High Temperatures and Net CO2 Uptake, Growth, and Stem Damage for the Hemiepiphytic Cactus Hylocereus undatus1

Hylocereus undatus, which is native to tropical forests experiencing moderate temperatures, would not be expected to tolerate the extremely high temperatures that can be tolerated by cacti native to deserts. Nevertheless, total daily net CO₂ uptake by this hemiepiphytic cactus, which is widely cultivated for its fruits, was optimal at day/night air temperatures of 30/20°C, temperatures that are higher than those optimal for daily net CO₂ uptake by cacti native to arid and semiarid areas. Exposure to 35/25°C for 30 weeks led to lower net CO₂ uptake than at 10 weeks; exposure to 40/30°C led to considerable necrosis visible on the stems at 6 weeks and nearly complete browning of the stems by 19 weeks. Dry mass gain over 31 weeks was greatest for plants at 30/20°C, with root growth being especially noteworthy and root dry mass gain representing an increasing percentage of plant dry mass gain as day/night air temperatures were increased. Viability of chlorenchyma cells, assayed by the uptake of the vital stain neutral red into the central vacuoles, was decreased 50 percent by a one‐hour treatment at 55°C compared with an average of 64°C for 18 species of cacti native to deserts. The lower high‐temperature tolerance for H. undatus reflected its low high‐temperature acclimation of only 1.4°C as growth temperatures were raised by 10°C compared with an average acclimation of 5.3°C for the other 18 species of cacti. Thus, this tropical hemiepiphytic cactus is not adapted to day/night air temperatures above ca 40/30°C, although its net CO₂ uptake is optimal at the relatively high day/night air temperatures of 30/20°C.     

BRANCHING PATTERNS OF COLUMNAR CACTI: INFLUENCES ON PAR INTERCEPTION AND CO2 UPTAKE

Field measurements and a computer model were used to determine how stem shape and arrangement of stems in space affect interception of photosynthetically active radiation (PAR) and CO₂ uptake under otherwise optimal conditions for four species of columnar cacti (Carnegiea gigantea, Lophocereus schottii, Pachycereus pringlei, and Stenocereus thurberi). In simulations where the number of widely spaced stems was increased from 1 to 19 but plant volume remained constant, surface area and PAR interception increased, leading to 3-fold increases in whole-plant CO₂ uptake. Increasing the distance between stems from 0 cm to infinity decreased self-shading and increased predicted CO₂ uptake 4-fold. Stem length, diam, ribbing characteristics, and spine coverage also influenced PAR interception. The model indicated that the observed higher frequency of branches on the south side of the trunk of C. gigantea had only a slight, though positive, effect on CO₂ uptake for single-branched plants. Because of its greater surface area (A), a five-stemmed plant of C. gigantea typical for a field site near Tucson, Arizona was predicted to have 52% more CO₂ uptake than a single-stemmed plant of the same volume (V). Although large A/V decreases water storage per unit transpiring area, whole-plant CO₂ uptake can be increased when A/V is increased by branching for these constant-volume plants. However, the stems must be arranged to avoid excessive self-shading and thus keep the area below PAR compensation small.     

Physiological responses of Opuntia ficus-indica to growth temperature

The influences of various day/night air temperatures on net CO₂ uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO₂ uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO₂ residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO₂ uptake at all growth temperatures. Nocturnal CO₂ uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO₂ uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO₂ uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.     

Leaf and Stem CO(2) Uptake in the Three Subfamilies of the Cactaceae

Net CO₂ uptake over 24-hour periods was examined for the leaves and for the stems of 11 species of cacti representing all three subfamilies. For Pereskia aculeata, Pereskia grandifolia, and Maihuenia poeppigii (subfamily Pereskioideae), all the net shoot CO₂ uptake was by the leaves and during the daytime. In contrast, for the leafless species Carnegiea gigantea, Ferocactus acanthodes, Coryphantha vivipara, and Mammillaria dioica (subfamily Cactoideae), all the shoot net CO₂ uptake was by the stems and at night. Similarly, for leafless Opuntia ficus-indica (subfamily Opuntioideae), all net CO₂ uptake occurred at night. For leafy members of the Opuntioideae (Pereskiopsis porteri, Quiabentia chacoensis, Austrocylindropuntia subulata), at least 88% of the shoot CO₂ uptake over 24 hours was by the leaves and some CO₂ uptake occurred at night. Leaves responded to the instantaneous level of photosynthetically active radiation (PAR) during the daytime, as occurs for C₃ plants, whereas nocturnal CO₂ uptake by stems of O. ficus-indica and F. acanthodes responded to the total daily PAR, as occurs for Crassulacean acid metabolism (CAM) plants. Thus, under the well-watered conditions employed, the Pereskioideae behaved as C₃ plants, the Cactoideae behaved as CAM plants, and the Opuntioideae exhibited characteristics of both pathways.     

CO2 uptake by the cultivated hemiepiphytic cactus, Hylocereus undatus

The climate of the native tropical forest habitats of Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, can help explain the response of its net CO2 uptake to environmental factors. Under wet conditions, about 85% of the total daily net CO₂ uptake occurs at night via Crassulacean acid metabolism, leading to a high water‐use efficiency. Total daily net CO₂ uptake is reduced 57% by only 10 days of drought, possibly involving stomatal closure induced by abscisic acid produced in the roots, which typically occupy a small substrate volume. Total daily net CO₂ uptake for H. undatus is maximal at day/night air temperatures of 30/20°C, optimal temperatures that are higher than those for desert cacti but representative of ambient temperatures in the tropics; its total daily net CO₂ uptake becomes zero at day/night air temperatures of 42/32°C. Stem damage occurs at 45°C for H. undatus, whose photosynthetic cells show little acclimation to high temperatures compared with other cacti and are also sensitive to low temperatures, ‐1.5°C killing half of these cells. Consistent with its shaded habitat, total daily net CO2 uptake is appreciable at a total daily PPF of only 2 mol m2 day' and is maximal at 20 mol m^?2 day^?1, above which photoinhibition reduces net CO₂ uptake. Net CO₂ uptake ability, which is highly correlated with stem nitrogen and chlorophyll contents, changes only gradually (halftimes of 2–3 months) as the concentration of applied N is changed. Doubling the atmospheric CO₂ concentration raises the total daily net CO₂ uptake of H. undatus by 34% under optimal conditions and by even larger percentages under adverse environmental conditions.     

ORIENTATION,PAR INTERCEPTION,AND NOCTURNAL ACIDITY INCREASES FOR TERMINAL CLADODES OF A WIDELY CULTIVATED CACTUS,OPUNTIA FICUS-INDICA

Terminal vertical cladodes (flattened stems) of Opuntia ficus-indica growing in widely separated locations were nonrandomly oriented. On plantations at 33°S latitude in Chile individual cladodes tended to orient in the same direction as the planted cladodes on which they developed. However, after 2 years unshaded new cladodes tended to face east-west. Terminal cladodes also tended to face east-west for irrigated O. ficus-indica in California (at 34°N) and in Israel (32 to 33°N), but cladodes developing in the winter tended to face north-south. Except for the residual effect of initial planting direction, the observed patterns tended to maximize the interception of photosynthetically active radiation (PAR). Specifically, east-west cladode orientation would maximize PAR interception, except for cladodes developing near the winter solstice at latitudes more than 27° from the equator. Nocturnal acidity increases and hence productivity would generally be light-limited, since the nocturnal increase in acidity was 90% saturated for a total daytime PAR of 24 mol m^–2 day^–1 and the PAR received on vertical surfaces is usually less than this. Topographical features can modify the orientation patterns, since at a site where PAR was considerably blocked by surrounding mountains the maximal nocturnal acidity increases and peak in cladode orientation occurred 20° from facing east–west. Laboratory studies showed that developing cladodes oriented toward a horizontal light and were rotated an average of 16° in a direction that increased PAR interception compared to the cladodes on which they developed. Such phototropic responses, the higher productivity of favorably oriented cladodes, and the tendency to orient similarly to the underlying cladode presumably accounts for the overall orientation patterns observed, where up to four times more cladodes may face in a particular direction than at right angles to it.     

Carbon dioxide concentrations within forest canopies—variation with time, stand structure, and vegetation type

Vertical CO₂ profiles (between 0.02 and 14.0 m) were studied in forest canopies of Pinus contorta, Populus tremuloides, and in a riparian forest with Acer negundo and Acer grandidentatum during two consecutive growing seasons. Profiles, measured continuously during 1- to 13-day periods in four to five stands differing in overstorey canopy area index (CAI < 4.5; including leaves, branches and stems), were well stratified, with highest [CO₂] just above the forest floor. Canopy [CO₂] profiles were influenced by stand structure (CAI, presence of understorey vegetation), and were highly dependent on vegetation type (deciduous and evergreen). A doubling of CAI in Acer spp. and P. tremuloides stands did not show an effect on upper canopy [CO₂], when turbulent mixing was high. However, increasing understorey biomass in Acer spp. stands had a profound effect on lower canopy [CO₂]. In open stands with a vigorous understorey layer, higher soil respiration rates were offset by increased understorey gas exchange, resulting in [CO₂] below those of the convective boundary layer (CBL). Midday depletions up to 20 ppmv below CBL values could be frequently observed in deciduous canopies. In evergreen canopies, [CO₂] stayed generally above the CBL background values, [CO₂] profiles were more uniform, and gradients were smaller than in deciduous stands with similar CAI. Seasonal changes of canopy [CO₂] reflected changes in soil respiration rates as well as plant phenology and gas exchange of both dominant tree and understorey vegetation. Seasonal patterns were less pronounced in evergreen than in deciduous forests.     

Carbon balance during CAM: an assessment of respiratory CO2 recycling in the epiphytic bromeliads Aechmea nudicaulis and Aechmea fendleri

Abstract The regulation of crassulacean acid metabolism (CAM) under controlled environmental conditions has been investigated for two tropical epiphytes, relating plant water and carbon balance to growth form and habitat preference under natural conditions. Aechmea fendleri is restricted to wet, upper montane regions of Trinidad, while A. nudicaulis has a wider distribution extending into more arid regions of the island. Morphological characteristics of these plants are related to habitat preference in terms of leaf succulence (0.44 and 0.94 kg m^?2 for the two species respectively) and a distinct layer of water storage parenchyma in A. nudicaulis In contrast, the thinner leaves of A. fendleri contain little water-storage parenchyma and less chlorenchyma per unit area, but the plants have a more open leaf rosette. The two species differ in expression of CAM, since the proportion of respiratory CO₂ recycled as part of CAM had been found to be much lower in A. fendleri This study compared the efficiency of water use and role of respiratory CO₂ recycling under two PAR regimes (300 and 120 μnol m^?2 s^?1) and three night temperatures (12, 18 and 25 °C). Dark CO₂ uptake rates for both species were comparable to plants in the field (maximum of 2.3 ± 0.2 μmol m^?2s^?1± SD, n= 3). Total net CO₂ uptake at night increased on leaf area basis with temperature for both species under higher PAR, although under the low PAR regime CO₂ uptake was maximal at 18 °C. Water-use efficiency (WUE) increased at 18 °C and 25 °C during dark CO₂ uptake (Phase I) and also during late afternoon photosynthesis (Phase IV) in both species. For A. fendleri, dawn to dusk changes in titrable acidity (ΔH ⁺) were similar under high and low PAR, although ΔH⁺ was correlated to night temperature and PAR in A. nudicaulis. The proportion of ΔH⁺ derived from respiratory CO₂ also varied with experimental conditions. Thus percentage recycling was lower in A. fendleri under high PAR (0–10%), but was only reduced at 18 °C under low PAR. Recycling by A. nudicaulis ranged from 32–42% under high PAR, but was also reduced to 6% under low PAR at 18 °C; at 12 °C and 25 °C, recycling was 37% and 52% respectively. Previous studies have suggested a relationship between the proportion of recycling and degree of water stress. This study indicated that CAM as a CO₂ concentrating mechanism regulates both water-use efficiency and plant carbon balance in these epiphytes, in response to PAR and night temperature. However, the precise relationship between respiratory processes and the balance between external and internal sources of CO₂ is as yet unresolved.     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 3. Canopy architecture and gas exchange

The response of whole-canopy net CO₂ exchange rate (CER) and canopy architecture to CO₂ enrichment and N stress during 1996 and 1997 for open-field-grown wheat ecosystem (Triticum aestivum L. cv. Yecora Rojo) are described. Every Control (C) and FACE (F) CO₂ treatment (defined as ambient and ambient +200 μmol mol⁻¹, respectively) contained a Low- and High-N treatment. Low-N treatments constituted initial soil content amended with supplemental nitrogen applied at a rate of 70 kg N ha⁻¹ (1996) and 15 kg N ha⁻¹ (1997), whereas High-N treatments were supplemented with 350 kg N ha⁻¹ (1996 and 1997). Elevated CO₂ enhanced season-long carbon accumulation by 8% and 16% under Low-N and High-N, respectively. N-stress reduced season-long carbon accumulation 14% under ambient CO₂, but by as much as 22% under CO₂ enrichment. Averaging both years, green plant area index (GPAI) peaked approximately 76 days after planting at 7.13 for FH, 6.00 for CH, 3.89 for FL, and 3.89 for CL treatments. Leaf tip angle distribution (LTA) indicated that Low-N canopies were more erectophile than those of High-N canopies: 48° for FH, 52° for CH, and 58° for both FL and CL treatments. Temporal trends in canopy greenness indicated a decrease in leaf chlorophyll content from the flag to flag-2 leaves of 25% for FH, 28% for CH, 17% for CL, and 33% for FL during 1997. These results indicate that significant modifications of canopy architecture occurs in response to both CO₂ and N-stress. Optimization of canopy architecture may serve as a mechanism to diminish CO₂ and N-stress effects on CER. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment

Leaf photosynthesis (Ps), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free‐air CO₂ enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO₂, respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloud‐free days (PPFD_day,sat) around the lowermost branches was 16·8 ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO₂ canopies, respectively. Elevated CO₂ significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD_day,sat throughout the canopies, and elevated CO₂ did not affect that relationship. Leaf Ps light‐response curves responded differently to elevated CO₂, depending upon canopy position. Elevated CO₂ increased Ps_sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO₂. Consequently, estimated daily integrated Ps on cloud‐free days (Ps_day,sat) was stimulated by elevated CO₂ only in the upper canopy. Ps_day,sat of the lowermost branches was actually lower with elevated CO₂ because of the darker light environment. The lack of CO₂ stimulation at the mid‐ and lower canopy was probably related to significant down‐regulation of photosynthetic capacity; there was no down‐regulation of Ps in the upper canopy. The relationship between Ps_day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole‐canopy Ps or photosynthetic N use efficiency. Elevated CO₂ had no effect on the optimization of canopy N allocation.     

Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis

Vertical structure of plant stands and canopies may change under conditions of elevated CO₂ due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO₂ on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO₂-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l⁻¹) and elevated (610 μl l⁻¹) CO₂. Plants in each stand were grown in a common soil as closed “artificial ecosystems” with a ground area of 6.7 m². LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO₂ treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO₂ the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO₂. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO₂. CO₂ enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO₂ at the stand-level. We suggest that, under elevated CO₂, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.     

Temperatures and thermal tolerances for cacti exposed to high temperatures near the soil surface

Abstract Soil surface temperatures in deserts can reach 70 °C, far exceeding the high-temperature tolerance of most vascular plants of about 55 °C. In this study a computer model indicated that the maximum temperatures of small spherical cacti would approach soil surface temperatures, in agreement with measurements on seedlings of Ferocactus acanthodes. Shortwave radiation was the most important environmental variable affecting maximum cactus temperatures: a 70% reduction in shortwave radiation by shading lowered both predicted and measured stem surface temperatures by 17 °C for plants 2 cm in diameter. High-temperature tolerance, measured as the temperature that halved the fraction of cells taking up a vital stain after a 1 h high-temperature treatment, could reach 60 °C for the detached stems of Opuntia bigelovii, which appears crucial for its vegetative reproduction, and 70 °C for O. ficus-indica, apparently the greatest high-temperature tolerance so far reported for higher vascular plants. Two-fold increases in shortwave absorptance from Epithelantha bokei to Mammillaria lasiacantha to Ariocarpus fissuratus led to a 5 °C predicted increase in maximum temperature. However, compensatory differences in high-temperature tolerances occurred for these dwarf cacti, helping to explain their occurrence in the same open habitat in the Chihuahuan Desert. All six species showed acclimation of their high-temperature tolerance as ambient temperatures were increased, including acclimation by the roots of the dwarf cacti, where the greater sensitivity to high temperatures of roots would exclude them from the upper 2 cm of the soil. Using the model, the observed high-temperature acclimation, and the temperatures needed to reduce stain uptake to zero, the three dwarf cacti were predicted to be able to survive soil surface temperatures of up to 74 °C.     

Seasonal variability in the effect of elevated CO2 on ecosystem leaf area index in a scrub-oak ecosystem

We report effects of elevated atmospheric CO₂ concentration (C_a) on leaf area index (LAI) of a Florida scrub‐oak ecosystem, which had regenerated after fire for between three and five years in open‐top chambers (OTCs) and was yet to reach canopy closure. LAI was measured using four nondestructive methods, calibrated and tested in experiments performed in calibration plots near the OTCs. The four methods were: PAR transmission through the canopy, normalized difference vegetation index (NDVI), hemispherical photography, and allometric relationships between plant stem diameter and plant leaf area. Calibration experiments showed: (1) Leaf area index could be accurately determined from either PAR transmission through the canopy or hemispherical photography. For LAI determined from PAR transmission through the canopy, ecosystem light extinction coefficient (k) varied with season and was best described as a function of PAR transmission through the canopy. (2) A negative exponential function described the relationship between NDVI and LAI; (3) Allometric relationships overestimated LAI. Throughout the two years of this study, LAI was always higher in elevated C_a, rising from, 20% during winter, to 55% during summer. This seasonality was driven by a more rapid development of leaf area during the spring and a relatively greater loss of leaf area during the winter, in elevated C_a. For this scrub‐oak ecosystem prior to canopy closure, increased leaf area was an indirect mechanism by which ecosystem C uptake and canopy N content were increased in elevated C_a. In addition, increased LAI decreased potential reductions in canopy transpiration from decreases in stomatal conductance in elevated C_a. These findings have important implications for biogeochemical cycles of C, N and H₂O in woody ecosystems regenerating from disturbance in elevated C_a.     

Net CO2 uptake rates for Hylocereus undatus and Selenicereus megalanthus under field conditions: Drought influence and a novel method for analyzing temperature dependence

Net CO₂ uptake rates (P _N) were measured for the vine cacti Hylocereus undatus and Selenicereus megalanthus under relatively extreme climatic conditions in Israel. Withholding water decreased rates and the daily amount of CO₂ uptake by about 10 % per day. Compared with more moderate climates within environmental chambers, the higher temperatures and lower relative humidity in the field led to a more rapid response to drought. The upper envelopes of scatter diagrams for P _N versus temperature for these Crassulacean acid metabolism species, which indicate the maximal rates at a particular temperature, were determined for both night time CO₂ uptake in Phase I (mediated by phosphoenolpyruvate carboxylase, PEPC) and early morning uptake in Phase II (mediated by ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO). As stem temperature increased above 13 °C, the maximal P _N increased exponentially, reaching maxima near 27 °C of 12 and 8 μmol m⁻² s⁻¹ for Phases I and II, respectively, for H. undatus and 6 and 4 μmol m⁻² s⁻¹, respectively, for S. megalanthus. Based on the Arrhenius equation, the apparent activation energies of PEPC and RuBPCO were 103 and 86 kJ mol⁻¹, respectively, for H. undatus and 77 and 49 kJ mol⁻¹, respectively, for S. megalanthus, within the range determined for a diverse group of species using different methodologies. Above 28 °C, P _N decreased an average of 58 % per °C in Phase I and 30 % per °C in Phase II for the two species; such steep declines with temperature indicate that irrigation then may lead to only small enhancements in net CO₂ uptake ability.     

Shrub canopies influence soil temperatures but not nutrient dynamics: An experimental test of tundra snow–shrub interactions

Shrubs are the largest plant life form in tundra ecosystems; therefore, any changes in the abundance of shrubs will feedback to influence biodiversity, ecosystem function, and climate. The snow–shrub hypothesis asserts that shrub canopies trap snow and insulate soils in winter, increasing the rates of nutrient cycling to create a positive feedback to shrub expansion. However, previous work has not been able to separate the abiotic from the biotic influences of shrub canopies. We conducted a 3‐year factorial experiment to determine the influences of canopies on soil temperatures and nutrient cycling parameters by removing ~0.5 m high willow (Salix spp.) and birch (Betula glandulosa) shrubs, creating artificial shrub canopies and comparing these manipulations to nearby open tundra and shrub patches. Soil temperatures were 4–5°C warmer in January, and 2°C cooler in July under shrub cover. Natural shrub plots had 14–33 cm more snow in January than adjacent open tundra plots. Snow cover and soil temperatures were similar in the manipulated plots when compared with the respective unmanipulated treatments, indicating that shrub canopy cover was a dominant factor influencing the soil thermal regime. Conversely, we found no strong evidence of increased soil decomposition, CO₂ fluxes, or nitrate or ammonia adsorbtion under artificial shrub canopy treatments when compared with unmanipulated open tundra. Our results suggest that the abiotic influences of shrub canopy cover alone on nutrient dynamics are weaker than previously asserted.     

Environmental influences on the productivity of three desert succulents in the south-western United States

Abstract Net CO₂ uptake over 24 h periods for shoots of Agave deserti, Ferocactus acanthodes, and Opuntia ficus-indica was measured under the ranges of water status, air temperature, and photo-synthetically active radiation (PAR) that occur in the south-western U.S.A. An environmental productivity index (EPI) was constructed indicating the overall influence of these three factors on net CO₂ uptake. Using growth chambers whose conditions were changed monthly to simulate the environmental conditions at a field site, the observed shoot dry weight increases per unit surface area changed in concert with monthly changes in EPI. The observed dry weight gain of the shoot was 17–19% lower than the predicted shoot net CO₂ uptake, which could be accounted for by carbon diversion to the roots. Mean monthly EPI was also predicted for 21 sites in the south-western U.S.A. All three species had low EPIs in the Colorado River basin, which has low annual rainfall and high summer temperatures, and in the north-central part of the region, which has low temperatures and low PAR during winter when water is available. The two native species, A. deserti and F. acanthodes, had high EPIs beyond their range in coastal southern California, where competition by other vegetation for PAR may limit net CO₂ uptake. Such regions as well as south-central California and south-central Arizona had high EPIs for all three species, indicating that these areas would be appropriate for the cultivation of O. ficus-indica.     

Would transformation of C3 crop plants with foreign Rubisco increase productivity? A computational analysis extrapolating from kinetic properties to canopy photosynthesis

Genetic modification of Rubisco to increase the specificity for CO₂ relative to O₂ (τ) would decrease photorespiration and in principle should increase crop productivity. When the kinetic properties of Rubisco from different photosynthetic organisms are compared, it appears that forms with high τ have low maximum catalytic rates of carboxylation per active site (k^c_c). If it is assumed that an inverse relationship between k^c_c and τ exists, as implied from measurements, and that an increased concentration of Rubisco per unit leaf area is not possible, will increasing τ result in increased leaf and canopy photosynthesis? A steady‐state biochemical model for leaf photosynthesis was coupled to a canopy biophysical microclimate model and used to explore this question. C₃ photosynthetic CO₂ uptake rate (A) is either limited by the maximum rate of Rubisco activity (V_cmax) or by the rate of regeneration of ribulose‐1,5‐bisphosphate, in turn determined by the rate of whole chain electron transport (J). Thus, if J is limiting, an increase in τ will increase net CO₂ uptake because more products of the electron transport chain will be partitioned away from photorespiration into photosynthesis. The effect of an increase in τ on Rubisco‐limited photosynthesis depends on both k^c_c and the concentration of CO₂ ([CO₂]). Assuming a strict inverse relationship between k^c_c and τ, the simulations showed that a decrease, not an increase, in τ increases Rubisco‐limited photosynthesis at the current atmospheric [CO₂], but the increase is observed only in high light. In crop canopies, significant amounts of both light‐limited and light‐saturated photosynthesis contribute to total crop carbon gain. For canopies, the present average τ found in C₃ terrestrial plants is supra‐optimal for the present atmospheric [CO₂] of 370 µmol mol^?1, but would be optimal for a CO₂ concentration of around 200 µmol mol^?1, a value close to the average of the last 400 000 years. Replacing the average Rubisco of terrestrial C₃ plants with one having a lower and optimal τ would increase canopy carbon gain by 3%. Because there are significant deviations from the strict inverse relationship between k^c_c and τ, the canopy model was also used to compare the rates of canopy photosynthesis for several Rubiscos with well‐defined kinetic constants. These simulations suggest that very substantial increases (> 25%) in crop carbon gain could result if specific Rubiscos having either a higher τ or higher k^c_c were successfully expressed in C₃ plants.     

Canopy CO2 concentrations and Crassulacean acid metabolism in Hoya carnosa in a subtropical rain forest in Taiwan: consideration of CO2 availability and the evolution of CAM in epiphytes

The potential importance of CO₂ derived from host tree respiration at night as a substrate for night time CO₂ uptake during CAM was investigated in the subtropical and tropical epiphytic vine Hoya carnosa in a subtropical rainforest in north-eastern Taiwan. Individuals were examined within the canopies of host trees in open, exposed situations, as well as in dense forests. Although night time CO₂ concentrations were higher near the epiphytic vines at night, relative to those measured during the day, presumably the result of CO₂ added to the canopy air by the host tree, no evidence for substantial use of this CO₂ was found. In particular, stable carbon isotope ratios of H. carnosa were not substantially lower than those of many other CAM plants, as would be expected if host-respired CO₂ were an important source of CO₂ for these CAM epiphytes. Furthermore, laboratory measurements of diel CO₂ exchange revealed a substantial contribution of daytime CO₂ uptake in these vines, which should also result in lower carbon isotope values than those characteristic of a CAM plant lacking daytime CO₂ uptake. Overall, we found that host-respired CO₂ does not contribute substantially to the carbon budget of this epiphytic CAM plant. This finding does not support the hypothesis that CAM may have evolved in tropical epiphytes in response to diel changes in the CO₂ concentrations within the host tree canopy.     

The influence of elevated CO2 on non-structural carbohydrate distribution and fructan accumulation in wheat canopies

We grew 2.4 m² wheat canopies in a large growth chamber under high photosynthetic photon flux (1000 μmol m⁻² s⁻¹) and using two CO₂ concentrations, 360 and 1200 μmol mol⁻¹. Photosynthetically active radiation (400–700 nm) was attenuated slightly faster through canopies grown in 360μmol mol⁻¹ than through canopies grown in 1200μmol mol⁻¹, even though high-CO₂ canopies attained larger leaf area indices. Tissue fractions were sampled from each 5-cm layer of the canopies. Leaf tissue sampled from the tops of canopies grown in 1200μmol mol⁻¹ accumulated significantly more total non-structural carbohydrate, starch, fructan, sucrose, and glucose (p≤ 0.05) than for canopies grown in 360μmol mol⁻¹. Non-structural carbohydrate did not significantly increase in the lower canopy layers of the elevated CO₂ treatment. Elevated CO₂ induced fructan synthesis in all leaf tissue fractions, but fructan formation was greatest in the uppermost leaf area. A moderate temperature reduction of 10 °C over 5d increased starch, fructan and glucose levels in canopies grown in 1200μmol mol⁻¹, but concentrations of sucrose and fructose decreased slightly or remained unchanged. Those results may correspond with the use of fructosyl-residues and release of glucose when sucrose is consumed in fructan synthesis.     

Scaling CO2-photosynthesis relationships from the leaf to the canopy

Responses of individual leaves to short-term changes in CO₂ partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO₂ are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO₂, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models — which are advocated here as being applicable to the evaluation of impacts of global change on the biosphere — simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO₂ exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO₂ assimilation. Predictions made by the model are consistent with canopy CO₂ exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.Abbreviations LAI leaf area index - NIR near infrared (700–3000 nm) radiation - PAR photosynthetically active (400–700 nm) radiation - PI photosynthetic irradiance (400–700 nm) - PPFD photosynthetic photon flux area density (400–700 nm) - PS I Photosystem I - PS II Photosystem II - Rubisco ribulose-1,5-bisphosphate carboxylase/oxygenase - RuP₂ ribulose-1,5-bisphosphate