Patterns Of Photosynthesis Research Articles

Gas Exchange and Water Relations of Lemmon's Willow and Nebraska Sedge

There is considerable interest in riparian zones in the western United States, yet little information is available on the autecology of plant species that dominate these areas. We measured gas exchange and xylem water potential of Nebraska sedge (Carex nebrascensis Dewey) and Lemmon's willow (Salix lemonii Bebb) growing in a streamside location in the northern Sierra Nevada over a 2 year period. Standing biomass of both species and leaf area index of Lemmon's willow was also determined. Rooting activity of Nebraska sedge was measured the second year of the study. Measurements were taken during 1988 and 1989 with growing season precipitation 46% and 110% of average, respectively. Photosynthesis was remarkably similar for the 2 species (10.9 and 11.1 11 micromoles m-2 second-1 for Nebraska sedge and Lemmon's willow, respectively) when averaged over all dates for the 2 years. However, the 2 species exhibited different seasonal and yearly patterns of photosynthesis. Nebraska sedge maintained higher rates of photosynthesis during the early portion of the growing season and Lemmon's willow had higher photosynthesis during mid to late summer. Mean seasonal rates of willow photosynthesis were higher than those of the sedge during the drought year, and the opposite was true during the average year. Yearly average photosynthesis varied more for the sedge than for the willow. However, mean seasonal photosynthesis rates for each species were higher in an average year compared to a drought year. Nebraska sedge almost always had more negative values of xylem water potential than Lemmon's willow (overall average was -2.6 MPa and -1.25 MPa for Nebraska sedge and Lemmon's willow, respectively). Trends in transpiration and conductance were similar among species, except that Nebraska sedge maintained higher rates than Lemmon's willow during the spring of 1989. Willow biomass was similar among years, but willow leaf area index and sedge biomass were slightly greater in the wet year (1989) compared to the dry year. Contrasting growth forms and morphology of the 2 species may help explain differences in gas exchange and xylem water potential. The ability of willows to tap groundwater and the concentration of sedge roots in the upper soil profile probably accounts for the differential response to drought.

Seasonal patterns of light-saturated photosynthesis and leaf conductance for mature and seedling Quercus rubra L. foliage: differential sensitivity to ozone exposure.

Extrapolation of the effects of ozone on seedlings to large trees and forest stands is a common objective of current assessment activities, but few studies have examined whether seedlings are useful surrogates for understanding how mature trees respond to ozone. This two-year study utilized a replicated open-top chamber facility to test the effects of subambient, ambient and twice ambient ozone concentrations on light-saturated net photosynthesis (P(max)) and leaf conductance (g(l)) of leaves from mature trees and genetically related seedlings of northern red oak (Quercus rubra L.). Gas exchange measurements were collected four times during the 1992 and 1993 growing seasons. Both P(max) and g(l) of all foliage followed normal seasonal patterns of ontogeny, but mature tree foliage had greater P(max) and g(l) than seedling foliage at physiological maturity. At the end of the growing season, P(max) and g(l) of the mature tree foliage exposed to ambient ( approximately 80-100 ppm-h) and twice ambient ( approximately 150-190 ppm-h) exposures of ozone were reduced 25 and 50%, respectively, compared with the values for foliage in the subambient ozone treatment ( approximately 35 ppm-h). In seedling leaves, P(max) and g(l) were less affected by ozone exposure than in mature leaves. Extrapolations of the results of seedling exposure studies to foliar responses of mature forests without considering differences in foliar anatomy and stomatal response between juvenile and mature foliage may introduce large errors into projections of the response of mature trees to ozone.

Gap Partitioning among Maples (Acer) in Central New England: Shoot Architecture and Photosynthesis

We measured shoot architecture and photosynthesis by three species of maple (Acer pensylvanicum, A. rubrum, A. saccharum) in response to understory and small canopy gaps in the mixed deciduous forests of central New England. Trees were felled to create six cleared gaps of two sizes (8 ° 12 m, 75 m2; 16 ° 24 m, 300 m2). Seedlings of the three species (2160 total, 720 per species) were transplanted into five plot locations (center plus northwest, northeast, southwest, and southeast gap edges) within all gaps and additional understory sites 1 yr before gap creation. Measurements of microclimates, architecture, photosynthetic performance and seedling survival and growth were made over 1 yr before, and 2 yr following, gap release. Architectural variation increased greatly over the 3—yr period. Striped maple (A. pensylvanicum) and red maple (A. rubrum) increased branch numbers, leaf numbers, and total leaf areas in gaps, especially large gaps, while sugar maple (A. saccharum) showed much smaller changes. Red maple tended to increase the number of leaves while leaf size decreased; striped maple increased leaf number but held leaf size constant. Diurnal patterns of photosynthesis by these species differed within and between gap and understory sites. Red maple showed higher photosynthetic rates per unit leaf area than striped and sugar maple in all site/plot combinations except the large—gap south plots, where striped maple exceeded red maple. Estimated diurnal shoot—level assimilation differentiated species more than unit area assimilation rates and also altered the rank order of performance, with striped maple > red maple > sugar maple in all microsites except the large gap north. Population—level assimilation vs. irradiance response curve exhibited a similar pattern, with red maple dominating unit area rates in most microsites. In contrast, shoot assimilation curves showed striped maple > red maple > sugar maple in all microsites except the large—gap north, where red maple > striped maple. Architectural variation among these species interacted with leaf—level assimilation rates to produce some differences among these species in shoot—level assimilation across the gap—understory microclimatic gradient. Since survival and growth patterns are usually correlated with differences in whole—plant carbon assimilation, our results suggest that there is some photosynthetic potential for gap partitioning among these three species of Acer.

Elevated CO 2: Impact on diurnal patterns of photosynthesis in natural microbial ecosystems

Algae, including blue-green algae (cyanobacteria), are the major source of fixed carbon in many aquatic ecosystems. Previous work has shown that photosynthetic carbon fixation is often enhanced in the presence of additional carbon dioxide (CO 2). This study was undertaken to determine if this CO 2 fertilization effect extended to microbial mats, and, if so, at what times during the day might the addition of CO 2 affect carbon fixation. Four microbial mats from diverse environments were selected, including mats from a hypersaline pond (area 5, Exportadora de Sal, Mexico), the marine intertidal ( Lyngbya , Laguna Ojo de Liebre, Mexico), an acidic hotspring ( Cyanidium , Nymph Creek, Yellowstone National Park), and an acidic stream at ambient temperature ( Zygogonium , Yellowstone National Park). Carbon fixation in the absence of additional CO 2 essentially followed the rising and failing sunlight levels, except that during the middle of the day there was a short dip in carbon fixation rates. The addition of CO 2 profoundly enhanced carbon fixation rates during the daylight hours, including during the midday dip. Therefore, it is unlikely that the midday dip was due to photoinhibition. Surprisingly, enhancement of carbon fixation was often greatest in the early morning or late afternoon, times when carbon fixation would be most likely to be light limited.

Diurnal patterns of leaf photosynthesis, conductance and water potential at the top of a lowland rain forest canopy in Cameroon: measurements from the Radeau des Cimes.

Diurnal patterns of leaf conductance, net photosynthesis and water potential of five tree species were measured at the top of the canopy in a tropical lowland rain forest in southwestern Cameroon. Access to the 40 m canopy was by a large canopy-supported raft, the Radeau des Cimes. The measurements were made under ambient conditions, but the raft altered the local energy balance at times, resulting in elevated leaf temperatures. Leaf water potential was equal to or greater than the gravitational potential at 40 m in the early morning, falling to values as low as -3.0 MPa near midday. Net photosynthesis and conductance were typically highest during midmorning, with values of about 10-12 micro mol CO(2) m(-2) s(-1) and 0.2-0.3 mol H(2)O m(-2) s(-1), respectively. Leaf conductance and net photosynthesis commonly declined through midday with occasional recovery late in the day. Photosynthesis was negatively related to leaf temperature above midday air temperature maxima. These patterns were similar to those observed in other seasonally droughted evergreen communities, such as Mediterranean-climate shrubs, and indicate that environmental factors may cause stomatal closure and limit photosynthesis in tropical rain forests during the midday period.

Canopy structure and vertical patterns of photosynthesis and related leaf traits in a deciduous forest.

Canopy structure and light interception were measured in an 18-m tall, closed canopy deciduous forest of sugar maple (Acer saccharum) in southwestern Wisconsin, USA, and related to leaf structural characteristics, N content, and leaf photosynthetic capacity. Light attenuation in the forest occurred primarily in the upper and middle portions of the canopy. Forest stand leaf area index (LAI) and its distribution with respect to canopy height were estimated from canopy transmittance values independently verified with a combined leaf litterfall and point-intersect method. Leaf mass, N and A max per unit area (LMA, N/area and A max/area, respectively) all decreased continuously by over two-fold from the upper to lower canopy, and these traits were strongly correlated with cumulative leaf area above the leaf position in the canopy. In contrast, neither N concentration nor A max per unit mass varied significantly in relation to the vertical canopy gradient. Since leaf N concentration showed no consistent pattern with respect to canopy position, the observed vertical pattern in N/area is a direct consequence of vertical variation of LMA. N/area and LMA were strongly correlated with A max/area among different canopy positions (r2=0.81 and r2=0.66, respectively), indicating that vertical variation in area-based photosynthetic capacity can also be attributed to variation in LMA. A model of whole-canopy photosynthesis was used to show that observed or hypothetical canopy mass distributions toward higher LMA (and hence higher N/area) in the upper portions of the canopy tended to increase integrated daily canopy photosynthesis over other LMA distribution patterns. Empirical relationships between leaf and canopy-level characteristics may help resolve problems associated with scaling gas exchange measurements made at the leaf level to the individual tree crown and forest canopy-level.

Patterns of Stem Photosynthesis in Two Invasive Legumes (Spartium junceum, Cytisus scoparius) of the California Coastal Region

Stem assimilation of Spartium junceum and Cytisus scoparius was measured over diurnal cycles during four time periods of 1992 in California. Spartium junceum plants were growing at an inland, coastal mountain site while the C. scoparius plants were growing at a coastal bluff habitat. Both species had positive stem assimilation that resulted in approximately 200 mmol m 2 day-1 carbon dioxide accumulation. Daily carbon gain decreased from spring to fall for S. junceum due to a decrease in shoot water potential. Although C. scoparius had the same relationship between assimilation and shoot water potential as that of S. junceum, carbon gain and water potential did not decrease during the year for C. scoparius. The major limitation for carbon gain of C. scoparius was light intensity because the coastal site was characterized by lower temperature, higher humidity, and more fog than the inland site. Although these species grew in habitats that had a different vapor pressure and temperature, the assimilation response to vapor pressure was similar between the species. Water use efficiency was higher and intercellular carbon dioxide was lower for S. junceum compared to C. scoparius. Although the leaves of both species are ephemeral, the canopy has a constant positive carbon balance because of stem assimilation. The constant carbon gain throughout the year, from stem assimilation, may enhance the growth capacity of these invasive species in disturbed habitats.

Seasonal Patterns of Photosynthesis in Alpine and Subalpine Populations of the Lichen Nephroma arcticum

Photosynthetic characteristics of an alpine and a subalpine population of Nephroma arcticum, assumed to represent different ecotypes, were measured to investigate possible adaptations to their respective environments. The light compensation levels in the alpine population were consistently higher than those in the subalpine ones, and the chlorophyll contents of the former were, on average, about half those of the latter, reflecting the differences in light regimes between sites. In both populations, altitude accounted for a significant part of the variation in photosynthesis at suboptimal thallus water contents assumed to correspond to levels resulting from normal in-situ hydration (...)

Internal and external control of net photosynthesis and stomatal conductance of mature eastern white pine (Pinusstrobus)

Leaf gas exchange and water relations were monitored in the upper canopy of two 25 m tall eastern white pine (Pinusstrobus L.) trees over two consecutive growing seasons (1986 and 1987). Examination of the seasonal and diurnal patterns of net photosynthesis and leaf conductance showed that both internal and external (environmental) factors were controlling net photosynthesis and leaf conductance. Internal control was indicated by a rapid increase and then decrease in the photosynthetic capacity of 1-year-old foliage during the development and maturation of current-year foliage, which was independent of environmental conditions. Large differences in net photosynthesis were observed between growing seasons due to seasonal differences in soil water availability, as indexed by predawn xylem pressure potential. Water stress reduced the maximum rate of net photosynthesis and altered the response of net photosynthesis and leaf conductance to absolute humidity deficit.

PHOTOBIO: Modeling the Stomatal and Biochemical Control of Plant Gas Exchange

Simulation models are increasingly being used to describe physiological processes in the plant sciences. These models, while useful for research purposes, also offer tremendous potential for demonstrating a wide array of scientific concepts to students. We have developed an educational software package that illustrates the stomatal and biochemical control of transpiration and photosynthesis. The simulation program uses a biochemical model of C assimilation that, when coupled to an empirical submodel describing stomatal conductance, can be solved iteratively for leaf photosynthesis, stomatal conductance, and transpiration. Graphic and tabular presentations, combined with on-screen requests for student input, serve to effectively convey the basic fundamentals of plant gas-exchange, and the diurnal patterns of photosynthesis and transpiration in response to fluctuating environmental conditions. More advanced topics focus on the biochemical limitations imposed on photosynthesis by Rubisco activity, electron transport capacity, and the regeneration of inorganic P. Also included is an exercise that challenges students to use the lessons learned to optimize C assimilation, while minimizing water losses, over a 3-d simulation period. Application of the program can assist instructors in illustrating important concepts regarding stomatal and biochemical control of plant gas-exchange.

Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.)

Continuous monitoring of steady-state carbon dioxide exchange rates in mature muskmelon (Cucumis melo L.) leaves showed diurnal patterns of photosynthesis and respiration that were translated into distinct patterns of accumulation and phloem export of soluble sugars and amino acids. Leaf soluble sugar patterns in general followed the pattern of photosynthetic activity observed in the leaf, whereas starch accumulated steadily throughout the light period. Sugar and starch levels declined through the dark phase. Phloem exudate analysis revealed that diurnal levels of the major transport sugars (stachyose and sucrose) in the phloem did not appear to correlate directly with the photosynthetic activity of the leaf but instead were inversely correlated with leaf starch accumulation and degradation. The amino acid pool in leaf tissues remained constant throughout the diurnal period; however, the relative contribution of individual amino acids to the total pool varied with the diurnal photosynthetic and respiratory activity of the leaf. In contrast, the phloem sap amino acid pool size was substantially larger in the light than in the dark, a result primarily due to enhanced export of glutamine, glutamate, and citrulline during the light period. The results indicate that the sugar and amino acid composition of cucurbit phloem sap is not constant but varies throughout the diurnal cycle in response to the metabolic activities of the source leaf.

Growth of cotton under continuous salinity stress: influence on allocation pattern, stomatal and non-stomatal components of photosynthesis and dissipation of excess light energy

Cotton (Gossypium hirsutum L.) plants were grown in flowing-culture solutions containing 0%, 26% and 55% natural seawater under controlled and otherwise identical conditions. Leaf Na(+) content rose to 360 mM in 55% seawater, yet the K(+) content was maintained above 100 mM. The K(+)/Na(+) selectivity ratio was much greater in the saline plants than in the control plants. All plants were healthy and able to complete the life cycle but relative growth rate fell by 46% in 26% seawater and by 83% in 55% seawater. Much of this reduction in growth was caused by a decreased allocation of carbon to leaf growth versus root growth. The ratio of leaf area/plant dry weight fell by 32% in 26% seawater and by 50% in 55 % seawater while the rate of carbon gain per unit leaf area fell by only 20% in 26% seawater and by as much as 66% in 55% seawater. Partial stomatal closure accounted for nearly all of the fall in the photosynthesis rate in 26% seawater but in 55% seawater much of the fall also can be attributed to non-stomatal factors. As a result of the greater effect of salinity on stomatal conductance than on CO2-uptake rate, photosynthetic water-use efficiency was markedly improved by salinity. This was also confirmed by stablecarbon-isotope analyses of leaf sugar and of leaf cellulose and starch. - Although non-stomatal photosynthetic capacity at the growth light was reduced by as much as 42% in 55% seawater, no effects were detected on the intrinsic photon yield of photosynthesis nor on the efficiency of photosystem II photochemistry, chlorophyll a/b ratio, carotenoid composition or the operation of the xanthophyll cycle. Whereas salinity caused in increase in mesophyll thickness and content of chloroplast pigments it caused a decrease in total leaf nitrogen content. The results indicate that the salinity-induced reduction in non-stomatal photosynthetic capacity was not caused by any detrimental effect on the photosynthetic apparatus but reflects a decreased allocation to enzymes of carbon fixation. - Rates of energy dissipation via CO2 fixation and photorespiration, calculated from gas-exchange measurements, were insufficient to balance the rate of light-energy absorption at the growth light. Salinity therefore would have been expected to cause the excess excitation energy to rise, leading to an increased nonradiative dissipation in the pigment bed and resulting increases in non-photochemical fluorescence quenching and zeaxanthin formation. However, no such changes could be detected, implying that salinity may have increased energy dissipation via a yet unidentified energy-consuming process. This lack of a response to salinity stress is in contrast to the responses elicited by short-term water stress which caused strong non-photochemical quenching and massive zeaxanthin formation.

Gas exchange response of western hemlock seedlings from various dormancy-induction treatments to reforestation site environmental conditions

Western hemlock ( Tsuga heterophylla (Raf.) Sarg.) seedlings from four dormancy-induction treatments (i.e. long-day dry, long-day wet, short-day dry, short-day wet) had needle conductance ( g wv) and net photosynthesis ( P n) patterns monitored over the first field growing season. The objective was to compare stocktype response with environmental conditions on a coastal reforestation site in British Columbia. Both P n and g wv response to photosynthetically active radiation (PAR) and vapor pressure deficit (VPD) were determined using boundary line analysis. This defined the stocktypes' maximum gas exchange response (i.e. P nMAX. and g wvMAX.) to field site environmental conditions. Short-day wet and long-day dry seedlings had greater P nMAX. with increasing PAR after frost. Short-day seedlings had greater P nMAX. with increasing PAR during summer conditions compared with long-day seedlings. The g wvMAX. response to PAR was suppressed after frost in all seedlings. Short-day seedlings had greater g wvMAX. response to PAR from March through June. All treatments showed a linear decrease in P nMAX. with increasing VPD, though short-day seedlings had the greatest P nMAX. with increasing VPD. One-year-old needles from seedlings in all treatments had very little change in g wvMAX. with increasing VPD. For current-year needles, all treatments had very little change in g wvMAX. as VPD increased, though short-day seedlings had the greatest g wvMAX.. Both P n and g wv decreased at lower shoot water potentials in all treatments, but between −0.5 and −1.0 MPa short-day seedlings had greater P n and g wv. At g wv measurements greater than 1.0 mm s −1, short-day wet seedlings had the least reduction in non-stomatal P n limitations implying a greater water-use efficiency.

Effect of Water Stress on Photosynthesis, Leaf Characteristics and Productivity of Field-GrownNicotiana tabacumL. Genotypes Selected for Survival at Low CO2

Dry mass production, leaf characteristics and diurnal photosynthesis of two N. tabacum L. genotypes selected for survival at low CO2 and the parent Wisconsin-38 (control plants) were measured on water-stressed and well-watered plants in the field. Differences in photosynthesis per unit leaf area were small and not significant between genotypes, but different patterns of photosynthesis were observed in stressed and non-stressed plants, with water stress reducing total net carbon fixation by 45% in all genotypes. More dry mass was produced by the selected genotypes than by Wisconsin-38 under irrigation. Production was smaller and the same for all three genotypes when stressed. The increased dry mass of the selected genotypes was related to greater total leaf area per plant which was accompanied by more cells per unit leaf area but smaller cell volume than in the control plants. The decrease in dry mass production under water stress was related to a decrease in total leaf area per plant and a decrease in cell number per unit leaf area; however, cell volume increased. © 1992 Oxford University Press.

An automated CO 2 control system for plant growth chambers and its use to estimate net plant CO 2 exchanges

Control of CO2 concentration within growth chambers is necessary to compensate photosynthetic CO2 depletion or to study the effects of elevated CO2 concentrations on plant physiology. A microcomputer-driven CO2 control system was developed to maintain independently, in four growth chambers, a CO2 concentration between 50 and 10 000 μmol mol−1. Air samples were taken sequentially every 12 min in each chamber and their CO2 concentrations measured by an infrared gas analyser. These measurements allow for the calculation of photosynthetic rate. Pure CO2 was delivered at the rate of its assimilation by plants by injecting a small, constant quantity at a frequency modulated by the previously calculated photosynthetic rates. The continually updated daily patterns of photosynthesis could be displayed on the computer screen. Examples of daily patterns of CO2 regulation and photosynthetic rates are given together with photosynthetic response of water hyacinth (Eichhornia crassipes) canopies to elevated CO2 levels. The latter example advocates the use of growth chambers equipped with process control systems for direct experiment rather than only for plant conditioning.

Soil temperature limitations on gas exchange in 1‐year‐old Pinus sylvestris (L) seedlings

Effects of soil temperature on gas exchange of Scots pine seedlings were studied to evaluate the significance of reduced gas exchange in seedlings planted in cold soils. The patterns of net photosynthesis during the 3‐week period at the two constant soil temperatures (8°C and 12°C) were quite similar but at 12°C the photosynthetic rate was higher. After U days differences were no more significant. Photosynthesis at the increasing soil temperature, from 5.5°C to 13°C, decreased for the first 18 days and then recovered up to the level of other treatments. The same patterns were found for transpiration, stomatal conductance, and photosynthetic efficiency. Xylem pressure potentials and relative resistance to water flow after 3 weeks did not differ among soil temperatures. Initiation and development of current‐year needles affected all the results of gas exchange parameters.

Seasonal Patterns of Photosynthesis and Physiological Parameters and the Effects of Emersion in Littoral Seaweeds

Seasonal Patterns of Photosynthesis and Physiological Parameters and the Effects of Emersion in Littoral Seaweeds

Low humidity can cause uneven photosynthesis in olive (Olea europea L.) leaves.

We examined the photosynthetic responses of olive (Olea europea L.) leaves exposed to either (a) two hours of high leaf-to-air vapor pressure difference (vpd) or (b) four 30-min cycles of high vpd separated by 15-min periods of recovery at low vpd. Neither treatment affected photosynthesis when vpd was less than 3.0 kPa. Photosynthesis by mature leaves was also insensitive to higher vpd, but photosynthesis of young leaves was reduced by both treatments at a vpd higher than 3.2 kPa. This effect of vpd was much smaller under high intercellular CO(2) pressure. Autoradiograms showed that under a vpd of 3.2 kPa, mature leaves photosynthesized uniformly, but patches of reduced CO(2) fixation occurred in the distal part of young leaves. We conclude that heterogeneities in photosynthesis along the length of the leaf caused the apparent reduction of photosynthesis in our experiments. This pattern of patchy photosynthesis was different from that observed in mesophytic herbs, but the effect on gas exchange analysis was the same. In this case, apparent biochemical effects of low humidity on photosynthesis of young olive leaves are likely an artifact.

SEASONAL PRIMARY PRODUCTION OF UNDERSTORY RHODOPHYTA IN AN UPWELLING SYSTEM1

ABSTRACTThe in situ primary production of three common under‐story members of the Rhodophyta in South African west coast kelp beds was determined monthly for a year using dissolved oxygen techniques. Strong seasonal patterns of photosynthesis and respiration were evident in all three species. Net photosynthesis of all three species was greatest in spring (October) and lowest in winter (June). Increasing photosynthesis in late winter coincided with increasing ambient irradiance and photoperiod, whereas decreasing photosynthesis in summer was not explained by changes in the environmental parameters measured. We suggest that this may he due to an innate pattern related to some other seasonal plant activity such as reproduction. Seasonal Pmax and Ik values reveal that the obligate understory species, B. prolifera and E. obtusa, are shade‐adapted whereas G. radula, a low intertidal and shallow subtidal dominant, is sun‐adapted. Low C: X ratios consistent with a high nutrient environment and high rates of productivity were found in all three species. Net photosynthesis to respiration (Pn:R) ratios were fairly constant for B. prolifera and E. obtusa, implying that then photosynthetic processes were governed more by seasonal variations in irradiance than by instantaneous light availability. The Pn: R ratio of G. radula was variable, suggesting that this species is more responsive to rapid fluctuations in irradiance and may therefore be adapted for rapid growth during periods of high irradiance.

Photosynthesis, Photorespiration, and Organic Carbon Release inNymphaea tuberosaPaine andNuphar variegatumEngelm

ABSTRACT Within two sympatric populations of Nymphaea tuberosa and Nuphar variegatum in a productive hardwater lake, both species exhibited similar seasonal patterns of photosynthesis, but the rates were much higher in Nymphaea than in Nuphar. Rates of photorespiration and organic carbon release were very low in both species during most of the growth season and were not a factor in the difference in photosynthetic rates. Both species were growing under optimal conditions, and the superior photosynthetic capacity of Nymphaea appeared to be associated with phenological differences permitting this species to compete successfully (i.e. coexist) with the earlier emerging, broader leaved Nuphar species.