LEAF GAS EXCHANGE IN QUERCUS MACROCARPA (FAGACEAE): RAPID STOMATAL RESPONSES TO VARIABILITY IN SUNLIGHT IN A TREE GROWTH FORM

Responses in net photosynthesis (A), stomatal conductance to water vapor (g), and leaf xylem pressure potential (ψ) were measured in the deciduous tree Quercus macrocarpa during alternating periods of sun (photosynthetic photon flux, PPF > 1,500 μmol m^-2 sec^-1) and shade (ca. 350 μmol m^-2 sec^-1 simulating cloud cover). Measurements were made on trees growing at the gallery forest-prairie edge on the Konza Prairie Research Natural Area in northeast Kansas. The region is near the westernmost extension of the range of Q. macrocarpa where this species experiences significant seasonal water stress (minimum ψ < -2.9 MPa). Quercus macrocarpa was chosen for study because it has relatively high A (15 μmol m^-2 sec^-1) and g (300 mmol m^-2 sec^-1) in contrast to the deciduous and evergreen subalpine trees previously studied. Both trees and large saplings of Q. macrocarpa responded to alternating several minute periods of sun and shade with relatively rapid changes in A and g. Reductions in g (110 mmol m^-2 sec^-1) during shade periods lowered transpirational water losses (E) by 13% (and reduced A by 5%) relative to estimates of A and E made assuming g remained constant. Partial stomatal closure during shade was correlated with moderate enhancement in ψ (0.31 MPa) in Q. macrocarpa. However, greater increases in ψ were measured in adjacent prairie grasses exposed to similar periods of shade (0.72 MPa in Andropogon gerardii, 0.61 MPa in Sorghastrum nutans). Reduced variability in ψ in tree growth forms may reflect greater buffering of water relations associated with the large size of trees, the amount of tissue devoted to water storage, and differences in hydraulic resistance relative to herbs. Nonetheless, the gas exchange and water relations responses in Q. macrocarpa were much more similar to those previously measured in herbaceous subalpine and grassland species than to those documented for subalpine trees. Thus, rapid gas exchange responses to variable PPF may also occur in tree growth forms.     

CONTRASTING STOMATAL RESPONSES TO VARIABLE SUNLIGHT IN TWO SUBALPINE HERBS

Gas exchange was measured in two subalpine herbs during alternating periods of sun and shade that simulated natural cloud patterns. Stomatal conductance (g) in the subalpine herb Helianthella quinquenervis was reduced by as much as 75% during 5-min shade periods that ranged in photosynthetic photon flux density (PPFD) from 100 to 1,100 μmol m^–2 sec^–1. In contrast, responses in g in another herb, Frasera speciosa, to fluctuations in sunlight were small, apparently due to slower stomatal responses. Based on an earlier hypothesis that water stress may strongly influence nonsteady state gas exchange responses to fluctuations in PPFD, net photosynthesis and g were measured in these herbs as seasonal water stress increased and compared with responses in irrigated plants. Stomatal conductance was relatively unresponsive to changes in PPFD in F. speciosa regardless of water stress. In contrast, substantial decreases in g occurred for H. quinquenervis during shade only when this species experienced water stress later in the season. Little response in g was measured early in the season or in irrigated plants. The seasonal shift in nonsteady state responses in H. quinquenervis from little response in g to shade when plants were nonstressed to more rapid reductions in g as water stress increased would maximize carbon gain early in the season when soil water was abundant while conserving water during periods of soil drought.     

Scaling carbon dioxide and water vapour exchange from leaf to canopy in a deciduous forest. I. Leaf model parametrization

In order to parametrize a leaf submodel of a canopy level gas-exchange model, a series of photosynthesis and stomatal conductance measurements were made on leaves of white oak (Quercus alba L.) and red maple (Acer rubrum L.) in a mature deciduous forest near Oak Ridge, TN. Gas-exchange characteristics of sun leaves growing at the top of a 30 m canopy and of shade leaves growing at a depth of 3–4 m from the top of the canopy were determined. Measured rates of net photosynthesis at a leaf temperature of 30°C and saturating photosynthetic photon flux density, expressed on a leaf area basis, were significantly lower (P = 0.01; n = 8) in shade leaves (7.9μmol m^?2 s^?1) than in sun leaves (11–5μmol m^?2 s^?1). Specific leaf area increased significantly with depth in the canopy, and when photosynthesis rates were expressed on a dry mass basis, they were not significantly different for shade and sun leaves. The percentage leaf nitrogen did not vary significantly with height in the canopy; thus, rates expressed on a per unit nitrogen basis were also not significantly different in shade and sun leaves. A widely used model integrating photosynthesis and stomatal conductance was parametrized independently for sun and shade leaves, enabling us to model successfully diurnal variations in photosynthesis and evapotranspiration of both classes of leaves. Key photosynthesis model parameters were found to scale with leaf nitrogen levels. The leaf model parametrizations were then incorporated into a canopy-scale gas-exchange model that is discussed and tested in a companion paper (Baldocchi & Harley 1995, Plant, Cell and Environment 18, 1157–1173).     

Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom Pseudo‐nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m^?2 · s^?1) and subsaturating (40 μmol photons · m^?2 · s^?1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO₃^?), ammonium (NH₄⁺), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO₃^?‐supported cells being significantly more toxic than both NH₄⁺‐ and urea‐supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO₃^? they exhibited maximum specific uptake rates (V_max) of 22.7, 29.9, 8.98 × 10^?3 · h^?1, half‐saturation constants (K_s) of 1.34, 2.14, 0.28 μg‐at N · L^?1, and affinity values (α) of 17.0, 14.7, 32.5 × 10^?3 · h^?1/(μg‐at N · L^?1) for NO₃^?, NH₄⁺ and urea, respectively. These labo‐ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo‐cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.     

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95nmol mol^?1, 0900 to 1700, 1 h mean) relative to 10 paired, untreated controls (45nmol mol^?1) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m^?2 d^?1) to deep shade (0.7mol m^?2 d^?1). Ozone reduced light-saturated rates of net photosynthesis (A_sat) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A_sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A_sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.     

Stomatal and photosynthetic responses during sun/shade transitions in subalpine plants: influence on water use efficiency

Summary Different response patterns in net photosynthesis (A) leaf conductance (g) and water use efficiency (WUE= a/transpiration) in three subalpine plants occurred during experimental sun/shade transitions that simulated natural cloudcover. In Frasera speciosa Dougl., a large-leaved herb characteristic of open sites, g was relatively insensitive to transitions in irradiance and variations in A. However, large decreases in leaf temperature resulted in reduced transpiration during shade intervals and relatively constant WUE throughout the experimental sun/shade regime. In the understory herb, Arnica cordifolia Hook., patterns of A and g were similar during sun/shade transitions, but WUE was substantially reduced compared to steady-state levels. A third, somewhat intermediate pattern of A, g, and WUE was found in Artemisia tridentata L., an open site shrub. Higher intercellular CO₂ values in A. tridentata suggested that internal, cellular limitations to A were high relative to stomatal limitations in this shrub when compared to the herbaceous species.     

Similar Susceptibility to Excess Irradiance in Sun and Shade Acclimated Saplings of Norway Spruce [Picea Abies (L.) Karst.] and Stone Pine (Pinus Cembra L.)

We compared the responses of sun and shade acclimated saplings of Picea abies and Pinus cembra to excess photosynthetic photon flux density (PPFD) equivalently exceeding the level for saturating net photosynthetic rate (P _N). Exposure for 2 h up to 2000 μmol(photon) m⁻² s⁻¹ did not affect radiant energy saturated P _N. Photoinhibition of photosynthesis was indicated by a small (10 %) reduction of the potential efficiency of photosystem 2 as derived from measurements of chlorophyll fluorescence (F_V/F_M). However, the extent of F_V/F_M reduction and half-time for recovery were similar in sun and shade acclimated saplings of both species. Furthermore, the effect on F_V/F_M was not stronger when the plants were exposed to excess PPFD at 5 °C instead of 15 °C. Frost-hardening of plants increased slightly their resistance to excess PPFD. Establishment of these conifer saplings usually acclimated to shade in their natural habitat may hardly be endangered by a sudden increase of PPFD, e.g., by gap formation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photosynthetic and leaf morphological characteristics in Leucaena leucocephala as affected by growth under different neutral shade levels

Morphological and physiological measurements on individual leaves of Leucaena leucocephala seedlings were used to study acclimation to neutral shading. The light-saturated photosynthetic rate (P_{n max}) ranged from 19.6 to 6.5 mol CO₂ m^–2 s^–1 as photosynthetic photon flux density (PPFD) during growth decreased from 27 to 1.6 mol m^–2 s^–1. Stomatal density varied from 144 mm^–2 in plants grown in high PPFD to 84 mm^–2 in plants grown in low PPFD. Average maximal stomatal conductance for H₂O was 1.1 in plants grown in high PPFD and 0.3 for plants grown in low PPFD. Plants grown in low PPFD had a greater total chlorophyll content than plants grown in high PPFD (7.2 vs 2.9 mg g^–1 on a unit fresh weight basis, and 4.3 vs 3.7 mg dm^–2 on a unit leaf area basis). Leaf area was largest when plants were grown under the intermediate PPFDs. Leaf density thickness was largest when plants were grown under the largest PPFDs. It is concluded that L. leucocephala shows extensive ability to acclimate to neutral shade, and could be considered a facultative shade plant.Abbreviations the initial slope of the photosynthesis vs PPFD curve - P_{n max} the light-saturated photosynthetic rate - PPFD photosynthetic photon flux density     

Susceptibility to photoinhibition of three deciduous broadleaf tree species with different successional traits raised under various light regimes

The susceptibility to photoinhibition of tree species from three different successional stages were examined using chlorophyll fluorescence and gas exchange techniques. The three deciduous broadleaf tree species were Betula platyphylla var. japonica, pioneer and early successional, Quercus mongolica, intermediate shade‐tolerant and mid‐successional, and Acer mono, shade‐tolerant and late successional. Tree seedlings were raised under three light regimes: full sunlight (open), 10% full sun, and 5% full sun. Susceptibility to photoinhibition was assessed on the basis of the recovery kinetics of the ratio of vaviable to maximum fluorescence (F_v/F_m) of detached leaf discs exposed to about 2000 μmol m^?1 s^?1 photon flux density (PFD) for 2 h under controlled conditions (25 to 28 °C, fully hydrated). Differences in susceptibility to photodamage among species were not significant in the open and 10% full sun treatments. But in 5% full sun, B. platyphylla sustained a significantly greater photodamage than other species, probably associated with having the lowest photosynthetic capacity indicated by light‐saturated photosynthetic rate (B. platyphylla, 9·87, 5·85 and 2·82; Q. mongolica, 8·05, 6·28 and 4·41; A. mono, 7·93, 6·11 and 5·08 μmol CO₂ m^?1 s^?1for open, 10% and 5% full sun, respectively). To simulate a gap formation and assess its complex effects including high temperature and water stress in addition to strong light on the susceptibility to photoinhibition, we examined photoinhibition in the field by means of monitoring ΔF/F′_m on the first day of transfer to natural daylight. Compared with ΔF/F′_m in AM, the lower ΔF/F′_m in PM responding to lower PFD following high PFD around noon indicated that photoinhibition occurred in plants grown in 10 and 5% full sun. The diurnal changes of ΔF/F′_m showed that Q. mongolica grown in 5% full sun was less susceptible to photoinhibition than A. mono although they showed little differences both in photosynthetic capacity in intact leaves and susceptibility to photoinhibition based on leaf disc measurements. These results suggest that shade‐grown Q. mongolica had a higher tolerance for additional stresses such as high temperature and water stress in the field, possibly due to their lower plasticity in leaf anatomy to low light environment.     

Net CO2 assimilation of taro and cocoyam as affected by shading and leaf age

Taro and cocoyam were grown outdoors in either full sun or under 40% shade. Leaves were tagged as they emerged and the effect of leaf age on net CO₂ assimilation rate (A) was determined. The effects of shading on A, transpiration (E), stomatal conductance for CO₂ (g_c) and H₂O (g_s), and water use efficiency (WUE) were also determined for leaves of a single age for each species. The effect of leaf age on A was similar for both species. Net CO₂ assimilation rates increased as leaf age increased up to 28 days with the exception of a sharp decline in A for 21 day-old leaves which corresponded to unusually low temperatures during the period of leaf expansion. A generally decreased as leaves aged beyond 28 days. Cocoyam had higher A rates than taro. Leaves of shade-grown plants had higher rates of A and E for both species at photosynthetic photon flux densities (PPFD) up to 1600 mol s^–1 m^–2. Shade-grown leaves of cocoyam had greater leaf dry weights per area (LW/A) and a trend toward higher g_c and g_s than sun-grown leaves. Shade leaves of taro had greater g_c and g₃ rates than sun-grown leaves. The data suggest that taro and cocoyam are highly adapted to moderate shade conditions.     

Seasonal variation in light-shade adaptation of natural populations of the symbiotic sea anemone Aiptasiapulchella (Carlgren, 1943) in Hawaii

The productivity and biomass parameters of the symbiotic anemone Aiptasia pulchella (Carlgren, 1943) from a shaded mangrove lagoon (maximum summer irradiance of 100 μE m⁻² · s⁻¹) and a sunlit reef flat (maximum summer irradiance of 1400 μE · m⁻² · s⁻¹) were examined in Hawaii. Light-shade adaptation was evident in the summer populations (1981) but not observed during the fall (1982). In the summer, zooxanthellae from the lagoon A. pulchella (shade anemones) contained 2.97 pg Chl a cell ⁻¹ and those from the reef flat (sun anemones) contained 1.70 pg Chl a · cell⁻¹; but Chl a : c₂ ratios were 2.5 in zooxanthellae from both shade and sun anemones. During the fall, there were no significant differences in Chl a and c₂ of zooxanthellae (2.25 pg Chl a · cell⁻¹) in shade and sun anemones, but Chl a : c₂ ratios averaged 3.9. During both seasons, shade anemones were larger and contained higher densities of zooxanthellae than sun anemones. In addition to differences between shade and sun habitats, there was localized photoadaptation of zooxanthellae within individual anemones due to microhabitat variations in ambient irradiance. Growth rates of zooxanthellae in A. pulchella differed in shade and sun anemones. Specific growth rates for zooxanthellae in situ were the same for shade populations in both summer and fall (0.016 day⁻¹). However, zooxanthellae in sun anemones grew four times faster in the fall (0.033 day⁻¹) than during the summer (0.008 day⁻¹). These results suggest that growth of zooxanthellae in these anemones was independent of ambient irradiance. Photosynthesis-irradiance (P-I) responses of shade and sun anemones during the summer showed that shade anemones had greater photosynthetic efficiencies (α) but lower photosynthetic capacities (P_max) than sun anemones. Dark-respiration rates of sun anemones were twice those obtained with shade anemones. In the fall, these populations of anemones did not exhibit P-I responses characteristic of light-shade adaptation. Both α and P_max of shade and sun anemones were higher in the fall, indicating that zooxanthellae in A. pulchella adapted to seasonal reduction in irradiance.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Is distribution of hydraulic constraints within tree crowns reflected in photosynthetic water-use efficiency? An example of <Emphasis Type="Italic">Betula pendula</Emphasis>

Spatial and daily variation in photosynthetic water-use efficiency was examined in leaves of Betula pendula Roth with respect to distribution of hydraulic conductance within the crown, morphological properties of stomata, and water availability. Intrinsic water-use efficiency (A _n/g _s) was determined from gas-exchange measurements performed both in situ in a natural forest stand and on detached shoots under laboratory conditions. In intact foliage, sun leaves demonstrated significantly higher (P < 0.001) A _n/g _s than shade leaves, as photosynthesis in the lower canopy was chronically limited by low light availability. However, this difference reversed in the mid-day period under sufficient irradiance (I > 800 μmol m⁻² s⁻¹): A _n/g _s averaged 28.8 and 24.0 μmol mol⁻¹ (P < 0.01) for shade and sun leaves, respectively. This last finding coincided with the data obtained in laboratory conditions: under equivalent leaf water supply and light, A _n/g _s in shade foliage was greater (P < 0.001) than in sun foliage across a wide range of irradiance. Thus, shade foliage of B. pendula is characterized by inherently higher A _n/g _s than sun foliage, associated with more conservative stomatal behavior, and lower soil-to-leaf (K _T) and leaf hydraulic conductances. Under unlimited light conditions, a within-crown trade-off between A _n/g _s and K _T becomes apparent. Differences in stomatal conductance between the detached shoots from sunlit and shaded canopy layers were largely attributable to the variation in stomatal morphology; significant relationships were established with characteristics combining stomatal size and density (relative stomatal surface, stomatal pore area index). Stomatal morphology is very likely involved in long-term adjustment of photosynthetic WUE.     

Chronic photoinhibition in seedlings of tropical trees

Seedlings of five canopy species of tropical trees from Costa Rica and Puerto Rico were grown in full shade (midday range of photosynthetic photon flux density [PPFD], 100–140 μmol m^?2 s^?1), partial shade (midday PPFD, 400–600 μmol m^?2 s^?1) and full sun (midday PPFD, 1 500–1 800 μmol m^?2 s^?1) for 3 months. The species were Ochroma lagopus (Bombacaceae), a pioneer species; Inga edulis (Fabaceae), found in secondary forest; and Dipteryx panamensis (Fabaceae), Hampea appendiculata (Malvaceae), and Manilkara bidentata (Sapotaceae), three species characteristic of primary forest. After the plants were placed in the dark overnight, chlorophyll fluorescence characteristics were measured for recently expanded and mature leaves. The ratio of variable fluorescence to maximum fluorescence (F_v/F_m) was used to estimate the degree of chronic photoinhibition. Only individuals of one species, Dipteryx panamensis, showed significant depression of F_v/F_m after long-term exposure to full sun. The depression was highly correlated with quantum yield of O₂ evolution which also declined after exposure to full sun. The decline may have been related to foliar N concentration. Although all plants were supplied with ample nutrients, foliar N did not increase significantly for Dipteryx seedlings in full sun, whereas it did for Ochroma and Inga. Leaf age affected F_v/F_m only in the cases of Manilkara, where it was slightly lower in recently expanded leaves, and of Dipteryx where it interacted with the effects of light regime. We conclude that chronic photoinhibition is not common in seedlings of canopy trees of tropical rain forests except when availability of mineral nutrients may be limiting.     

GENDER-RELATED DIFFERENCES IN GAS EXCHANGE ARE NOT RELATED TO HOST QUALITY IN THE XYLEM-TAPPING MISTLETOE,PHORADENDRON JUNIPERINUM (VISCACEAE)

Male and female individuals of dioecious species often differ in morphology, physiology, growth, and habitat distribution. Where habitat distribution differences have been demonstrated, female plants generally occupy those habitats with greater resource availability (“rich” habitats). Gender-specific habitat preferences are often presumed to be a consequence of greater resource requirements, per gamete, of female reproduction. Previous work has shown that Phoradendron juniperinum, a xylem-tapping dioecious mistletoe that parasitizes Juniperus species in western North America, displays the opposite pattern: males are relatively more numerous than females in richer sites (i.e., branches with relatively high light and low evaporative demand within the host tree). We report here differences in host (“site”) quality and gas-exchange properties between the sexes. To minimize environmental variation, all measurements were made on sunlit foliage between 9:00 a.m. and 2:00 p.m. Males had significantly higher photosynthetic rates (4.0 [SE = 0.2] μmol m^-2 sec^-1) than either females (2.9 [0.3] μmol m^-2 sec^-1) or nonreproductive individuals (3.0 [0.2] μmol m-² sec^-1). Female photosynthetic rates were not statistically different from those of nonreproductive individuals. No concomitant differences in stomatal conductance were observed. Gas exchange data were independently confirmed by significant differences in carbon isotope ratio (δ¹³C). Gender-related differences were not related to host quality as measured by foliar N, foliar δ¹³C, or water potential of the host tree. The fate of the additional photosynthate in males is unknown, but we discuss the possibility that carbon costs of reproduction in males have been underestimated in past work.     

Growth, biomass allocation and photosynthesis of invasive and native Hawaiian rainforest species

Growth, biomass allocation, and photosynthetic characteristics of seedlings of five invasive non-indigenous and four native species grown under different light regimes were studied to help explain the success of invasive species in Hawaiian rainforests. Plants were grown under three greenhouse light levels representative of those found in the center and edge of gaps and in the understory of Hawaiian rainforests, and under an additional treatment with unaltered shade. Relative growth rates (RGRs) of invasive species grown in sun and partial shade were significantly higher than those for native species, averaging 0.25 and 0.17 g g⁻¹ week⁻¹, respectively, while native species averaged only 0.09 and 0.06 g g⁻¹ week⁻¹, respectively. The RGR of invasive species under the shade treatment was 40% higher than that of native species. Leaf area ratios (LARs) of sun and partial-shade-grown invasive and native species were similar but the LAR of invasive species in the shade was, on average, 20% higher than that of native species. There were no differences between invasive and native species in biomass allocation to shoots and roots, or in leaf mass per area across light environments. Light-saturated photosynthetic rates (Pmax) were higher for invasive species than for native species in all light treatments. Pmax of invasive species grown in the sun treatment, for example, ranged from 5.5 to 11.9 μmol m⁻² s⁻¹ as compared with 3.0−4.5 μmol m⁻² s⁻¹ for native species grown under similar light conditions. The slope of the linear relationship between Pmax and dark respiration was steeper for invasive than for native species, indicating that invasive species assimilate more CO₂ at a lower respiratory cost than native species. These results suggest that the invasive species may have higher growth rates than the native species as a consequence of higher photosynthetic capacities under sun and partial shade, lower dark respiration under all light treatments, and higher LARs when growing under shade conditions. Overall, invasive species appear to be better suited than native species to capturing and utilizing light resources, particularly in high-light environments such as those characterized by relatively high levels of disturbance. Received: 30 December 1997 / Accepted: 1 September 1998     

Carbon uptake and respiration in above-ground parts of a Larix decidua × leptolepis tree

Summary Shade needles of hybrid larch (Larix decidua × leptolepis) had the same rates of photosynthesis as sun needles per dry weight and nitrogen, and a similar leaf conductance under conditions of light saturation at ambient CO₂ (A_max). However, on an area basis, A_max and specific leaf weight were lower in shade than in sun needles. Stomata of sun needles limited CO₂ uptake at light saturation by about 20%, but under natural conditions of light in the shade crown, shade needles operated in a range of saturating internal CO₂ without stomatal limitation of CO₂ uptake. In both needle types, stomata responded similarly to changes in light, but shade needles were more sensitive to changes in vapor pressure deficit than sun needles. Despite a high photosynthetic capacity, the ambient light conditions reduced the mean daily (in summer) and annual carbon gain of shade needles to less than 50% of that in sun needles. In sun needles, the transpiration per carbon gain was about 220 mol mol^–1 on an annual basis. The carbon budget of branches was determined from the photosynthetic rate, the needle biomass and respiration, the latter of which was (per growth and on a carbon basis) 1.6 mol mol^–1 year^–1 in branch and stem wood. In shade branches carbon gains exceeded carbon costs (growth + respiration) by only a factor of 1.6 compared with 3.5 in sun branches. The carbon balance of sun branches was 5 times higher per needle biomass of a branch or 9 times higher on a branch length basis than shade branches. The shade foliage (including the shaded near-stem sun foliage) only contributed approximately 23% to the total annual carbon gain of the tree.     

Chlorophyll Fluorescence and Photosynthetic Gas Exchange Responses to Irradiance of Tree of Heaven (Ailanthus Altissima) in Contrasting Urban Environments

Sun-and shade-adapted plants of Ailanthus altissima utilized thermal-dissipative photoprotection (NPQ) across a range of photosynthetic photon flux densities (PPFD), with higher NPQ and lower maximum quantum yield of photosystem 2 photochemistry (F_v/F_m) in sun-adapted individuals, suggesting increased engagement of antennae-based quenching. Photosynthetic quantum requirements (Q_req; number of photons per CO₂) were similar in sun and shade plants, but were low and comparable to forest understory species. Diurnal measurements showed that PPFDs in both habitats were consistently above photosynthetic compensation irradiance, and frequently exceeded saturating values. In addition, sun- and shade-adapted individuals possessed stomata that tracked short-term fluctuations in PPFD. Thus A. altissima may be unique in that it couples high, shade-plant like photosynthetic efficiency with high photosynthetic capacity in high-irradiance, while stomatal attributes that optimize water use efficiency are maintained in the shade. These features may contribute to success of A. altissima in establishing in disturbance-prone urban systems, and facilitate its spread into more PPFD-limited and competitive natural ecosystems.     

Andreaea Hartmanii Thed.—A histological study

Abstract

Photosynthetic responses to light intensity were studied under laboratory conditions in seven bryophyte species from evergreen laurel forest, a threatened habitat, on Terceira island in the Azores. Four mosses (Andoa berthelotiana, Echinodium prolixum, Fissidens serrulatus, Myurium hochstetteri) and three liverworts (Bazzania azorica, Frullania tamarisci, Lepidozia cupressina) were selected to encompass a range of potential responses to variations in the forest light environment. Carbon dioxide exchange measurements were made, using an infra-red gas-analyser, at photosynthetic photon flux densities (PPFD) of 0-900 µmol m^-2 s^-1 and a mean temperature of 21°C in fully hydrated shoots. Most species achieved light saturation of photosynthesis below 30 µmol m^-2 s^-1, the lowest value being for A. berthelotiana (20 µmol m^-2 s^-1) and the highest for M. hochstetteri (68 µmol m^-2 s^-1). The liverwort F. tamarisci had the highest maximum photosynthetic rate (P_max, 23 µmol CO₂ g^-1 h^-1) whereas P_max was lowest in the mosses E. prolixum and M. hochstetteri (10 µmol CO₂ g^-1 h^-1). Dark respiration rate, a critical factor in toleration of shade by forest floor plants, was highest in the species with the highest values for P_max. Compensation point was extremely low (7 µmol photons m^-2 s^-1) in Fissidens serrulatus, a species found in the deep shade of forest ravines and caves, and highest in M. hochstetteri a moss restricted to better illuminated habitats within and outside the forest. No photoinhibition was detected during the relatively short exposures to high irradiances. Comparison of these responses with data on the forest light environment indicates that, despite the possession of considerable shade adaptations, during winter in the evergreen laurel forest, low light levels may often limit photosynthetic rates of the bryophytes.     

Photosynthetic acclimation to light in juveniles of two cloud forest tree species

The photosynthetic response of juveniles of Decussocarpus rospigliosii, an emergent primary forest species and shade tolerant in its juvenile stages and Alchornea triplinervia, a gap-colonizing species of tropical cloud forest in Venezuela was studied. Daily courses of microenvironmental variables and gas exchange under contrasting light conditions (gap and understory) were carried out in their natural environment and transplanted to different light regimes (shade and sun) in the field. The photosynthetic response and some anatomical characteristics of plants from different treatments were analyzed in the laboratory. Photosynthetic rates were low for both species, and were negative during some diurnal periods, related to the low photosynthetically active radiation levels obtained at both gap (6% of total radiation) and understory (2%). A. triplinervia shows higher rates (1.5–3.0 molm^{-2 -1}) than D. rospigliosii (0.7–1.1 molm^-2s^-1). Both species showed increased photosynthetic rates when grown in gaps. A. triplinervia did not adjust its maximum photosynthetic rates to the prevailing light conditions. In contrast, D. rospigliosii responded to increased light levels. Both species showed low light compensation points when grown under total shade. There was a partial stomatal closure generally during midday in D. rospigliosii. A. triplinervia presented lower leaf conductances, transpiration rates and lesser stomatal control. Some leaf anatomical characteristics, in both species, were affected by variations in the light regime (i.e. increased leaf thickness, leaf specific weight and stomatal density). These results suggest that both species have the ability to respond to variations in their natural light environments, therefore maintaining a favorable carbon balance during the day.