Stomatal versus biochemical limitations to dynamic photosynthetic performance in four tropical rainforest shrub species

Photosynthetic performance under dynamic light regimes was assessed in four different species of tropical shrubs from the family Rubiaceae via field gas exchange measurements conducted on Barro Colorado Island, Panamá. Rates of photosynthetic induction and induction loss were assessed throughout the day in both the wet and dry seasons in order to determine the relative roles of stomata and biochemistry in limiting photosynthetic performance under transient light conditions. A high degree of coordination was observed between stomatal conductance and biochemical capacity for CO₂ assimilation during induction. Rates of biochemical and overall photosynthetic induction sharply decreased when initial stomatal conductance fell below a narrow range of critical values. Time of day or season did not affect rates of biochemical deactivation upon shading, but did influence stomatal closure, which often exerted a significant influence over induction loss in the darkness. In measurements of total assimilation due to a 60-s light pulse, both biochemical activity and stomatal conductance were linearly related to total CO₂ uptake. Only during the mornings of the wet season was stomatal conductance consistently high enough to be non-limiting to dynamic photosynthetic performance. At all other times, stomatal behavior exercised significant influence over induction times, photosynthetic induction loss, and total CO₂ uptake from 60-s light pulses. Received: 17 March 1999 / Accepted: 26 October 1999     

Photosynthetic sunfleck utilization potential of understory saplings growing under elevated CO<Subscript>2</Subscript> in FACE

Few studies have evaluated elevated CO₂ responses of trees in variable light despite its prevalence in forest understories and its potential importance for sapling survival. We studied two shade-tolerant species (Acer rubrum, Cornus florida) and two shade-intolerant species (Liquidambar styraciflua, Liriodendron tulipifera) growing in the understory of a Pinus taeda plantation under ambient and ambient+200 ppm CO₂ in a free air carbon enrichment (FACE) experiment. Photosynthetic and stomatal responses to artificial changes in light intensity were measured on saplings to determine rates of induction gain under saturating light and induction loss under shade. We expected that growth in elevated CO₂ would alter photosynthetic responses to variable light in these understory saplings. The results showed that elevated CO₂ caused the expected enhancement in steady-state photosynthesis in both high and low light, but did not affect overall stomatal conductance or rates of induction gain in the four species. Induction loss after relatively short shade periods (<6 min) was slower in trees grown in elevated CO₂ than in trees grown in ambient CO₂ despite similar decreases in stomatal conductance. As a result leaves grown in elevated CO₂ that maintained induction well in shade had higher carbon gain during subsequent light flecks than was expected from steady-state light response measurements. Thus, when frequent sunflecks maintain stomatal conductance and photosynthetic induction during the day, enhancements of long-term carbon gain by elevated CO₂ could be underestimated by steady-state photosynthetic measures. With respect to species differences, both a tolerant, A. rubrum, and an intolerant species, L. tulipifera, showed rapid induction gain, but A. rubrum also lost induction rapidly (c. 12 min) in shade. These results, as well as those from independent studies in the literature, show that induction dynamics are not closely related to species shade tolerance. Therefore, it cannot be concluded that shade-tolerant species necessarily induce faster in the variable light conditions common in understories. Although our study is the first to examine dynamic photosynthetic responses to variable light in contrasting species in elevated CO₂, studies on ecologically diverse species will be required to establish whether shade-tolerant and -intolerant species show different photosynthetic responses in elevated CO₂ during sunflecks. We conclude that elevated CO₂ affects dynamic gas exchange most strongly via photosynthetic enhancement during induction as well as in the steady state. Received: 1 April 1999 / Accepted: 16 August 1999     

Shade does not ameliorate drought effects on the tree fern species Dicksonia antarctica and Cyathea australis

We examined the responses of two tree fern species (Dicksonia antarctica and Cyathea australis) growing under moderate and high light regimes to short-term water deficit followed by rewatering. Under adequate water supply, morphological and photosynthetic characteristics differed between species. D. antarctica, although putatively the more shade and less drought adapted species, had greater chlorophyll a/b ratio, and greater water use efficiency and less negative δ¹³C. Both species were susceptible to water deficit regardless of the light regime showing significant decreases in photosynthetic parameters (A _max, V _cmax, J _max) and stomatal conductance (g _s ) in conjunction with decreased relative frond water content (RWC) and predawn frond water potential (Ψ_predawn). During the water deficit period, decreases in g _s in both species started one day later, and were at lower soil water content, under moderate light compared with high light. D. antarctica under moderate light was more vulnerable to drought than all other plants as was indicated by greater decreases in Ψ_predawn, lowest stomatal conductance, and photosynthetic rates. Both tree fern species were able to recover after a short but severe water stress.     

Coupled response of stomatal and mesophyll conductance to light enhances photosynthesis of shade leaves under sunflecks

Light gradients within tree canopies play a major role in the distribution of plant resources that define the photosynthetic capacity of sun and shade leaves. However, the biochemical and diffusional constraints on gas exchange in sun and shade leaves in response to light remain poorly quantified, but critical for predicting canopy carbon and water exchange. To investigate the CO₂ diffusion pathway of sun and shade leaves, leaf gas exchange was coupled with concurrent measurements of carbon isotope discrimination to measure net leaf photosynthesis (A_n), stomatal conductance (g_s) and mesophyll conductance (g_m) in Eucalyptus tereticornis trees grown in climate controlled whole‐tree chambers. Compared to sun leaves, shade leaves had lower A_n, g_m, leaf nitrogen and photosynthetic capacity (A_max) but g_s was similar. When light intensity was temporarily increased for shade leaves to match that of sun leaves, both g_s and g_m increased, and A_n increased to values greater than sun leaves. We show that dynamic physiological responses of shade leaves to altered light environments have implications for up‐scaling leaf level measurements and predicting whole canopy carbon gain. Despite exhibiting reduced photosynthetic capacity, the rapid up‐regulation of g_m with increased light enables shade leaves to respond quickly to sunflecks.     

Stomatal dynamics and its importance to carbon gain in two rainforest Piper species

The effects of leaf-air vapor pressure deficit (VPD) on the transient and steady-state stomatal responses to photon flux density (PFD) were evaluated in Piper auritum, a pioneer tree, and Piper aequale, a shade tolerant shrub, that are both native to tropical forests at Los Tuxtlas, Veracruz, México. Under constant high-PFD conditions, the stomata of shade-acclimated plants of both species were sensitive to VPD, exhibiting a nearly uniform decrease in g_s as VPD increased. Acclimation of P. auritum to high light increased the stomatal sensitivity to VPD that was sufflcient to cause a reduction in transpiration at high VPD's. At low PFD, where g_s was already reduced, there was little additional absolute change with VPD for any species or growth condition. The stomatal response to 8-min duration lightflecks was strongly modulated by VPD and varied between the species and growth light conditions. In P. aequale shade plants, increased VPD had no effect on the extent of stomatal opening but caused the rate of closure after the lightfleck to be faster. Thus, the overall response to a lightfleck changed from hysteretic (faster opening than closure) to symmetric (similar opening and closing rates). Either high or low VPD caused g_s not to return to the steady-state value present before the lightfleck. At high VPD the value after was considerably less than the value before whereas at low VPD the opposite occurred. Shade-acclimated plants of P. auritum showed only a small g_s response to lightflecks, which was not affected by VPD. Under sunfleck regimes in the understory, the stomatal response of P. aequale at low VPD may function to enhance carbon gain by increasing the induction state. At high VPD, the shift in the response enhances water use efficiency but at the cost of reduced assimilation.     

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.Abbreviations g_s = stomatal conductance - HL = high light - LL = low light - P_i = intercellular CO₂ partial pressure - PFD = photon flux density     

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.     

Do fruit sunburn control measures affect leaf photosynthetic rate and stomatal conductance in ‘Royal Gala’ apple?

A field study was conducted on a 5-year-old orchard of ‘Royal Gala’ apple (Malus domestica Borkh.) in Stellenbosch, South Africa, to investigate whether the measures employed to control sunburn in fruit, viz., evaporative cooling, Surround WP and 20% black shade net affect leaf photosynthetic gas exchange attributes in comparison to untreated control during the 2003/2004 season. Shade net significantly reduced midday leaf net photosynthetic rate (A) compared to evaporative cooling. Furthermore, shade net and Surround WP significantly reduced midday leaf stomatal conductance (g_s) compared to evaporative cooling and control. Evaporative cooling increased light saturated photosynthetic rate by 27 and 24% compared to shade net and Surround WP, respectively. Light compensation point and dark respiration of shaded leaves were about a third of the other treatments and about 50% less than the control leaves, respectively. Shade net down-regulated photosynthetic capacity of the leaves as evidenced by lower maximum rate of carboxylation and light saturated rate of electron transport compared to control leaves. Sunburn control treatments reduced day respiration by 60–70% compared to the control. Response of A and g_s to increasing temperature showed only slight increase in both A and g_s with increasing temperature from 20 to 30 °C. A declined at 35 °C in Surround WP and shade net leaves while it declined at 40 °C in evaporatively cooled and control leaves. Evaporative cooling and control had higher g_s than shade net and Surround WP at all leaf temperatures. In conclusion, shade net down-regulated photosynthetic reactions and Surround WP and shade net reduced leaf g_s and increased the vulnerability of leaf A and g_s to high temperature compared to evaporative cooling and control.     

Stomatal and nonstomatal limitations of photosynthesis in relation to the drought and shade tolerance of tree species in open and understory environments

 Light saturated photosynthesis (A) in field saplings of shade tolerant, intermediate, and intolerant tree species was analyzed for stomatal and nonstomatal limitations to test differences between species and sun and shade phenotypes during drought. Throughout the study, photosynthesis was highest and mesophyll limitations of A (L_m) lowest in the intolerant species in both open and understory habitats. The shade tolerant species exhibited the only drought-related decreased A and increased L_m in the open, and the greatest drought-related decreased A and increased L_m in the understory. Few species exhibited significant habitat or drought-related differences in stomatal conductance to CO₂ (g_c), but even slight decreases in g_c during drought were associated with large increases in stomatal limitations to A (L_g). Combined changes in L_m and L_g resulted in increased relative stomatal limitation to A (l _g) in several species during drought. Nevertheless, the overall lack of stomatal closure allowed for nonstomatal limitations to play a major role in reduced A during drought. Higher leaf N was associated with shallower slope of the l _g versus g_c relationship, an indication of greater A capacity. Photosynthetic capacity tended to be greater in the intolerant species than the tolerant species, and it tended to decrease during drought primarily in the shade tolerant species in the understory. Findings in the literature suggest that carbon reduction reactions may be more susceptible to drought than photosynthetic light reactions. If so, reduced carbon reduction capacity of shade tolerant species or shade phenotypes may predispose them to drought conditions, which suggests a mechanism behind the well-recognized tradeoff between drought tolerance and shade tolerance of temperate tree species. Received: 20 October 1995 / Accepted: 20 February 1996     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

Photosynthetic characteristics of dipterocarp seedlings in three tropical rain forest light environments: a basis for niche partitioning?

In three tropical rain forest light environments in Sabah, Malaysia, we compared photosynthesis in seedlings of ten climax tree species with putatively differing shade tolerances. The objectives of the study were (a) to characterise the range of photosynthetic responses in ten species of the Dipterocarpaceae and (b) to elucidate those photosynthetic characteristics that might provide a basis for niche partitioning. Seedlings were acclimated (c. 7 months) in three light environments; understorey, partial shade and a gap (140 m²). The light environments represented a gradation in median diurnal (0630–1830 hours) photon flux density (PFD) ranging from understorey (4.7 μmol m⁻² s⁻¹), through partial shade (21.2 μmol m⁻² s⁻¹) to gap (113.7 μmol m⁻² s⁻¹). Integrated diurnal PFD were in the sequence gap > partial shade > understorey (15.2, 4.7, 1.3 mol m⁻² day⁻¹, respectively). In gap-acclimated plants, species differed in the photosynthetic light-response variables apparent quantum yield, dark respiration rate, light compensation point, net saturated leaf assimilation rate (A _sat), and in stomatal conductance (g _s sat) when assimilation rate (A) was saturated. A light-demanding pioneer species (Macaranga hypoleuca) and a shade-demanding understorey species (Begonia sp.) had, respectively, higher and lower A _sat and g _s sat than the dipterocarp species. In high-light conditions A _sat and g _s sat were strongly positively correlated in dipterocarp species. Differing photosynthetic characteristics of gap-acclimated plants suggest that, in these dipterocarp species, different rates of carbon fixation may be an important factor contributing towards niche partitioning. Mean integrated diurnal A (A _diurnal) in the gap, partial shade and understory were, respectively, 122.9, 52.7, 20.5 mmol m⁻² day⁻¹. Differences occurred in A _diurnal of dipterocarp species between light environments. When Macaranga was included, differences in A _diurnal were evident in the gap and partial shade, and in both cases were attributed to the pioneer. For the variable A _diurnal, there was of a shift in the rank position of Macaranga among light environments, but a shift did not occur among the dipterocarp species. Results from this study are consistent with the idea that rates of carbon fixation per unit leaf area may contribute towards niche differentiation between the climax and single pioneer species, but not within the group of climax species. Other physiological and/or carbon allocation factors may be involved in any niche partitioning; dipterocarp species often have inherently different growth rates and susceptibility to herbivory. As an alternative to niche partitioning, dipterocarp species may co-exist in natural light environments as a result of habitat disequilibrium or purely stochastic processes. Received: 2 April 1997 / Accepted: 13 July 1997     

Comparative life history and physiology of two understory Neotropical herbs

Summary Demography and physiology of two broad-leaved understory tropical herbs (Marantaceae) were studied in gaps and shaded understory in large-scale irrigated and control treatments during the dry season at Barro Colorado Island (BCI), Panama. Because photosynthetic acclimation potential may not predict light environments where tropical species are found, we studied a suite of physiological features to determine if they uniquely reflect the distribution of each species. Calathea inocephala and Pleiostachya pruinosa grow and reproduce in gaps, persist in shade, and have equivalent rates of leaf production. Calathea leaves survived 2 to 3 times as long as leaves of Pleiostachya and plants of Pleiostachya were 6 to 8 times more likely to die as plants of Calathea during 3.5 years of study. Pleiostachya had lowest survival in shade and when not irrigated during the dry season, while Calathea survived well in both habitats and both treatments. Pleiostachya had higher photosynthetic capacity and stomatal conductance than Calathea and acclimated to gaps by producing leaves with higher photosynthetic capacity. Calathea had lower mesophyll CO₂ concentrations than Pleiostachya. Both species had similar dark respiration rates and light compensation points, and water-use and nitrogen-use efficiencies were inversely related between species. Species showed no differences in leaf osmotic potentials at full turgor. Calathea roots were deeper and had tuberous swellings.Leaf-level assimilation and potential water loss are consistent with where these species are found, but photosynthetic acclimation to high light does not reflect both species' abilities to grow and reproduce in gaps. Pleiostachya's gap-dependent, rapid growth and reproduction require high rates of carbon gain in short-lived leaves, which can amortize their cost quickly. High rates of water loss are associated with reduced longevity during drought. Calathea's roots may confer greater capacitance, while its leaves are durable, long-lived and have lower water loss, permitting persistence long after gap closure.     

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

In order to separate the net effect of growth at elevated [CO₂] on stomatal conductance (g_s) into direct and acclimatory responses, mid‐day values of g_s were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO₂], which averaged 350 μmol mol^?1 in the daytime, and at ambient + 350 μmol mol^?1[CO₂] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing g_s measured at 700 μmol mol^?1[CO₂] for plants grown at the two [CO₂]. The direct effect of increasing [CO₂] from 350 to 700 μmol mol^?1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with g_s. For all species, growth at the higher [CO₂] significantly reduced g_s measured at 700 μmol mol^?1[CO₂]. The reduction in g_s caused by growth at the higher [CO₂] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO₂] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO₂]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, g_s consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in g_s when averaged over measurement dates. The net effect of growth at elevated [CO₂] on mid‐day g_s resulted from unique combinations of direct and acclimatory responses in the various species.     

Leaf gas exchange responses to abrupt changes in light intensity for two invasive and two non-invasive C4 grass species

Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance to water vapor (g_s)) to marked changes in photosynthetic photon flux density (PPFD) were studied for two invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C₄ grass species from semiarid temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 μmol photon m⁻² s⁻¹, the two invasive species reduced g_s to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE = A/g_s) at low, compared to high-light conditions. For non-invasives, a comparable drop in g_s and A led to invariant PWUE, which was lower than that for the invasive group under low light. The duration and speed of stomatal closure was similar for the four species. However, unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species’ ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for non-invasives. These results indicate more efficient water use under variable light for invasive compared to non-invasive perennial C₄ grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less importance in such environments.     

Leaf gas exchange responses to abrupt changes in light intensity for two invasive and two non-invasive C4 grass species

Transient and steady state responses of leaf gas exchange (photosynthesis (A) and stomatal conductance to water vapor (g_s)) to marked changes in photosynthetic photon flux density (PPFD) were studied for two invasive [Cynodon dactylon (L.) Pers. and Sorghum halepense (L.) Pers.] and two non-invasive, native [Bothriochloa ischaemum (L.) Keng and Chrysopogon gryllus (Torn.) Trin.] perennial C₄ grass species from semiarid temperate grasslands or croplands. Following an abrupt drop in PPFD from 1300 to 270 μmol photon m^?2 s^?1, the two invasive species reduced g_s to a greater extent than A, resulting in higher intrinsic photosynthetic water use efficiency (PWUE = A/g_s) at low, compared to high-light conditions. For non-invasives, a comparable drop in g_s and A led to invariant PWUE, which was lower than that for the invasive group under low light. The duration and speed of stomatal closure was similar for the four species. However, unlike the other grasses, the noxious weed S. halepense exhibited a negligible net loss in PWUE during the high-to-low light transition. Responses of the native B. ischaemum were mostly intermediate between those of the two invasive species and the non-invasive C. gryllus, which is in agreement with the species’ ecological intermediacy: non-invasive but often reaches local dominance following a disturbance. With a sudden reverse change in PPFD photosynthetic light induction was not faster for invasives than for non-invasives. These results indicate more efficient water use under variable light for invasive compared to non-invasive perennial C₄ grasses which may contribute to their success in semiarid temperate habitats with a heterogeneous light regime. Yet, rapid photosynthetic light induction appears to be of less importance in such environments.     

Differential coordination of stomatal conductance,mesophyll conductance,and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s) and mesophyll conductance (g_m) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf) across species, under both steady‐state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf, gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf, and anatomical traits varied widely across species. Under light‐saturated conditions, the A, g_s, g_m, and K_leaf were strongly correlated across species. However, the response patterns of A, g_s, g_m, and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark‐adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light‐adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.     

Limitation of coffee leaf photosynthesis by stomatal conductance and light availability under different shade levels

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A _n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A _n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A _n by low light availability (photon flux density, PFD) and stomatal conductance (g _s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A _n were related to PFD and g _s in order to calculate envelope functions which were used to establish PFD or g _s limitations to A _n. Under the growing conditions of the present trial, mean leaf A _n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A _n due to g _s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A _n by PFD and g _s in response to shade explain the absence of a decrease in coffee leaf A _n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.     

Photosynthetic responses of Microstegium vimineum (Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments

Microstegium vimineum (Trin.) A. Camus, a shade-tolerant C₄ grass, has spread throughout the eastern United States since its introduction in 1919. This species invades disturbed understory habitats along streambanks and surrounding mesic forests, and has become a major pest in areas such as Great Smoky Mountains National Park. The focus of this study was to characterize the photosynthetic induction responses of M. vimineum, specifically its ability to utilize low light and sunflecks, two factors that may be critical to invasive abilities and survival in the understory. In addition, we were curious about the ability of a grass with the C₄ photosynthetic pathway to respond to sunflecks. Plants were grown under 25% and 50% ambient sunlight, and photosynthetic responses to both steady-state and variable light were determined. Plants grown in both 25% and 50% ambient sun became 90% light saturated between 750–850 μmol m⁻² s⁻¹; however, plants grown in 50% ambient sun had significantly higher maximum steady-state photosynthetic rates (16.09 ± 1.37 μmol m⁻² s⁻¹ vs. 12.71 ± 1.18 μmol m⁻² s⁻¹). Both groups of plants induced to 50% of the steady-state rate in 3–5 min, while it took 10–13 min to reach 90% of maximum rates, under both flashing and steady-state light. For both groups of plants, stomatal conductance during induction reached maximum rates in 6–7 min, after which rates decreased slightly. Upon return to low light, rates of induction loss and stomatal closure were very rapid in both groups of plants, but were more rapid in those grown in high light. Rapid induction and the ability to induce under flashing light may enable this species to invade and dominate mesic understory habitats, while rapid induction loss due to stomatal closure may prevent excess water loss when low light constrains photosynthesis. The C₄ pathway itself does not appear to present an insurmountable barrier to the ability of this grass species to respond to sunflecks in an understory environment. Received: 21 February 1997 / Accepted: 10 October 1997     

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.     

Physiological determinants of genotypic differences in carbon isotope discrimination in potato grown in well-watered conditions

Carbon isotope discrimination (A), leaf conductance (g_s), photosynthetic capacity, and plant growth were measured in well-watered, glasshouse-grown potato plants of clones from a cross made between diploid Solanum tuberosum and Solanum vernei. Clones showed significant differences (P < 0.001) in g_s, Δ, stomatal density, root growth, and total dry matter production. Carbon isotope discrimination of genotypes was positively correlated (P < 0.001) with g_s. There was no correlation between g_s and stomatal density indicating that differences in g_s reflected differences in stomatal aperture. Differences in rooting characteristics or in root/shoot ratio did not contribute to differences in g_s or A. Genotypic differences in photosynthetic capacity were not statistically significant, and there was no correlation between A and photosynthetic capacity. Total dry matter production and A were positively correlated (P < 0.001) when differences in the time of plant emergence were included in the regression model. It is concluded that differences in A among potato genotypes is largely determined by g_s, but confounding of g_s and photosynthetic capacity reduces genotypic variation in A compared with that in g_s. Total dry matter production is largely determined by processes other than carbon assimilation rate per unit area in individual leaves. Effective use of A as a character for selection in plant breeding depends on elucidating the effects that differences in stomatal characteristics have on crop production both in well-watered and in water-limited crops.