Lower Dark Respiration Rates Research Articles

Acclimation of tropical tree species to hurricane disturbance: ontogenetic differences

We investigated acclimation responses of seedlings and saplings of the pioneer species Cecropia schreberiana Miq. and three non-pioneer species, Dacryodes excelsa Vahl, Prestoea acuminata (Willdenow) H.E. Moore var. montana (Graham) Henderson and Galeano, and Sloanea berteriana Choisy ex DC, following a hurricane disturbance in a lower montane wet forest in Puerto Rico. Measurements were made, shortly after passage of the hurricane, on leaves expanded before the hurricane (pre-hurricane leaves) and, at a later time, on recently matured leaves that developed after the hurricane (post-hurricane leaves) from both seedlings and saplings at sites that were severely damaged by the hurricane (disturbed sites) and at sites with little disturbance (undisturbed sites). Pre-hurricane leaves of the non-pioneer species had relatively low light-saturated photosynthetic rates (A(max)) and stomatal conductance (g(s)); neither A(max) nor g(s) responded greatly to the increase in irradiance that resulted from the disturbance, and there were few significant differences between seedlings and saplings. Pre-hurricane leaves of plants at undisturbed sites had low dark respiration rates per unit area (R(d)) and light compensation points (LCP), whereas pre-hurricane leaves of plants at disturbed sites had significantly higher R(d) and LCP. Post-hurricane leaves of plants at disturbed sites had significantly higher A(max) and R(d) than plants at undisturbed sites. Compared with seedlings, saplings had higher A(max) and R(d) and showed greater acclimation to the increase in irradiance that followed the disturbance. Post-hurricane leaves of the non-pioneer species had significantly lower A(max) and were less responsive to changes in irradiance than the pioneer species C. schreberiana. Variation in A(max) across light environments and stages was strongly related to differences in leaf mass per unit area (LMA), especially in the non-pioneer species. As indicated by V(cmax) or J(max) per unit nitrogen, light acclimation of A(max) was determined by leaf morphology (LMA) for the non-pioneer species and by both leaf morphology and leaf biochemistry for C. schreberiana. Ontogenetic changes in A(max) were attributable to changes in leaf morphology. The ontogenetic component of variation in A(max) across light environments and stages differed among species, ranging from 36 to 59% for the non-pioneer species (D. excelsa, 59.3%; P. acuminata var. montana, 44.7%; and S. berteriana, 36.3%) compared with only 17% in the pioneer species C. schreberiana.

The relationship between isoprene emission rate and dark respiration rate in white poplar (Populus alba L.) leaves

In past studies, it was hypothesized that reductions in chloroplast isoprene emissions at high atmospheric CO(2) concentrations were caused by competition between cytosolic and mitochondrial processes for the same substrate, possibly phosphoenolpyruvate (PEP). We conducted field and laboratory experiments using leaves of white poplar (Populus alba L.) to identify whether an inverse relationship occurs between the dark respiration rate (a mitochondrial process) and the isoprene emission rate. Field experiments that were carried out in a free-air CO(2)-enriched (FACE) facility showed no clear effect of elevated CO(2) on either isoprene emission rate or respiration rate by leaves. In young, not yet fully expanded leaves, low isoprene emission and high dark respiration rates were measured in both ambient and elevated CO(2). In these leaves, isoprene emission was inversely correlated with dark respiration. It is possible to interpret from these results that, in young leaves, high rates of growth respiration compete with isoprene biosynthesis for the same substrate. However, it is also possible that the negative correlation reflects the contrasting reductions in growth respiration and increases in expression of the enzyme isoprene synthase at this final stage of leaf maturation. In contrast to our observations on young leaves, respiration rate and isoprene emission rate were positively correlated in older, fully expanded leaves (8 and 11 from apex). A positive correlation was also found between respiration rate and isoprene emission rate when these parameters were modulated using different ozone exposure, growth light intensity, growth temperature and exposure to different leaf temperatures in laboratory experiments. These data show that competition for substrate between isoprene biosynthesis and leaf respiration does not determine the rate of isoprene emission in most circumstances that affect both processes. A negative correlation was observed across all experiments between isoprene emission rate and the activity of phosphoenolpyruvate carboxylase (PEPc), a cytosolic enzyme that competes with isoprene biosynthesis for substrate. The cytosolic metabolite, PEP, occurs at a metabolic branch point from which substrate flows into three processes: (1) the production of pyruvate for mitochondrial respiration, (2) the production of oxaloacetate (OAA) by PEPc for anabolic support of mitochondrial respiration and (3) transport into the chloroplast to support chloroplastic demands for pyruvate, including isoprenoid biosynthesis. The results of our observations suggest that only the second process competes for substrate with isoprenoid synthesis, while the partitioning of PEP between mitochondrial respiration and chloroplast isoprenoid biosynthesis is controlled in a way that retains balance in substrate demand.

Photosynthetic Responses of Plant Communities to Sand Burial on the Machair Dune Systems of the Outer Hebrides, Scotland

The effects of both short-term (2 weeks) and long-term (6 weeks) burial on the photosynthetic efficiency of four typical plant sub-communities of the machair sand dunes of the Outer Hebrides are described. Previous studies have examined the photosynthetic responses on individual species rather than the response at the community level. Three replicate turves from four different sub-community types (foredune grassland, dune slack, three-year fallow and unploughed grassland) were subjected to short- and long-term burial treatments after acclimatisation in an unheated greenhouse for approximately 10 weeks. Three replicate control turves from each sub-community were left unburied. After treatment, photosynthetic rate was measured at 16-20 h and 40-44 h after re-exposure, using an infra-red gas analyser, with standardization by total leaf area for each turf. Effects of sub-community type, burial duration and time since re-exposure were analysed by 3-factor split-plot analysis of variance (ANOVA) with repeated measures for time since re-exposure in the subplots. Buried turves were characterized by a low dark respiration rate, which may represent a maintenance response to burial. After removal of sand, each machair sub-community showed some capacity for an elastic photosynthetic response. There were no differences between the effects of short- and long-term burial on the photosynthetic efficiency of machair vegetation, although turves buried for 6 weeks generally attained photosynthetic rates approaching those of control rates sooner than turves buried for 2 weeks. Photosynthetic responses to burial varied between sub-communities, with the slack turves exhibiting the poorest capacity for recovery within the investigated 44-h period. In the machair environment, the ability to maintain photosynthetic equipment whilst buried, and the ability to bring about a relatively rapid reinstatement of photosynthetic mechanisms on emergence or exposure, is an important adaptation for survival. Survival is closely related to the ability of a plant to replenish carbohydrate reserves before the next burial event.

Leaf and canopy photosynthetic characteristics of cotton ( Gossypium hirsutum) under elevated CO 2 concentration and UV-B radiation

Increases in both atmospheric CO2 concentration ([CO2]) and ultraviolet-B (UV-B) radiation on the Earth's surface are features of current climate change patterns. An experiment was conducted in sunlit, controlled environment chambers known as Soil-Plant-Atmosphere-Research (SPAR) units to determine interactive effects of elevated [CO2] and UV-B radiation on leaf and canopy photosynthetic characteristics of cotton. Six treatments were comprised of two CO2 levels of 360 (ambient) and 720 (elevated) microL L(-1) and three levels of 0 (control), 8, and 16 kJ m(-2) d(-1) biologically effective UV-B radiation. Treatments were imposed for 66 days from crop emergence through three weeks after the first flower stage. Plants grown in elevated [CO2] had significantly greater leaf area, higher leaf and canopy net photosynthetic rates (PN), lower dark respiration rate (Rd), and lower light compensation point (LCP) than plants grown in ambient [CO2]. There was no difference in CO2 compensation point (gamma), maximum rate of Rubisco activity (Vcmax), or light-saturated rate of electron transport (Jmax) between ambient and elevated CO2 treatments. When plants were grown in 8 kJ m(-2) d(-1) UV-B radiation, most of the measured photosynthetic parameters did not differ from control plants. High UV-B (16 kJ) radiation, however, caused 47-50% smaller leaf area, 38-44% lower leaf PN, 72-74% lower Vcmax, and 61-66% lower Jmax compared to the control. There were no interactive effects of [CO2] and UV-B radiation on most of the photosynthetic parameters measured. From the results, it is concluded that decreased canopy photosynthesis due to enhanced UV-B radiation in cotton is associated with both smaller leaf area and lower leaf PN, and loss of Rubisco activity and electron transport are two major factors in UV-B inhibition of leaf PN.

Comparison of Photosynthetic Traits Between Two Typical Shrubs: Legume and Non-Legume in Hunshandak Sandland

In Huanshandak Sandland, China, net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (gs), intercellular CO2 concentration (Ci), water use efficiency (WUE), photochemical efficiency of photosystem 2 (Fv/Fm), and leaf nitrogen content were compared for Hedysarum fruticosum var. mongolicum (H.f.m.), a nitrogen fixing shrub, and Salix gordejevii (S.g.), a nitrogen non-fixing shrub. PN, E, and gs of the two shrubs were similar in trends, i.e. two peaks were observed in diurnal courses. However, except Ci, other parameters of H.f.m. were higher during the measured days than those of S.g. The midday depression of PN was mainly due to decrease in stomata conductance and to reduction of Fv/Fm at midday. The higher PN of H.f.m. was consistent with the higher leaf N content and there was a positive relation between them. In addition, several C4 traits were found in H.f.m., i.e. high saturation irradiance and WUE, low dark respiration rate, and Ci, which partly resulted in higher PN. This seems to indicate that the C3 plant H.f.m. may have C4 photosynthesis pathway or C4 enzymes.

Observations of the Photosynthetic Physiology of Tree Species within the C3 Monocotyledon Genus Pandanus, and Comparison with Dicotyledon C3 Tree Species

Photosynthetic characteristics of tree species from the tropical C3 monocotyledon genus Pandanus were compared with C3 dicotyledon species growing in similar environments. The Pandanus species had similar maximum photosynthetic rates (Amax) to dicotyledon tree species in leaves from both sun and shaded environments when Amax was expressed on an area basis. Because of the low specific leaf area of the schlerophyllous leaves of the Pandanus compared to the dicotyledon species, the similarity in Amax was no longer evident when Amax was expressed on a dry-weight basis. Leaf dark respiration rates of the Pandanus on a leaf area and weight basis were generally lower than the shade-intolerant dicotyledons and similar to the shade-tolerant dicotyledon species. Low dark respiration rates and low specific leaf area of the Pandanus may be important characteristics for growth and survival in environments where resource levels are low and the likelihood of tissue damage is high.

Photosynthetic acclimation and the estimation of temperate ice algal primary production in Saroma-ko Lagoon, Japan

Abstract Temporal changes in the sea ice environment, ice algal biomass and photosynthetic characteristics were studied at Saroma-ko Lagoon in Japan, the area where the southernmost seasonal sea ice in the northern hemisphere occurs. In 1992, the sea ice started to develop in early January and covered the entire lagoon surface in late January, when water temperatures at the center of the lagoon decreased below −1.7°C. High concentrations of ice algae in the bottom layer of the sea ice, where light levels were 0.5–2.8% of the surface irradiance, were visually confirmed in mid-February. The biomass increased in late February to a maximum of 38.25 mg Chl am−2 then suddenly decreased during stormy weather in early March. Afterwards it remained rather constant, with high values of 20–30 mg Chl am−2 until mid-March. Photosynthesis vs. light analysis revealed that ice algae in this lagoon had a low dark respiration rate of 0.024 mg C mg Chl a−1h−1 on average while the increase of photosynthesis at light levels lower than 25 μmol m−2s−1 showed gentle linear increases with increments of light intensity. However, the maximum photosynthetic rate and the efficiency of the photosynthesis at low light levels were rather low compared with values from previous studies in the polar sea ice areas. Nevertheless, in situ estimates of net diel photosynthesis and production, which were calculated with a numerical model using the photosynthetic parameters and hourly averaged light at the ice algal habitat, suggested that large positive values were expected throughout this study. In temperate sea ice areas like Saroma-ko, where there are day/night light cycles, ice algae that have a small net loss of carbon at night due to dark respiration could achieve positive photosynthesis and growth even though they do not show the efficient photosynthesis under low light as shown by polar ice algae.

Adaptation of winter cereal species to shade and competition in a winter/spring cereal forage mixture

Spring planted mixtures of spring and winter cereals maximize dry matter yield and provide fall pasture by regrowth of the winter cereal. However, delay of initial harvest may reduce the winter cereal component and therefore subsequent regrowth yield. Research was conducted at Lacombe, Alberta to investigate the effect of time of initial cut (stage), winter cereal species (species) and cropping system (monocrop and mixture) on winter cereal shoot weight, leaf carbon exchange efficiency and shoot morphology. These parameters may be related to adaptation of winter cereals to growth and survival in the mixture. Winter cereal plants were grown in pails embedded in monocrop plots of fall rye (Secale cereale L.), winter triticale (X Triticosecale Wittmack) and winter wheat (Triticum aestivum L.) and in binary mixtures with Leduc barley (Hordeum vulgare L.). The plants were removed when the barley reached the boot (B), heads emerged (H), H + 2, H + 4 and H + 6 wk stages. Shoot weight was generally smaller in the mixture than in the monocrop and wheat was reduced more than fall rye and triticale in the mixture compared to the monocrop. Dark respiration rate (r = −0.54) and carbon exchange (r = 0.36) under low light intensity were correlated (P < 0.05) to shoot size in the mixture. Fall rye and winter triticale had lower dark respiration rates than winter wheat. Leaf area index (LAI) was closely correlated (r = 0.83 and 0.84) with shoot weight in both the mixture and monocrop. While species failed to exhibit clear cut differences for LAI, fall rye and winter triticale were reduced less than winter wheat in the mixture relative to the monocrop. Stage was the dominant factor affecting winter cereal growth in both cropping systems, but fall rye and triticale exhibited superior morphological features, and their carbon exchange responses to light were more efficient than wheat, which should allow them to be sustained longer under the shaded conditions of a mixture. Key words: Delayed harvest, shade, spring and winter cereal mixtures, adaptation, carbon exchange, respiration

Relation between photosynthesis and dark respiration in cereal leaves

Summary From the light response curves of net photosynthesis at 30 °C, the values of dark respiration, apparent quantum yield and light compensation point were calculated for leaves of Triticum turgidum, Hordeum chilense (H1, H35 and H71 lines), and tritordeum HT31, an amphiploid from H. chilense and T. turgidum . Tritordeum HT31 has an unusually low light compensation point and dark respiration rate; at high irradiance, its rate of O 2 evolution was within the range of a typical sun plant. In the five genotypes, both the light compensation point and the apparent quantum yield were positively correlated with the dark respiration rate. T. turgidum and H. chilense H71 had a larger chlorophyll content than tritordeum HT31. Leaf chlorophyll content per unit fresh mass was correlated to dark respiration rate, apparent quantum yield and light compensation point. These results indicate that in leaves from cereals grown under identical environmental conditions, there is a close association between chlorophyll content, dark respiration rate and apparent quantum yield.

Nitrogen Use Efficiency among Diverse Sorghum Cultivars

Genetic diversity for N use efficiency (NUE) has been demonstrated in sorghum [Sorghum bicolor (L.) Moench] with some of the most efficient types being cultivars that evolved from low‐fertility environments. The objectives of this study were to compare diverse sorghum cultivars for their ability to cope with limited supplies of N. A Nigerian landrace (SC566), an Indian improved line (M35‐1), two U.S. pure lines (CK60 and Martin), and a commercial sorghum hybrid (Dekaib E59 4‐) were compared in solution culture and under two levels of limited N availability in a Nebraska field environment. Dry matter production and NUE (grants of dry matter per gram of N) were determined among all environments. In the field environments, the vertical canopy profile was divided into three layers and repeated observations were made of leaf morphology, internal leaf anatomy, N content, and CO2 gas exchange rates under light saturating and dark conditions. As the nature of N stress changed and increased among environments, SC566 and M35‐1 becmne superior to all U.S. cultivars in NUE. Conversely, U.S. cultivars were among the most responsive in total dry matter production and grain yield as N supply increased. In general, traits related to high NUE included larger canopies comprised of fewer but larger leaves with low N concentration, thicker leaves, larger leaf phloem transsectional area, rapid solubilization and remobilization of N from older to younger leaves, and lower dark respiration rates. These cluwacteristics did not necessarily seem related to the level of domestication or origin of a cultivar, though M35‐1 was found superior in NUE among all environments. Unique and possibly useful traits were found among both landrace and domesticated cultivars.

Photosynthetic and Growth Responses of Variegated Ornamental Species to Elevated CO2

Variegated and completely green cultivars of oleander (Nerium oleander L.) and willow myrtle (Agonis flexuosa (Willd.) Sweet) were grown in controlled environment cabinets for 3 and 5 months, respectively, under either ambient levels of CO2 or with supplementary CO2 to a partial pressure of 800 μbar. Photosynthesis of entirely green leaves and the green portions of variegated leaves on both species was greatly stimulated by high CO2 and there was no evidence of downward adjustment (acclimation) of photosynthetic rates to high CO2 during the experiment. Dark respiration rates of these leaves were lowered by high CO2. The yellow portions of willow myrtle leaves showed a low level of photosynthetic activity which was stimulated by high CO2; however, dark respiration rates showed little response to elevated CO2. Green and yellow areas on variegated leaves of willow myrtle had much lower dark respiration rates than completely green leaves, but this difference was not evident for oleander. Yellow portions of oleander leaves showed little evidence of photosynthetic capacity. This was also confirmed by a low photochemical efficiency as determined by chlorophyll fluorescence. A major effect of variegation was to slow overall plant growth compared with completely green plants. The respective 3-fold and 6-7-fold differences in biomass between fully green and variegated cultivars of oleander and willow myrtle was closely related to estimated net carbon gain per day by the plant canopy. Variegation for both species averaged close to 50:50, green:yellow areas. Variegated plants developed about twice the leaf area ratio and specific leaf area compared with their completely green counterparts. The relative growth response to high CO2 was significantly greater for the variegated plants compared to the completely green plants.

Photosynthetic acclimation in a terrestrial CAM bromeliad, Bromelia humilis Jacq.

Photosynthetic acclimation of a terrestrial CAM bromeliad -Bromelia humilis Jacq. to irradiance and nitrogen supply during growth was investigated under controlled conditions. Considerable acclimation to shade was realized through an increase in total chlorophyll, accessory pigments, an increase in chloroplast size and appressed to non-appressed thylakoid membrane ratio, and a reduction in chlorophyll a/b ratio. A relatively high apparent photon yield in shade was associated with a low light-compensation point and low dark-respiration rate. Light saturation of integrated dark CO2 uptake occurred at a relatively low total daily photosynthetic photon flux density (PPFD) and was not influenced by growth PPFD level. Maximum photosynthetic capacity expressed per unit chlorophyll and nitrogen use efficiency were significantly higher in high-PPFD-grown plants. In addition, apparent photon yield was only slightly reduced in high-PPFD-grown plants with nitrogen, while maximum photosynthetic rate remained the same indicating radiationless dissipation of excess light. Very high levels of recycling of respiratory carbon were observed under nitrogen deficiency in plants grown in both high and low PPFD. The extensive photosynthetic acclimation of B. humilis is discussed in terms of contrasting exposure and limitations of its natural environment.

Comparisons of photosynthesis and photoinhibition in the CAM vine Hoya australis and several C3 vines growing on the coast of eastern Australia

Abstract. The CAM vine Hoya australis and three C3 vines, Smilax australis, Ipomoea pes‐caprae and Kennedta rubicunda, were studied at a site on the coast of northeastern New South Wales, Australia. The level of CAM activity (nocturnal acid accumulation) was comparable in H. australis growing in full sunlight and in deep shade. Acclimation to shade by H. australis was indicated by thinner leaves, increased chlorophyll content, decreased chlorophyll a/b ratios, lower dark respiration rates, and lower light compensation points. When growing in full sunlight H. australis exhibited reductions in photochemical efficiency, as indicated by reduced quantum yields and Fv/Fm fluorescence from PS II as well as low rates of photosynthesis at high light. Sun leaves of H. australis experienced a massive quenching of fluorescence from PS II during normal exposure to midday irradiance which was rapidly reversible under low irradiance conditions in the late afternoon. This quenching indicated a reduction in photochemical efficiency, part of which could be accounted for by an increase in non‐radiative energy dissipation, while part of it was due to one or more processes not yet identified. Changes in PS II fluorescence from shade H. australis exposed to full sunlight suggest a decrease in the rate constant for photochemistry indicative of damage to the reaction centre, as well as an increase in non‐radiative energy dissipation. The C3 vine S. australis was also shade tolerant, but exhibited little evidence of photoinhibition when growing in full sunlight. Ipomoea pescaprae and K. rubicunda, both of which were apparently shade intolerant (being found only in full sunlight), possessed high quantum yields and much higher photosynthetic capacities than either H. australis or S. australis. From this study, and several others, it appears that plants possessing CAM experience photoinhibition to a greater degree than do C3 species in full sunlight under natural conditions, which is probably exacerbated by some degree of CAM‐idling.

Photosynthetic characteristics and chloroplast ultrastructure of C3 and C4 tree species grown in high- and low-light environments

Plants of the C4 tree species, Euphorbia forbesii, Sherff and the C3 tree species, Claoxylon sandwicense Muell-Arg., were grown in a full sun and a shade environment designed to simulate the understory of their native Hawiian forest habitat. When grown under shade conditions, both species exhibited a photosynthetic light response typical of shade plants with low light compensation points and low dark respiration rates. E. forbesii, however, exhibited greater acclimation of light saturated photosynthetic rates and no evidence of photoinhibition in high light. In contrast, quantum yields for CO2 uptake and chlorophyll contents were reduced in the high-light as compared to the low-light grown C. sandwicense plants. Both species exhibited similar changes in the intercellular CO2 response curves and chloroplast whole-chain electron transport capacities, suggesting that the underlying mechanisms of light acclimation are similar. Chloroplasts of E. forbesii exhibited large changes in ultrastructure, with much greater thylakoid membrane development in low than high light. In contrast, C. sandwicense exhibited different starch contents, but otherwise similar membrane development in high and low light. The results show that E. forbesii possesses a very flexible photosynthetic apparatus which may account for its ability to survive in the understory of shaded forests.

Photosynthetic potential of sun and shade Viola species

The photosynthetic responses of a forest floor violet (Viola blanda) and a related meadow species (Viola flmbriatula) grown under controlled conditions were measured to test the prediction that these two species were photosynthetically shade and sun adapted, respectively. Based on their low photosynthetic and dark respiration rates, and low light saturation and compensation points, both violets can be classified as shade-tolerant. The forest species was photosynthetically and morphologically inflexible when grown under high light conditions, which led to chlorosis and greatly decreased photosynthetic performance. Conversely, the meadow species was both photosynthetically and morphologically flexible; its photosynthetic performance allowed it to grow well under both high and low light regimes. As a consequence, morphological flexibility may play a greater role than physiological (i.e., photosynthetic) plasticity in regulating the distribution of these two violets under field conditions.

Photosynthesis and Growth of Bermudagrass Swards. I. Carbon Dioxide Exchange Characteristics of Swards Mowed at Weekly and Monthly Intervals1

Bermudagrass (Cynodon dactylon L. Pers.) is a highly productive subtropical species widely grown in the southern United States for pasture and hay. This work was undertaken to determine the response of CO2 exchange rate (CER) of the canopy to photosynthetic photon flux density (PPFD) and to leaf area index (LAI) as influenced clipping frequency. An experiment was conducted during the growing season on an established sward of ‘Coastal’ bermudagrass. One series of plots was mowed to a height of 6 cm every week and another series every month. Quadrats, 0.25 m2 in size, were hand harvested weekly to ground level to estimate phytomass yields and LAI. Carbon dioxide exchange rates were measured weekly over a wide range of PPFD with a chamber technique. The response of CER of bermudagrass canopies to PPFD followed a rectangular hyperbola with the curves becoming more linear and maximum CER increasing as LAI increased. Rates at near full sunlight (2,000/μmol‐2 sec‐1 PPFD) w ere about 9.5 g m−2 hour−1 for plots with the highest LAI. Dark respiration rates of the canopy plus soil ranged from 1.3 to 1.9 g m−2 hour−1 and were not influenced by LAI. Optimum LAI was 4.7 at 1,600‐2,000 μ m−2 sec−1 PPFD; the critical LAI (95% PPFD interception based on a canopy PPFD extinction coefficient of 0.62) was 4.8. Swards mowed weekly had much lower CER than plots mowed monthly, except just after the monthly mowing when LAI was low and the treatment effect was reversed. For weekly‐mowed plots LAI usually ranged from 1 to 3, whereas on monthly‐mowed plots LAI ranged from near zero to 6.5. Lower CER of swards mowed weekly was attributed mostly to the lower LAI, although at comparable LAI values CER was also lower in weekly mowed plots. Plants in weeklymowed plots had lower dark respiration rates and lower initial slopes of the light response curves than monthly‐mowed plants. These observations confirm the lower productivity often reported for frequently harvested grass swards and indicate the cause to be low LAI.

Photosynthetic responses of C3 and C4 species from cool shaded habitats in Hawaii.

The C4 species, Euphorbia forbesii, and the C3 species, Claoxylon sandwicense, occupy cool, shaded habitats in Hawaii. Both of these species exhibit the photosynthetic characteristics of typical shade plants: low light-saturated photosynthetic rates, low dark respiration rates, low light levels for saturation of photosynthesis, and low light compensation points. In addition, the quantum yields of the two species are similar at leaf temperatures near 22°C, reflecting a significant increase in the quantum yield of E. forbesii over that of C4 species from open habitats. C. sandwicense has a lower dark respiration rate than E. forbesii. Hence, since the quantum yields of the two species are similar at cool temperatures, C. sandwicense has a higher photosynthetic rate than E. forbesii at low incident photon flux densities. As a consequence, C. sandwicense should have a greater carbon gain than E. forbesii under the diffuse radiation conditions of their native habitat. However, since E. forbesii has a higher light-saturated photosynthetic rate than C. sandwicense, E. forbesii may have a greater carbon gain than C. sandwicense during sunflecks.

Seasonal patterns of the CO2 exchange characteristics of understory plants from a deciduous forest

The seasonal patterns of CO2 exchange characteristics were determined for six deciduous forest understory species that differed substantially in time of photosynthetic activity with regard to the time of overstory canopy development. Species that were photosynthetically active only before overstory canopy development were characterized by high maximum light-saturated net photosynthetic rates and high leaf and mesophyll conductances. These early species were, however, active for only a relatively brief period of time. Species that were present during overstory canopy closure initially had moderate light-saturated net CO2 uptake rates that dropped markedly in response to the reduced light intensities after overstory canopy closure. All species present during the summer under the strongly reduced light intensities had low rates of light-saturated net CO2 uptake and low leaf and mesophyll conductances. These species, however, possessed low dark respiration rates which resulted in higher rates of net CO2 uptake at low light intensities than those of the early species. Light-saturated photosynthetic rates were correlated with ribulose-1,5-diphosphate (RuDP) carboxylase activities in mature but not in young expanding leaves.