Plant Carbon Gain Research Articles

Gas exchange responses of a desert herbaceous perennial to variable sunlight in contrasting microhabitats

Over the course of a day, aridland plants experience a range of incident photosynthetic photon flux (PPF) spanning three orders of magnitude. Rapid photosynthetic responses to changes in PPF have large effects on individual plant carbon gain and water use patterns, hence are important to plant distribution and success. We investigated the response over time of photosynthesis ( A), stomatal conductance ( g), and inter-cellular CO 2 concentration ( C i) to step changes in PPF in a long-lived aridland perennial that typically occurs in two contrasting microhabitats, shade under shrubs of other species and unshaded inter-shrub spaces. An initial rapid response in A and C i for plants in both microhabitats occurred after abrupt changes in PPF. This was followed by slower changes in these parameters during the rest of the light or dark period. Stomatal conductance responded more gradually to step changes in PPF. The initial increase in A after a dark cycle was significantly greater for plants under shrubs than for plants in inter-shrub spaces, but other changes in A, g and C i did not differ. We attribute the similar responses in plants from different microhabitats to natural variations in solar radiation and limited selection for differentiation due to population gene pools dominated by plants in the open. Our results support the hypothesis that variable light regimes select for photosynthetic gas exchange processes that closely track changes in incident PPF. Our data also support the hypothesis that gas exchange responses to variable light regimes in aridland plants minimize trade-offs between carbon gain and water loss.

Altitudinal differences in flower traits and reproductive allocation

We tested whether alpine plants increase their effort to attract pollinators to compensate for assumed pollinator scarcity at high altitude. A three times larger fraction of the shoot was allocated to flowers in alpine plants (30 species, 2700m asl) compared to lowland plants (20 species, 600m asl), while leaf mass fraction did not differ between the altitudes. At high elevation, a three times smaller fraction of the shoot was allocated to stems, which was accompanied by a change in its function from leaf support for photosynthesis at low altitude to support for flowers at high altitude. Although shoot mass is massively reduced at high altitudes, display area and biomass of individual flowers were remarkably similar at both altitudes. All flowers together attracted pollinators with about the same total display area relative to overall plants size, but generally alpine plants maintain their flowers longer. Together with decreased plant height this leads to an increased self-shading which is likely to cause reductions in carbon gain in alpine plants. The results of this field survey emphasize the importance of outcrossing in alpine plants and its priority over growth

ADVANCES IN ECOLOGICAL STUDIES ON LEAF LIFESPAN AND ASSOCIATED LEAF TRAITS

It is well known that plants with long leaf lifespans are found to grow mostly in habitats where nutrient availability is low and/or water is limited, while plants with short leaf lifespans usually occur on sites with relatively h igh nutrient availability. Considerable research conducted worldwide indicates t hat leaf mass-based nitrogen concentrations (N mass ) are positively rel ated to ma ximum photosynthetic rates, and specific leaf area (leaf area per unit dry weigh t) shows a negative relationship with leaf construction cost, and both decrease with increased leaf lifespan. These relationships among leaf traits generally ex ist across wide ranges of plant populations, communities and biomes, apparently reflecting convergent adaptation of plants to the given climate and/or other env ironmental constraints. Therefore, knowledge about leaf traits is important t o further understand the structure and function of plant ecosystems. With increasing altitude, leaf lifespan generally increases. Associated leaf tra its also show general altitudinal trends: leaf area-based nitrogen concentration s and maximum photosynthetic rates both increase, whereas specific leaf area dec reases with increasing elevation. Plants with long leaf lifespans are thought to be adapted to environmental stresses, such as low temperatures, short growing s e ason length or low light and nutrient availability, while those with short leaf lifespans tend to be associated with fast rates of relative growth and carbon fi xation in response to seasonal stresses of drought and/or cold winters. Under gi ven environmental constraints, the cost-benefit hypothesis explains that variati ons in leaf lifespan are controlled by the trade-offs between leaf carbon costs and benefits for maximizing carbon gain of plants. Cost-benefit simulations nice ly predict a global bimodal distribution of evergreen forests. Accordingly, leaf lifespan and its related leaf traits appear to provide a conceptual link betwee n processes at short-term leaf scales and long-term whole plant and stand-level scales. Knowledge about patterns of leaf lifespan and associated leaf traits acr oss biomes would help us to develop a processing link between biogeography model s and biogeochemistry models and to further understand mechanisms of vegetation dynamics in response to global change. However, there is limited knowledge on ho w leaf characteristics might be used to deduct ecosystem function information and how this scaling task could be accomplished. The traditional focus of plant ecophysiology, understanding how plants cope with often-stressful habitats, is organism centered. It is still difficult to quantitatively describe the relati onships among community characteristics, climatic factors and leaf characteristi cs due to a lack of field data at the ecosystem level. In China, there has been very little work on the ecology of leaf lifespan and its related traits, so it's a challenge and fruitful area for the future research.

Chlorophyll fluorescence of submerged and floating leaves of the aquatic resurrection plant Chamaegigas intrepidus.

The role of submerged and floating leaves in plant photosynthetic performance of the aquatic resurrection plant Chamaegigas intrepidus Dinter was investigated by monitoring chlorophyll fluorescence under the fluctuating natural field conditions that characterise the extreme habitat of this species. The performance of the two different leaf types during desiccation-rehydration cycles in the field was examined. PSII quantum efficiency indicates a similar regeneration capacity in both leaf types after water stress. Electron transport rates under controlled light conditions were 3-4 times higher in floating leaves than in submerged leaves. The two leaf types showed specific adaptations to their ambient photosynthetic photon flux densities (PPFD), shade tolerance in the submerged leaves and adaptation to high PPFD in floating leaves. These results imply a significant role of the floating leaves for total plant carbon gain. It is concluded that the combination of high N content of floating leaves and a high availability of CO2 and light at the water surface contributes to the importance of this leaf type for photosynthesis in C. intrepidus.

Effects of precipitation and soil water potential on drought deciduous phenology in the Kalahari

AbstractWe utilized an ecosystem process model to investigate the influence of precipitation and soil water potential on vegetation phenology in the semi‐arid, drought‐deciduous ecosystems in the Kalahari region of South Africa. The timing of leaf flush was assumed to be the first day during which a rainfall event exceeded that day's estimate of potential evapotranspiration after a defined dry season. Leaf senescence was assumed to be a dynamic feedback between soil water potential and net plant carbon gain and was determined by dynamically modeling the effects of concomitant trends in soil water potential and net primary production on leaf area index (LAI). Model predictions of LAI were compared with satellite‐derived normalized difference vegetation indices (NDVI) for 3 years at two sites along the Kalahari transect. The mean absolute error for the prediction of modeled leaf flush date compared with leaf flush dates estimated from NDVI were 10.0 days for the Maun site and 39.3 days for the Tshane site. Correlations between model predicted 10‐day average LAI and 10‐day composite NDVI for both Maun and Tshane were high (ρ=0.67 and 0.74, respectively, P<0.001), suggesting that this method adequately predicts intra‐annual leaf area dynamics in these dry tropical ecosystems.

Ecosystem UV-B experiments in terrestrial communities: a review of recent findings and methodologies

Ecosystem-level UV-B experiments have revealed a variety of responses in terrestrial communities. These include decreases in the growth of some plant species (usually small changes but occasionally >25%), both decreases and increases in herbivory, occasional altered decomposition patterns, changes in populations of fungi and invertebrates, and morphological changes in the growth patterns of mosses. These field experiments have been pursued both with solar UV-B exclusion using selective filters and with supplementation of UV-B using lamps. For UV-B-exclusion studies it is critical that the UV-B-excluding and UV-B-transmitting filters pass the same amount of photosynthetically active radiation (PAR, 400–700 nm) and also infrared. Canopy photosynthesis models indicate that even small differences in PAR transmittance can, under many circumstances, have significant effects on plant carbon gain. It is also important that filters in UV-B-exclusion studies allow precipitation to pass uniformly to the plots unless there is some form of irrigation underneath the filters. For UV-B-supplementation studies with lamps, many factors are involved in the realism of the ozone depletion simulation. We believe the major lamp-system error is excessive supplementation of UV-B by timer-controlled lamps when weather conditions (primarily clouds) decrease ambient solar UV-B irradiance. The choice of biological spectral weighting functions (BSWF) used in adjusting lamp flux is also critical in determining the level of ozone depletion simulated. In addition, shading by the lamp arrays influences plant growth and becomes particularly important when BSWF with substantial weighting in the UV-A are employed.

Gene Duplication, Neofunctionalization, and the Evolution of C4Photosynthesis

The evolution of C4 photosynthesis provides one of the most interesting examples of evolutionary novelty in plants. As an adaptation that enhances plant carbon gain in warm climates with high light and relatively low atmospheric CO2 concentration, the complex interactions between C4 anatomy and biochemistry appear to have evolved over thirty times independently within the angiosperms. Past theories have explained the multiple appearances of C4 photosynthesis solely on the basis of global decreases in atmospheric CO2 concentration during the past 50 million years. The premise of such theories is that the C4 pathway provides selective advantages in terms of plant carbon gain in an atmosphere of low CO2 concentration. These “carbon balance” theories, however, are limited in their ability to explain why or how C4 photosynthesis evolved so many times independently and why certain patterns in the taxonomic distribution of C4 photosynthesis exist; e.g., the absence of C4 photosynthesis in canopy‐forming forest tree species and the paucity of C4 species within eudicots compared to monocots. In this review, I present the case that one of the most often overlooked aspects of C4 evolution is the potential for genetic limitation, specifically that associated with gene duplication and subsequent modification, which is crucial to the evolution of C4 biochemistry. I describe the research to date that provides insight into the origins of C4 genes, and I derive the conclusion that the evolution of C4 photosynthesis is largely a story of gene duplication while plants are still in the ancestral, C3 state. Once a reservoir of key, duplicated, and preserved C3 genes is present, a small amount of subsequent modification within gene promoter regions is all that is necessary to transform certain C3 patterns of gene expression to C4 patterns. Quantitative theory predicts that the most likely factors to be associated with the accumulation of a reservoir of duplicated C3 genes are large population size, short generation time, and frequent recruitment of sexually produced individuals. When combined with the selective pressures of reduced atmospheric CO2 concentration, consideration of population and life history factors, and the genetic constraints that they impose, could help explain certain patterns of C4 distribution.

Patterns of dynamic irradiance affect the photosynthetic capacity and growth of dipterocarp tree seedlings.

In the deeply shaded understorey of S.E. Asian rain forests the growth and survival of dipterocarp seedlings is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain in understorey plants. To test the sensitivity of photosynthesis and growth to variation in the pattern of dynamic irradiance, dipterocarp tree seedlings (Shorea leprosula and Hopea nervosa) were grown for 370 days under shaded forest light treatments of equal total daily photosynthetic photon flux density (approximately 3.3 mol m(-2) day(-1)), but characterised by either long flecks (LF) or short flecks (SF). Seedling growth was more than 4-fold greater under LF, compared with SF, in both species. Variation in the relative growth rates (RGR) and light saturated rates of photosynthesis (A(max)) were strongly positively correlated with the mean duration of sunflecks. Variation in RGR was strongly correlated with greater unit leaf rate growth, indicating that photosynthetic carbon gain per unit leaf area was greater under LF. The accumulation of starch in leaves over the diurnal period was 117% greater in both species under LF, compared with SF. Greater carbon gain in seedlings under LF is likely to have resulted from the combination of (1) greater A(max) (S. leprosula 35%, H. nervosa 40%), (2) more efficient dynamic photosynthesis, and (3) greater incident photosynthetic quantum yield, compared with seedlings receiving the SF irradiance treatment. The pattern of dynamic irradiance received by seedlings may significantly impact their growth and survival to a previously unrecognised extent, with important consequences for regeneration processes and hence forest structure and composition.

Seasonal and annual variations in the photosynthetic productivity and carbon balance of a central Siberian pine forest

We present a first analysis of data (June 1998 to December 2000) from the long-term eddy covariance site established in a Pinus sylvestris stand near Zotino in central Siberia as part of the EUROSIBERIAN CARBONFLUX project. As well as examining seasonal patterns in net ecosystem exchange ( N E ) , daily, seasonal and annual estimates of the canopy photosynthesis (or gross primary productivity, G P ) were obtained using N E and ecosystem respiration measurements. Although the forest was a small (but significant) source of CO 2 throughout the snow season (typically mid-October to early May) there was a rapid commencement of photosynthetic capacity shortly following the commencement of above-zero air temperatures in spring: in 1999 the forest went from a quiescent state to significant photosynthetic activity in only a few days. Nevertheless, canopy photosynthetic capacity was observed to continue to increase slowly throughout the summer months for both 1999 and 2000, reaching a maximum capacity in early August. During September there was a marked decline in canopy photosynthesis which was only partially attributable to less favourable environmental conditions. This suggests a reduction in canopy photosynthetic capacity in autumn, perhaps associated with the cold hardening process. For individual time periods the canopy photosynthetic rate was mostly dependent upon incoming photon irradiance. However, reductions in both canopy conductance and overall photosynthetic rate in response to high canopy-to-air vapour differences were clearly evident on hot dry days. The relationship between canopy conductance and photosynthesis was examined using Cowan's notion of optimality in which stomata serve to maximise the marginal evaporative cost of plant carbon gain. The associated Lagrangian multiplier (λ) was surprisingly constant throughout the growing season. Somewhat remarkably, however, its value was markedly different between years, being 416 mol mol −1 in 1999 but 815 mol mol −1 in 2000. Overall the forest was a substantial sink for CO 2 in both 1999 and 2000: around 13 mol C m −2 a −1 . Data from this experiment, when combined with estimates of net primary productivity from biomass sampling suggest that about 20% of this sink was associated with increasing plant biomass and about 80% with an increase in the litter and soil organic carbon pools. This high implied rate of carbon accumulation in the litter soil organic matter pool seems unsustainable in the long term and is hard to explain on the basis of current knowledge. DOI: 10.1034/j.1600-0889.2002.01487.x

A simulation study on the importance of size-related changes in leaf morphology and physiology for carbon gain in an epiphytic bromeliad.

This study addresses the question of how size-related changes in leaf morphology and physiology influence light absorption and carbon gain of the epiphytic bromeliad Vriesea sanguinolenta. A geometrically based computer model, Y-plant, was used for the three-dimensional reconstruction of entire plants and for calculation of whole plant light interception and carbon gain. Plants of different sizes were reconstructed, and morphological and physiological attributes of young and old leaves, and small and large plants were combined to examine the individual effects of each factor on light absorption and carbon gain of the plant. The influence of phyllotaxis on light absorption was also explored. Departure of measured divergence angles between successive leaves from the ideal 137.5 degrees slightly decreased light absorption. The only morphological parameter that consistently changed with plant size was leaf shape: larger plants produced more slender foliage, which substantially reduced self-shading. Nevertheless, self-shading increased with plant size. While the maximum rate of net CO(2) uptake of leaves increased linearly with plant size by a factor of two from the smallest to the largest individual, the potential plant carbon gain (based on total foliage area) showed a curvilinear relationship, but with similar numerical variation. We conclude that leaf physiology has a greater impact on plant carbon gain than leaf and plant morphology in this epiphytic bromeliad.

Seasonal and annual variations in the photosynthetic productivity and carbon balance of a central Siberian pine forest

We present a first analysis of data (June 1998 to December 2000) from the long-term eddy covariance site established in a Pinus sylvestris stand near Zotino in central Siberia as part of the EUROSIBERIAN CARBONFLUX project. As well as examining seasonal patterns in net ecosystem exchange (NE), daily, seasonal and annual estimates of the canopy photosynthesis (or gross primary productivity, GP) were obtained using NE and ecosystem respiration measurements.Although the forest was a small (but significant) source of CO2 throughout the snow season (typically mid-October to early May) there was a rapid commencement of photosynthetic capacity shortly following the commencement of above-zero air temperatures in spring: in 1999 the forest went from a quiescent state to significant photosynthetic activity in only a few days. Nevertheless, canopy photosynthetic capacity was observed to continue to increase slowly throughout the summer months for both 1999 and 2000, reaching a maximum capacity in early August. During September there was a marked decline in canopy photosynthesis which was only partially attributable to less favourable environmental conditions. This suggests a reduction in canopy photosynthetic capacity in autumn, perhaps associated with the cold hardening process. For individual time periods the canopy photosynthetic rate was mostly dependent upon incoming photon irradiance. However, reductions in both canopy conductance and overall photosynthetic rate in response to high canopy-to-air vapour differences were clearly evident on hot dry days. The relationship between canopy conductance and photosynthesis was examined using Cowan's notion of optimality in which stomata serve to maximise the marginal evaporative cost of plant carbon gain. The associated Lagrangian multiplier (λ) was surprisingly constant throughout the growing season. Somewhat remarkably, however, its value was markedly different between years, being 416 mol mol−1 in 1999 but 815 mol mol−1 in 2000. Overall the forest was a substantial sink for CO2 in both 1999 and 2000: around 13 mol C m−2 a−1. Data from this experiment, when combined with estimates of net primary productivity from biomass sampling suggest that about 20% of this sink was associated with increasing plant biomass and about 80% with an increase in the litter and soil organic carbon pools. This high implied rate of carbon accumulation in the litter soil organic matter pool seems unsustainable in the long term and is hard to explain on the basis of current knowledge.

Limitations on photosynthesis of competing individuals in stands and the consequences for canopy structure.

The canopy structure of a stand of vegetation is determined by the growth patterns of the individual plants within the stand and the competitive interactions among them. We analyzed the carbon gain of individuals in two dense monospecific stands of Xanthium canadense and evaluated the consequences for intra-specific competition and whole-stand canopy structure. The stands differed in productivity, and this was associated with differences in nitrogen availability. Canopy structure, aboveground mass, and nitrogen contents per unit leaf area (N area) were determined for individuals, and leaf photosynthesis was measured as a function of N area. These data were used to calculate the daily carbon gain of individuals. Within stands, photosynthesis per unit aboveground mass (P mass) of individual plants increased with plant height, despite the lower leaf area ratios of taller plants. The differences in P mass between the tallest most dominant and shortest most subordinate plants were greater in the high-nitrogen than in the low-nitrogen stand. This indicated that competition was asymmetric and that this asymmetry increased with nitrogen availability. In the high-nitrogen stand, taller plants had a higher P mass than shorter ones, because they captured more light per unit mass and because they had higher photosynthesis per unit of absorbed light. Conversely, in the low-nitrogen stand, the differences in P mass between plants of different heights resulted only from differences in their light capture per unit mass. Sensitivity analyses revealed that an increase in N area, keeping leaf area of plants constant, increased whole-plant carbon gain for the taller more dominant plants but reduced carbon gain in the shorter more subordinate ones, which implies that the N area values of shorter plants were greater than the optimal values for maximum photosynthesis. On the other hand, the carbon gain of all individual plants, keeping their total canopy N constant, was positively related to an increase in their individual leaf area. At the same time, however, increasing the leaf area for all plants simultaneously reduced the carbon gain of the whole stand. This result shows that the optimal leaf area index (LAI), which maximizes photosynthesis of a stand, is not evolutionarily stable because at this LAI, any individual can increase its carbon gain by increasing its leaf area.

Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine

AbstractRecent work has shown that stomatal conductance (gs) and assimilation (A) are responsive to changes in the hydraulic conductance of the soil to leaf pathway (KL), but no study has quantitatively described this relationship under controlled conditions where steady‐state flow is promoted. Under steady‐state conditions, the relationship between gs, water potential (Ψ) and KL can be assumed to follow the Ohm's law analogy for fluid flow. When boundary layer conductance is large relative to gs, the Ohm's law analogy leads to gs = KL (Ψsoil−Ψleaf)/D, where D is the vapour pressure deficit. Consequently, if stomata regulate Ψleaf and limit A, a reduction in KL will cause gs and A to decline. We evaluated the regulation of Ψleaf and A in response to changes in KL in well‐watered ponderosa pine seedlings (Pinus ponderosa). To vary KL, we systematically reduced stem hydraulic conductivity (k) using an air injection technique to induce cavitation while simultaneously measuring Ψleaf and canopy gas exchange in the laboratory under constant light and D. Short‐statured seedlings (< 1 m tall) and hour‐long equilibration times promoted steady‐state flow conditions. We found that Ψleaf remained constant near − 1·5 MPa except at the extreme 99% reduction of k when Ψleaf fell to − 2·1 MPa. Transpiration, gs, A and KL all declined with decreasing k (P < 0·001). As a result of the near homeostasis in bulk Ψleaf, gs and A were directly proportional to KL (R2 > 0·90), indicating that changes in KL may affect plant carbon gain.

Whole Plant Carbon Gain of an Endangered Herbaceous Species Aster kantoensis and the Influence of Shading by an Alien Grass Eragrostis curvula in its Gravelly Floodplain Habitat

Aster kantoensis, an endangered plant species, is endemic to gravelly floodplains of a few large rivers in central Japan. In recent years, competitive exclusion by alien perennial grasses in its natural habitat has been suspected to be one of the major factors threatening this species. In the River Kinu, increased shading by the perennial alien grass Eragrostis curvula reduces light availability for A. kantoensis. To reveal the influence of shading on the establishment and growth of A. kantoensis rosettes, the potential carbon gain of A. kantoensis in its natural habitat was estimated using microenvironmental data and whole plant photosynthetic and respiratory responses to light and temperature. Whole plant CO2exchange responses were measured with a specifically designed ‘double chamber’, which enabled measurement of the CO2gas exchange rates of the foliage (F) and the culm (C; stem and roots) separately. It was demonstrated that in a plant with average C/F ratio, positive carbon gain could be maintained only in the microsites where the relative PPFD (photosynthetically active photon flux density) was above 15 or 30% of unshaded conditions in early- or mid-summer, respectively. Increasing C/F ratio, caused by an increase in root biomass as an adaptive response to drought, resulted in a large reduction in the carbon gain irrespective of microsite, weather and season. The high light requirement of A. kantoensis is interpreted as a cost of the morphological responses necessary to avoid stresses characteristic of this gravelly floodplain habitat.

Effects of rhizospheric bicarbonate on net nitrate uptake and partitioning between the main nitrate utilising processes in Populus canescens and Sambucus nigra

Increased levels of rhizospheric dissolved inorganic carbon have repeatedly been demonstrated to enhance plant growth by up to 80%, although carbon from dark fixation accounts for only 1–3% of total plant carbon gain. This study, therefore, aimed at investigating the effects of bicarbonate on nitrate uptake, assimilation and translocation to shoots. Clonal saplings of poplar (Populus canescens(Ait.) Sm.) and elder (Sambucus nigraL.) were grown hydroponically for 35 days in a nutrient solution containing 0, 0.5 and 1 mM bicarbonate and 2 mM nitrate as the sole nitrogen source at pH 7.0. Net nitrate uptake, root nitrate accumulation and reduction, and export of nitrogenous solutes to shoots were measured after incubating plants with 15N-labelled nitrate for 24 h. Net nitrate uptake increased non-significantly in plant species (19–61% compared to control plants) in response to 1 mM bicarbonate. Root nitrate reduction and nitrogen export to shoots increased by 80 and 95% and 15 and 44% in poplar and elder, respectively. With enhanced root zone bicarbonate, both species also exhibited a marked shift between the main nitrate utilising processes. Poplar plants increasingly utilised nitrate via nitrate reduction (73–88% of net nitrate uptake), whereas the proportions of export (20–9%) and storage in roots (7–3%) declined as plants were exposed to 1 mM external bicarbonate. On the other hand, elder plants exhibited a significant increase of root nitrate reduction (44–66%) and root nitrate accumulation (6–25%). Nitrate translocation to elder shoots decreased from 50 to 8% of net nitrate uptake. The improved supply of nitrogen to shoots did not translate into a significant stimulation of growth, relative growth rates increased by only 16% in poplar saplings and by 7% in elder plants.

Phenological and physiological responses of heavily dusted creosote bush (Larrea tridentata) to summer irrigation in the Mojave Desert

In the Mojave Desert, numerous human activities damage soil structure, often resulting in increased wind erosion and deposition of dust on plant leaves. Especially susceptible to dust coating are the resinous, evergreen leaves of the dominant creosote bush ( Larrea tridentata ) wherever these shrubs grow along unpaved roads and trails and at the margins of dry playas. A yearlong field study was conducted, begun during summer drought, to investigate effects of summer irrigation on heavily dusted plants of L. tridentata . Plant water relations, quantitative phenology, and gas exchange were monitored for sets of five plants under three conditions: dusted nonirrigated, dusted irrigated, and undusted nonirrigated (control). Following watering, dusted irrigated plants experienced rapid shoot growth and had significantly higher predawn shoot water potentials (Ψ = −2.5 MPa) than dusted nonirrigated plants (−5.5 MPa), and midday values were −3.1 to −4.1 MPa versus −5.5 to −6.4 MPa, respectively. For dusted plants, irrigation resulted in marked increases of assimilation (14.0 versus 1.24μmol m −2 s −1 ), stomatal conductance (0.19 versus 0.03 mol m −2 s −1 ), and linear growth, and accompanying new growth the old, dusted leaves were mostly abscised. All measures of water-use efficiency (WUE) were higher in controls than in dusted plants, and during the summer, dust was associated with a 40–90% reduction in shoot growth. Nonirrigated controls showed much greater water stress than dusted irrigated plants, but when irrigation was discontinued, the formerly irrigated plants rapidly converged on the physiological and phenological characteristics of not watered control plants. Dust deposition may reduce plant carbon gain by decreasing WUE and impairing tolerance of water stress. This experiment demonstrated that creosote bushes can recover rapidly from acute heavy dust deposition if irrigation, simulating heavy summer rainfall, is provided, and these results may be useful in measures to minimize the ecological impact of human activities in desert ecosystems.

Carbon gain in a multispecies canopy: the role of specific leaf area and photosynthetic nitrogen‐use efficiency in the tragedy of the commons

Models have been formulated for monospecific stands in which canopy photosynthesis is determined by the vertical distribution of leaf area, nitrogen and light. In such stands, resident plants can maximize canopy photosynthesis by distributing their nitrogen parallel to the light gradient, with high contents per unit leaf area at the top of the vegetation and low contents at the bottom. Using principles from game theory, we expanded these models by introducing a second species into the vegetation, with the same vertical distribution of biomass and nitrogen as the resident plants but with the ability to adjust its specific leaf area (SLA, leaf area∶leaf mass). The rule of the game is that invaders replace the resident plants if they have a higher plant carbon gain than those of the resident plants. We showed that such invaders induce major changes in the vegetation. By increasing their SLA, invading plants could increase their light interception as well as their photosynthetic nitrogen‐use efficiency (PNUE, the rate of photosynthesis per unit organic nitrogen). By comparison with stands in which canopy photosynthesis is maximized, those invaded by species of high SLA have the following characteristics: (1) the leaf area index is higher; (2) the vertical distribution of nitrogen is skewed less; (3) as a result of the supra‐optimal leaf area index and the more uniform distribution of nitrogen, total canopy photosynthesis is lower. Thus, in dense canopies we face a classical tragedy of the commons: plants that have a strategy to maximize canopy carbon gain cannot compete with those that maximize their own carbon gain. However, because of this strategy, individual as well as total canopy carbon gain are eventually lower. We showed that it is an evolutionarily stable strategy to increase SLA up to the point where the PNUE of each leaf is maximized.

Optimization theory of stomatal behaviour: I. A critical evaluation of five methods of calculation

There are two principal aims in this first manuscript, first, to compare five methods for calculating the marginal unit water cost of plant carbon gain (E/A) of leaves of two Australian tropical tree species, and second, to test the hypothesis that E/A of tropical tree leaves is constant when leaf-to-air vapour pressure difference (D) varies. Few differences in the absolute values of E/A or the form of the response were found between species. However differences did exist between methods of calculation. Importantly, E/A was rarely constant with changing D. Stomatal limitations of net photosynthesis caused by a reduction in internal carbon dioxide concentration as stomatal conductance declined caused E/A to increase when D was increased. Some methods are applicable to canopy scale field measurements whilst others could only be used in laboratory settings where sufficient control of the environment is possible. The best method of calculation of E/A varied according to the criteria used to judge best. However, overall, despite the large numbers of independent data sets required, Method 1 was judged best for calculating E/A for individual leaves as, although it relies on the largest number of independently derived relationships, it has the fewest assumptions. Methods 2 and 3 were applicable to the field when a number of simplifying assumptions were made.

Optimization theory of stomatal behaviour: II. Stomatal responses of several tree species of north Australia to changes in light, soil and atmospheric water content and temperature

In a companion paper several methods of calculating the marginal unit water cost of plant carbon gain (∂E/∂A) were tested to determine whether stomata were behaving optimally in relation to regulating leaf gas exchange. In this paper one method is applied to several tropical tree species when leaf-to-air vapour pressure difference (D), photosynthetic photon flux density, leaf temperature, and atmospheric soil water availability were manipulated. The response of leaves that had expanded during the dry season were also compared to that of leaves that had expanded in the wet season. Few differences in absolute value of ∂E/∂A, or the form of the relationship, were observed between species or between seasons. In the majority of species, ∂E/∂A increased significantly as either leaf-to-air vapour pressure difference increased, at a leaf temperature of either 33°C or 38°C, or as photosynthetic photon flux density increased. In contrast, as leaf temperature increased at constant D, ∂E/∂A was generally constant. As pre-dawn water potential declined, ∂E/∂A declined, indicating increased efficiency of stomatal behaviour. The relationship between ∂E/∂A and D did not differ whether internal or ambient carbon dioxide concentration were kept constant. It is concluded that stomata are only behaving optimally over a very small range of D. If a larger range of D is used, to incorporate values that more closely reflect those experienced by tropical trees in a savanna environment, optimization is incomplete.

Increased Organic Matter in the Growing Medium Decreases Benlate DF Phytotoxicity.

Some benzimidazole fungicides are phytotoxic to bedding plants. Organic pesticides are bound to the organic matter fraction in the root zone and their availability to plants depends on the composition of the growing medium. Thus, pesticide phytotoxicity may be affected by the fraction of organic matter in the growing medium. We conducted two studies to examine the relationship between benzimidazole phytotoxicity and organic matter content of the growing medium. In the first study, plants were grown in diatomaceous earth, containing no organic matter, and drenched with different fungicides. Benlate DF reduced carbon accumulation (growth) of the plants by 32 and 73% at the 0.5× and 1× label rate, respectively. Carbon gain of plants drenched with either Derosal or 3336 WP was similar to that of the control plants. Both Benlate DF and 3336 WP significantly decreased the number of flowers on the plants. The second study quantified the phytotoxicity of Benlate DF in media containing different amounts of organic matter. The growth of Benlate DF-treated plants was strongly affected by the amount of peat. Net photosynthesis decreased and the severity of visual symptoms (chlorosis) of Benlate DF phytotoxicity increased in media containing less peat. Benlate DF phytotoxicity strongly depends on the amount of organic matter in the growing medium, probably due to sorption of the active ingredient of Benlate DF and/or its breakdown products to the organic matter.