii during the midsummer might contribute to its much greater abundance in lowland sites in the shortgrass steppe. The C4 grass,B. gracilis, occurs in dry upland sites in addition to the more mesic lowland sites.

Plant population dynamics and life cycle characteristics are presented for two genetically related, yet ecologically distinct, chromosomal races of Machaeranthera gracilis (Nutt.) Shinners. Seedling mortality was very high for a field population of the desert race (n=2) as only 8% of the maximum number of seedlings produced did survive to the period of peak reproduction. Lower mortality occurred in the foothills race (n=4) as 15% of the maximum number of seedlings produced did survive until the period of peak reproduction. The desert race also exhibited the capacity to assume the perennial growth habit. Perennial plants of the desert race produced 4.4 heads/plant (116 heads/m2) versus 0.2 heads/plant (4 heads/m2) for annual plants of the desert race under field conditions. Annual plants of the foothills race produced 2.7 heads/plant (75 heads/m2). The perennial habit did not occur in field populations of the foothills race. When grown in identical glasshouse conditions the desert race reached anthesis of the first four flower heads ca. 25 days prior to the foothills race. This acceleration to anthesis was accompanied by a greater potential reproductive allocation. The latter response was due in part to greater biomass allocation to reproductive structures at the expense of vegetative structures, and the uncoupling of net photosynthesis from phenological events.

Non-mycorrhizal plants of the alpine sedge, Kobresia myosuroides, take up the amino acid glycine from nutrient solutions at greater rates than NOinf3sup-or NHinf4sup+. The amino acids glutamate and proline were also taken up at high rates. Total plant biomass was twice as high after 4 months of growth on glycine, compared to NH4NO3, with significant increases in both root and leaf biomass. By taking advantage of differences in the d13C signature of air in the growth chamber and the glycine used for growth, a two-member mixing model was used to estimate that a significant amount of the glycine was taken up as intact molecules, enough to contribute 16% of the total carbon assimilation over a 4-month growing period. Glycine uptake was inhibited when roots were exposed to N2 in place of air, and when the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) was added to the root solution. From these results it is concluded that glycine uptake occurs through active transport. Glycine uptake exhibited a Q10 of 2.0 over the temperature range 5–15° C, with relatively high rates maintained at the lowest temperature measured (5° C). Roots of Kobreasia were not capable of taking up NHinf4sup+at measureable rates. To our knowledge, this is the first report of a plant whose non-mycorrhizal roots cannot take up NHinf4sup+. Measurements of three N fractions (NOinf3sup-, NHinf4sup+, and total amino acids) in the soil pore water were made over two growing seasons in two Kobresia dry meadows using microlysimeters. At the West Knoll site, which is characterized by soils with average amounts of organic matter, the dominant forms of N in the soil pore water were NOinf3sup-and NHinf4sup+(0–450 µmol L-1). Amino acid concentrations were generally less than 20 µmol L-1 at this site. At the East Knoll site, which is characterized by soils with higher than average amounts of organic matter, amino acids were generally present at higher concentrations (17–100 µmol L-1), compared to NOinf3sup-and NHinf4sup+. The most abundant amino acids were glycine (10–100 µmol L-1), glutamate (5–70 µmol L-1), and late in the season cystein (5–15 µmol L-1). The results demonstrate that this sedge, which dominates dry meadow communities in many alpine ecosystems, is capable of taking up intact amino acids as a principal N source, and has access to high amino acid concentrations in certain alpine soils. Such uptake of organic N may accommodate plant N demands in the face of slow alpine N mineralization rates due to cold soil temperatures.

Measurements of leaf gas exchange were conducted in situ for the C3‐C4intermediate plant Flaveria floridana. Leaves exhibited measurable CO2assimilation at atmospheric CO2concentrations as low as 20 μmol/mol. This result demonstrates that the low CO2compensation points observed in past studies of greenhouse‐grown C3‐C4intermediate plants also exist in plants growing in their natural habitat. Photosynthesis rates in F. floridanawere near their maximum at intercellular CO2concentrations as low as 112 μmol/mol. The existence of near‐maximum photosynthesis rates at such low intercellular CO2concentrations is interpreted as evidence for the existence of a CO2‐concentrating mechanism in F. floridana. Such a mechanism would also explain the observed lack of response in photosynthesis rates to reductions in stomatal conductance and intercellular CO2concentration as the leaf‐to‐air water vapor concentration gradient is increased. Photosynthetic rates were relatively high at leaf temperatures between 35 and 40 C, compared to most C3plants. At midday during May, when leaf temperatures were between 35 and 42 C, F. floridanaleaves exhibited photosynthesis rates that were four times higher than a sympatric C3species (Eustoma exaltatum) of similar growth form and ecological habit. The high photosynthesis rates at high leaf temperatures in F. floridanawere not due to higher leaf nitrogen contents, but rather to its reduced rate of photorespiration. These results confirm that C3‐C4intermediate photosynthesis can provide plants with an advantage at high leaf temperatures, compared to C3plants.

As part of an intensive study of heritable differences among the progeny of Pinus ponderosaparents from two contrasting habitats (coastal vs. interior, continental), we examined the potential for differences in photosynthesis rate, stomatal conductance, and photosynthetic water‐use efficiency. Plants from a cross between two coastal parents (ponderosa × ponderosa) exhibited lower photosynthetic water‐use efficiencies, relative to plants from a coastal × interior cross (ponderosa × scopulorum). The lower water‐use efficiencies in the ponderosa × ponderosaplants were evident as a lower ratio of external to intercellular CO2concentrations and higher stomatal conductances at any given rate of photosynthesis. The ponderosa × scopulorumplants exhibited lower stomatal conductances over a range of leaf‐to‐air water vapor concentration differences, which was partially explained by lower stomatal densities. The ponderosa × scopulorumplants also exhibited lower maximum photosynthesis rates and lower needle nitrogen concentrations. Taken together, the results suggest that in adapting to drier habitats, P. ponderosahas acquired improved water‐use efficiencies and lower transpiration rates, but at the expense of reduced maximum photosynthesis rates.

We studied fourteen morphological attributes of 511 seedling stage progeny involving crosses between Pinus ponderosa ponderosaparents as well as intervarietal crosses of P. p. ponderosa × P. p. scopulorum.Intravarietal progeny were distinctly differentiated genetically from intervarietal sibships in seven of the fourteen characters examined. The patterns of differentiation observed strongly suggest a syndrome of selective responses to increased water stress in the progeny with a scopulorum parent and for comparatively more rapid, columnar growth in the progeny of the intravariety crosses which involved only P. p. ponderosaparents. These differences are consistent with known ecological distinctions between the habitats of the two varieties. Narrow‐sense heritability estimates, obtained from half‐sib progeny analysis, indicated that considerable levels of additive genetic variance remain present in several traits. We could detect no relationship between the amount of additive genetic variance present for a particular character and its presumed relevance to fitness.

Intact chloroplasts were obtained from mesophyll protoplasts isolated from Mesembryanthemum crystallinum in the C3 or Crassulacean acid metabolism (CAM) photosynthetic mode, and examined for the influence of inorganic phosphate (Pi) on aspects of bicarbonate-dependent O2 evolution and CO2 fixation. While the chloroplasts from both modes responded similarly to varying Pi, some features appear typical of chloroplasts from species capable of CAM, including a relatively high capacity for photosynthesis in the absence of Pi, a short induction period, and resistance to inhibition of photosynthesis by high levels of Pi. In the absence of Pi the chloroplasts retained 75–85% of the 14CO2 fixed and the total export of dihydroxyacetone phosphate was low compared with the rate of photosynthesis. In CAM plants the ability to conduct photosynthesis and retain most of the fixed carbon in the chloroplasts at low external Pi concentrations may enable storage of carbohydrates which are essential for providing a carbon source for the nocturnal synthesis of malic acid. At high external Pi concentrations (e.g. 10 25 mM), the amount of total dihydroxyacetone phosphate exported to the assay medium relative to the rate of photosynthesis was high while the products of 14CO2 fixation were largely retained in the chloroplasts which indicates starch degradation is occurring at high Pi levels. Starch degradation normally occurs in CAM plants in the dark; high levels of Pi may induce starch degradation in the light which has the effect of limiting export of the immediate products of photosynthesis and thus the degree of Pi inhibition of photosynthesis with the isolated chloroplast.

Yucca glauca in the Colorado shortgrass prairie undergoes a pronounced midday depression in net photosynthesis and stomatal conductance under summer field conditions. This phenomenon can be duplicated in the laboratory using potted plants by simulating a typical summer daily pattern of leaf temperature and leaf-to-air water vapor concentration difference (?w). The decrease in photosynthetic rate appears to be due primarily to high leaf temperatures, while the decrease in stomatal conductance can be attributed mainly to high ?w values. Stomatal conductance also decreases when leaf temperatures exceed a critical threshold value, even when ?w is artificially maintained at a constant level. The threshold temperature is commonly attained for leaves in situ, but only after substantial stomatal closure has already occurred as a result of high ?w values.

Several aspects of photosynthetic adaptation to temperature were examined in four graminoid species from the Colorado shortgrass steppe. The experimental species were chosen to provide examples of a variety of in situ seasonal phenology patterns. The cool season grass, Agropyron smithii (C3), exhibited higher photosynthesis rates when grown in a cool temperature regime (20/15°C), and compared to warm grown plants (35/15°C). The warm season species, Bouteloua gracilis (C4) and Buchloe dactyloides (C4), exhibited higher photosynthetic capacities when grown in the warmer temperature regime. The sedge, Carex eleocharis (C3), which exhibits seasonal growth potential during the cool and warm portions of the growing season, exhibited a marked capacity for photosynthetic temperature acclimation. Differential effects of growth temperature on the intracellular conductance to CO2 appeared to have a greater regulatory role in these responses for the two C3 species, relative to stomatal conductance or photorespiration (O2 inhibition of photosynthesis). In the two C4 species decreases in the intracellular conductance in cool grown plants were correlated with the decreased photosynthetic capacity in normal air for B. gracilis, but not for B. dactyloides. Analysis of the Arrhenius relationship for CO2 saturated net photosynthesis at low leaf temperatures (4.5–17°C) indicated sharp breaks in the apparent energy of activation at 5.8–9.0°C in the warm season species B. gracilis and B. dactyloides. Leaves of A. smithii and C. eleocharis exhibited no significant low temperature limitations according to this analysis. The low temperature limitations in the warm season species were partially reflected in an inhibition of the quantum yield for CO2 uptake after 2 h at 5–6°C in the presence of high photon flux densities. Temperature dependent increases in the chlorophyll fluorescence yield at high temperatures revealed the lowest breakpoint values for A. smithii, and the highest values for B. gracilis. The differential patterns of temperature adaptation among the species further extend the proposal of Kemp and Williams (1980; Ecology 61:846–858) that seasonal temperature gradients in the shortgrass steppe have a regulatory role in maintaining offset patterns of resource utilization and decreasing interspecific competition.

The temperatures at which chlorophyll fluorescence yield is substantially increased and the temperatures at which the quantum yield for CO2 uptake is irreversibly inhibited were measured for three shortgrass prairie species. The experimental taxa include, a cool season species (Agropyron smithii), a warm season species (Bouteloua gracilis), and a species which grows throughout the cool and warm seasons (Carex stenophylla). Agropyron smithii exhibited lower high temperature damage thresholds (43°C in cool grown plants, 46°C in warm grown plants), relative to the other two species. Bouteloua gracilis exhibited the highest tolerance to high temperature, with threshold values being 44–49°C for cool grown plants and 53–55°C for warm grown plants. Carex stenophylla exhibited threshold values which were intermediate to the other two species (43–47°C for cool grown plants, and 51–53°C for warm grown plants). Seasonal patterns in the fluorescence rise temperatures of field grown plants indicated acclimation to increased temperatures in all three species. The results demonstrate a correlation between the high temperature thresholds for damage to the photosynthetic apparatus, and in situ seasonal phenology patterns for the three species.

Abstract: Studies were conducted to determine whether herbivore-induced synthesis of monoterpenes occurs in the needles of ponderosa pine (Pinus ponderosa Lawson), lodgepole pine (P. contorta Douglas var. latifolia Engelmann), white fir (Abies concolor Lindl. and Gordon) and Engelmann spruce [Picea engelmanii (Parry) Engelm.]. In the needles of all species except Engelmann spruce, simulated herbivory significantly induced the activity of monoterpene cyclases 4–8 days after wounding. In ponderosa pine, real herbivory by last-instar tiger moth larvae (Halisdota ingens Hy. Edwards: Lepidoptera) induced a significantly larger response (4.5-fold increase in monoterpene cyclase activity) than did simulated herbivory (2.5-fold increase). To our knowledge, this is the first report of herbivore-induced increases in monoterpene synthesis in needle tissue. Despite this increase in monoterpene synthesis, we observed no significant increase in total monoterpene pool size in wounded needles compared to controls. Large increases in the rate of monoterpene volatilization were observed in response to wounding. We conclude that the volatile losses caused by tissue damage compensate for herbivore-induced monoterpene synthesis, resulting in no change in pool size. Tiger moth larvae consume ponderosa pine needles in a pattern that begins at the tip and proceeds downward to midway along the needle, at which point they move to an undamaged needle. Constitutive monoterpene concentrations and monoterpene cyclase activities were highest in the lower half of ponderosa pine needles. The monoterpene profile also differed between the upper and lower needle halves, the lower half possessing an additional one to four monoterpene forms. We propose that the increasing gradient in monoterpene concentrations and number of monoterpenes along the needle from tip to base deters feeding beyond the midway point and provides time for the induction of increased cyclase activity and production of new monoterpenes. The induction of new monoterpene synthesis may have a role in replacing monoterpenes lost through damage-induced volatilization and preventing extreme compromise of the constitutive defense system.

Females of dioecious species usually have higher reproductive effort than males because they produce fruits in addition to flowers. Since females have higher reproductive effort, they are expected to be more negatively affected than males by low resource availability. We tested that assumption by growing females and males of Silene latifoliaunder low levels of light, water, nitrogen, phosphorus, and potassium. Gas exchange of the sexes did not respond differently to low resource availability; higher female reproductive effort relative to males did not differentially affect their ability to assimilate carbon. However, male photosynthesis rates and stomatal conductances were slightly, but consistently, higher than those of females. The intersexual difference in photosynthesis rate may be a proximate result of reproduction if females translocate nutrients, particularly nitrogen, from their leaves to developing fruits. Alternatively (or perhaps additionally), higher male photosynthesis and stomatal conductances relative to females may be the ultimate result of sexual selection. This could be the case if 1) reproductive effort as estimated by biomass allocation is misleading and males actually invest more in reproduction than females, or 2) females experience stronger selection than males to conserve water late in the growing season, when soil moisture is likely to be low but females need to complete fruit maturation. Our results indicated that females had slightly lower leaf nitrogen but higher photosynthetic water‐use efficiency than males, so it is possible that both proximate and ultimate factors are operating simultaneously to cause lower female photosynthesis rates.

Studies were conducted to examine the importance of nitrogen storage to seasonal aboveground growth in the alpine herb Bistorta bistortoides. Stored reserves accounted for 60% of the total nitrogen allocated to the shoot during the growing season. The stored nitrogen was equally partitioned between preformed buds of the shoot and the roots/rhizome. Reliance on stored N was similar in populations of a 105-day growing season site and of a 75-day growing season site. Contrary to our initial hypothesis, stored nitrogen reserves were not used to extend the growing season of this species into the late-spring when soils are still cold, and saturated with snow-melt water. The time at which stored nitrogen was used to initiate shoot growth coincided with the time of root initiation, rapid soil warming, and near maximum soil concentrations of NOinf3sup-and NHinf4sup+. Thus, nitrogen demand and soil nitrogen supply were both high at the same time. The importance of nitrogen storage in this species appeared to be in satisfying the high demand of simultaneous vegetative and reproductive growth during the early-growing season after soils thawed. The initiation of rapid leaf and inflorescence growth occurred in mid-June in both sites. The maximum pool size of shoot nitrogen (maximum nitrogen demand) occurred only 12 days later in the long season site, and 28 days later in the short season site. The early-season utilization of nitrogen stores allows plants of this species to initiate reproductive allocation at the same time vegetative tissues are exhibiting maximal growth rates. By releasing vegetative and reproductive growth from competition for nitrogen, seeds could mature early in the alpine growing season, before the frost probability sharply increases in mid-August.

Mesophyll and bundle sheath protoplasts were differentially isolated for the first time from leaves of a C3-C4 intermediate, Flaveria ramosissima. Protoplasts were partially purified from leaf digests following differential centrifugation and flotation on dextran step-gradients. Two mesophyll and one bundle sheath fraction were obtained, with relative purities of the preparations determined visually as >95% for mesophyll and >80% for bundle sheath. Representative C3 and C4 photosynthetic enzymes had substantial activities, on a chlorophyll basis, in all three protoplast preparations. The activity of phosphoenolpyruvate carboxylase was highest in the lower density mesophyll fraction and lowest in the bundle sheath fraction. Conversely, the activity of NADP-malic enzyme was highest in the bundle sheath, and lowest in the lighter mesophyll preparation. Ribulose 1,5-bisphosphate carboxylase/oxygenase had similar activity in all three preparations, as did glycolate oxidase. However, glycine decarboxylase was about 3-fold enriched in the bundle sheath fraction. The data indicate that the partial compartmentation of photorespiratory metabolism may contribute along with limited C4 photosynthesis to reducing photorespiration in this intermediate species.