Decline of photosynthetic capacity with leaf age and position in two tropical pioneer tree species

The effect of leaf age on photosynthetic capacity, a critical parameter in the theory of optimal leaf longevity, was studied for two tropical pioneer tree species, Cecropia longipes and Urera caracasana, in a seasonally dry forest in Panama. These species continuously produce short-lived leaves (74 and 93 d, respectively) during the rainy season (May-December) on orthotropic branches. However, they differ in leaf production rate, maximum number of leaves per branch, light environment experienced by the leaves, leaf mass per unit area, and nitrogen content. Light-saturated photosynthetic rates for marked leaves of known ages (±1 wk) were measured with two contrasting schemes (repeated measurements vs. chronosequence within branch), which overall produced similar results. In both species, photosynthetic rates and nitrogen use efficiency were negatively correlated with leaf age and positively correlated with light availability. Photosynthetic rates declined faster with leaf age in Cecropia than in Urera as predicted by the theory. The rate of decline was faster for leaves on branches with faster leaf turnover rates. Nitrogen per unit leaf area decreased with leaf age only for Urera. Leaf mass per unit area increased with leaf age, either partly (in Cecropia) or entirely (in Urera) due to ash accumulation.     

Photosynthetic light acclimation in two rainforest Piper species with different ecological amplitudes

Summary Piper auritum (H.B. & K.), a pioneer tree restricted to open sites and Piper hispidum (Swartz), a shrub common in sites ranging from recent clearings to shaded understory, both adjust photosynthetic characteristics in response to light availability during growth. The sensitivity of photosynthetic capacity to light availability during growth was indistinguishable for the two species growing in their natural habitat. Photosynthetic capacity was strongly correlated with leaf nitrogen in both species, and the relationship was similar between species. Dark respiration and leaf specific mass were more sensitive to light during growth in P. hispidum, the species with the broad habitat ange, than in P. auritum. In general, similarities between the species were more striking than differences between them. The differences in dark respiration could have important implications for carbon balance. The difference in the responsiveness of leaf specific mass to light indicates that the broad-ranging species maintains access to modes of response little utilized by the open-site specialist. We did not and, in the gas exchange characteristics, any evidence that the open site specialist is better suited than the generalist to high-light sites.This CIW DPB publication number 962     

Light gap detection by the photoblastic seeds ofCecropia obtusifolia andPiper auritum,two tropical rain forest trees

The photoblastic seeds of two pioneer trees from the tropical rain forest-Cecropia obtusifolia andPiper auritum — require long diurnal periods of red light to germinate. In the field, the establishment of the trees takes place only in light gaps produced by falling trees. Experiments were performed in order to ascertain the relationship between the light requirements of the seeds and the germination control in the field. Results show that phytochrome regulated germination allows the detection of light gaps.     

Competition and patterns of resource use among seedlings of five tropical trees grown at ambient and elevated CO2

Summary Seedlings of five tropical trees, Cecropia obtusifolia, Myriocarpa longipes, Piper auritum, Senna multijuga and Trichospermum mexicanum, were grown both as individuals, and in competition with each other at ambient (350) and two levels of elevated CO₂ (525 and 700 l l^-1) for a period of 111 days. Growth, allocation, canopy architecture, mid-day leaf water potential and soil moisture content were assessed three times over this period for individually grown plants, and at the end of the experiment for competitively grown plants. In addition, leaf photosynthesis and conductance were assessed for the individually grown plants midway through the experiment, and light profile curves were determined for the competitive arrays at three stages of development. Elevated CO₂ did not affect photosynthesis or overall growth of the individually-grown plants but did affect canopy architecture; mean canopy height increased with CO₂ in Piper and Trichospermum and decreased in Senna. Stomatal conductance decreased slightly as CO₂ increased from 350 to 525 l l^-1 but this had no significant effect upon whole plant water use of leaf water potential. Soil moisture content for the individuals increased marginally as CO₂ increased, but this did not occur in the competitive arrays. There was a marked effect of CO₂ upon species composition of the competitive arrays; Senna decreased in importance as CO₂ increased while Cecropia, Trichospermum and Piper increased in importance. Stepwise regression analysis using competitive performance as the independent variable, and the various morphological and physiological parameters measured on the individually grown plants as independent variables, suggested that canopy height was the single most important variable determining competitive ability. Also significant were photosynthetic rate (particularly at low light levels) and allocation to roots early in the experiment. Light profiles in the canopy revealed that less than 15% of incident light penetrated to the level of mean canopy height. Results suggest that competition for light was the major factor determining community composition, and that CO₂ affected competitive outcome through its affect upon canopy architecture.This study was supported by a grant from the US Department of Energy     

Leaf dynamics,self-shading and carbon gain in seedlings of a tropical pioneer tree

We examined leaf dynamics and leaf age gradients of photosynthetic capacity and nitrogen concentration in seedlings of the tropical pioneer tree, Heliocarpus appendiculatus, grown in a factorial design under controlled conditions with two levels each of nutrients, ambient light (light levels incident above the canopy), and self-shading (the gradient of light levels from upper to lower leaves on the shoot). Correlations among these parameters were examined in order to determine the influence of self-shading, and the regulation of standing leaf numbers, on leaf longevity and its association with leaf photosynthetic capacity. Leaf longevity and the number of leaves on the main shoot were both reduced in high light, while in the low light environment, they were reduced in the steeper self-shading gradient. In high nutrients, leaf longevity was reduced whereas leaf number increased. Leaf initiation rates were higher in the high nutrient treatment but were not influenced by either light treatment. Maximum-light saturated photosynthetic rate, on an area basis, was greater in the high light and nutrient treatments, while the decline in photosynthetic capacity in realtion to leaf position on the shoot was more rapid in high light and in low nutrients. Leaf longevity was negatively correlated among treatments with initial photosynthetic capacity. The leaf position at which photosynthetic capacity was predicted to reach zero was positively correlated with the number of leaves on the shoot, supporting the hypothesis that leaf numbers are regulated by patterns of self-shading. The negative association of longevity and initial photosynthetic capacity apparently arises from different associations among gradients of photosynthetic capacity, leaf numbers and leaf initiation rates in relation to light and nutrient availability. The simultaneous consideration of age and position of leaves illuminates the role of self-shading as an important factor influencing leaf senescence and canopy structure and dynamics.     

Dynamic stomatal behavior and its role in carbon gain during lightflecks of a gap phase and an understory Piper species acclimated to high and low light

Summary Steady-state and dynamic stomatal and assimilation responses to light transients were characterized in sun- and shade-acclimated plants of Piper auritum, a pioneer tree, and Piper aequale a shade-tolerant shrub from a tropical forest at Los Tuxtlas, Veracruz, México. Despite essentially identical steady-state responses of stomatal conductance to PFD of P. aequale and P. auritum shade plants, the dynamic responses to lightflecks were markedly different and depended on the growth regime. For both species from both growth environments, the increase in stomatal conductance occurring in response to a lightfleck continued long after the lightfleck itself so that the maximum stomatal conductance was not reached until 20–40 min after the lightfleck. Closing then occurred until stomatal conductance returned to near its original value before the lightfleck. Plants that were grown under light regimes similar to those of their natural habitat (high light for P. auritum and shade for P. aequale) had large maximum excursions of stomatal conductance and slower closing than opening responses. Plants grown under the opposite conditions had smaller excursions of stomatal conductance, especially in P. auritum, and more symmetrical opening and closing. The large and hysteretic response of stomatal conductance of P. aequale shade plants to a lightfleck was shown to improve carbon gain during subsequent lightflecks by 30–200%, depending on lightfleck duration. In contrast the very small stomatal response to lightflecks in P. auritum shade plants, resulted in no significant improvement in use of subsequent lightflecks.     

Prolonged leaf senescence in Clusia multiflora H.B.K.

 Leaf aging and senescence in Clusia multiflora H.B.K. was investigated by artificial treatments, such as floating leaf discs on water in darkness, or darkening leaves attached to the parent plant in situ in trees living in a tropical cloud forest. In both cases several parameters modified by age were evaluated such as nitrogen levels, chlorophyll content, succulence and carbohydrates levels. A prolonged senescence (nearly 3 months in floating leaf discs) was observed, contrasting with species such as Heliocarpus americanus (5 days) and Cecropia palmatisecta (20 days), characterized by low values of leaf weight per area, but similar to species with high leaf weight per area and with high levels of organic acids such as Clusia minor and Fourcroya humboldtiana, where acids may act as a reserve of C and energy. After 30 days in darkness C. multiflora leaf samples collected in the field did not show differences in comparison to non-darkened opposite leaves with respect to chlorophyll, titratable protons and carbohydrates, and leaves performed photosynthesis after 2 months in darkness. The effect of age in leaves was evaluated in a gradient of leaves, sampled at different positions from the apex and ranging in age from 15 days to 2 years old. The study of senescence in tropical wild plants is uncommon, but it is important knowledge for understanding foliar development, and response to internal rather than environmental regulation in climates where seasons are not strongly marked as is the case in the tropical mountain forest, where C. multiflora constitutes an important component in the early successional vegetation. Received: 21 October 1996 / Accepted: 12 November 1996     

Plasticity and acclimation to light in tropical Moraceae of different sucessional positions

Summary We evaluated both the photosynthetic plasticity and acclimation to light of seedlings of five co-occurring tropical tree species in the Moraceae,Cecropia obtusifolia, Ficus insipida, Poulsenia armata, Brosimum alicastrum, andPseudolmedia oxyphyllaria. Distinct differences in the species' abilities to respond to increasing irradiance correlated with their known habitat breadths and successional status. The early successinalsCecropia andFicus exhibited the highest photosynthetic rates and conductance values in high light. There was a several-fold difference in assimilation across light regimes, consistent with a high physiological plasticity. When individuals grown at low light were transferred to higher irradiances, seedlings of bothCecropia andFicus produced leaves which photosynthesized at rates as high or higher than those of plants continuously grown in high light, indicating a high photosynthetic acclimation potential. In contrast, the late successionals were characterized by both a more restricted physiological plasticity and acclimation potential. Higher light levels resulted in only moderate increases in assimilation among the late successionals, and onlyBrosimum acclimated fully to increased irradiances. NeitherPoulsenia norPseudolmedia increased appreciably their photosynthetic rates when transferred to high light. This suggests that acclimation potential cannot always be inferred from plasticity responses, and calls for a reevaluation of arguments developed solely from plasticity studies. Finally, differences between the early and late successional species in the allocation of nitrogen into RuBP carboxylase and thylakoid nitrogen pools or non-photosynthetic compounds are suggested by the distinct relationships between maximum photosynthetic capacity and nitrogen content.     

Partitioning of inorganic nitrogen assimilation between the roots and shoots of cerrado and forest trees of contrasting plant communities of South East Brasil

Summary Woody plants growing in cerrado and forest communities of south-east Brasil were found to have low levels of nitrate reductase activity in their leaves suggesting that nitrate ions are not an important nitrogen source in these communities. Only in the leaves of species growing in areas of disturbance, such as gaps and forest margins, were high levels of nitrate reductase present. When pot-grown plants were supplied with nitrate, leaves and roots of almost all species responded by inducing increased levels of nitrate reductase. Pioneer or colonizing species exhibited highest levels of nitrate reductase and high shoot: root nitrate reductase activities. Glutamine synthetase, glutamate synthase and glutamate dehydrogenase were present in leaves and roots of the species examined.¹⁵N-labelled nitrate and ammonium were used to compare the assimilatory characteristics of two species:Enterolobium contortisiliquum, with a high capacity to reduce nitrate, andCalophyllum brasiliense, of low capacity. The rate of nitrate assimilation in the former was five times that of the latter. Both species had similar rates of ammonium assimilation. Results for eight species of contrasting habitats showed that leaf nitrogen content increased in parallel with xylem sap nitrogen concentrations, suggesting that the ability of the root system to acquire, assimilate or export nitrate determines shoot nitrogen status. These results emphasise the importance of nitrogen transport and metabolism in roots as determinants of whole plant nitrogen status.     

Self-shading, carbon gain and leaf dynamics: a test of alternative optimality models

A simple model of shoot-level carbon gain is presented addressing the optimal number and life span of leaves in relation to alternative optimality criteria: (1) maximizing carbon export from the shoot, or (2) maximizing the rate of leaf production at the shoot tip. Additionally, the processes that cause declining assimilation with leaf age are considered in relation to (1) leaf position on the shoot (e.g., self-shading) versus (2) leaf age per se. Using these alternative scenarios, only a model based on position-dependent assimilation and maximization of leaf production rates resulted in quantitative predictions for all aspects of leaf dynamics on the shoot (i.e., leaf number, life span, and birth rate), while other approaches predicted that one or more parameters would be infinite. This formulation of the model also predicted that leaves should be maintained on the shoot until the diurnal carbon balance declines to zero, in contrast with other scenarios which predict that leaves should be shed while maintaining a positive carbon balance. Predictions of the model were supported by the results of a field study of carbon gain and leaf dynamics in saplings of three species of tropical pioneer trees (Carica papaya, Cecropia obtusifolia, and Hampea nutricia) which differ in the number of leaves per shoot. The results illustrate that in these fast-growing plants, leaf production and height growth may be more appropriate measures of performance than net carbon export from the shoot, and suggest that leaf senescence is primarily a function of the position of a leaf within the canopy, rather than its chronological age. Received: 13 October 1998 / Accepted: 27 January 1999     

High vapour pressure deficit exacerbates xylem cavitation and photoinhibition in shade-grown Piper auritum H.B. & K. during prolonged sunflecks

Water relations dynamics during simulated sunflecks at high (36°C) and medium (27°C) temperatures and high and low vapour pressure deficits beween leaf and air (VPD) were studied on shade-grown Piper auritum H.B. & K. plants, a pioneer tree, common in gaps and clearings of tropical rain forests. The leaves of P. auritum wilt rapidly when exposed to high light. Exposure to high VPD and high light caused substantial and rapid dehydration of leaves. Dehydration could be prevented under high humidity irrespective of temperature. Water stored in leaf cells served as initial source for transpiration upon high light exposure. This effect increased with increasing VPD and temperature. The pronounced decrease in leaf water content over time in high light caused a rapid decrease in leaf water potential (Ψ_l) and a concomitant increase in water potential gradient (ΔΨ/Δx) between trunk and leaf, yet the high leaf elasticity (small bulk elastic modulus, ε) allowed turgor maintenance under most conditions. Under high VPD and high temperature, stomata remained open and ΔΨ/Δx frequently exceeded 0.95 MPa · m⁻¹, the cavitation-inducing threshold (ΔΨ/Δx _cav) causing high rates of acoustic emissions from stems and leaf petioles and leading to concomitant losses in hydraulic conductance per leaf area (k _l). At medium temperature (high VPD), stomatal closure contained xylem embolism by keeping ΔΨ/Δx at or below this threshold. We argue that wilting substantially contributes to creating a sufficient driving force for water uptake from the soil, and reducing the VPD (through a decrease in radiation load and thus leaf temperature) to avoid excessive dehydration. Received: 3 March 1996 / Accepted: 10 November 1996     

Light,leaf age,and leaf nitrogen concentration in a tropical vine

Summary Tropical vines in the Araceae family commonly exhibit alternating periods of upward and downward growth, decoupling the usual relationship between decreasing light environment with increasing age among the leaves on a shoot. In this study I examined patterns of light, leaf specific mass, and leaf nitrogen concentration in relation to leaf position, a measure of developmental age, in field collected shoots of Syngonium podophyllum. These data were analyzed to test the hypothesis that nitrogen allocation parallels within-shoot gradients of light availability, regardless of the relationship between light and leaf age. I found that leaf nitrogen concentration, on a mass basis, was weakly correlated with leaf level light environment. However, leaf specific mass, and consequently nitrogen per unit leaf area, were positively correlated with gradients of light within the shoot, and either increased or decreased with leaf age, providing support for the hypothesis that nitrogen allocation parallels gradients of light availability.     

Interspecific and intraspecific differences in shoot and leaf lifespan of four Carex species which differ in maximum dry matter production

The effect of N supply on shoot and leaf lifespan was investigated in established stands of four herbaceous Carex species which differed in maximum dry matter production. These species were, in rank order of increasing maximum dry matter production (per unit ground area): Carex diandraC. rostrata. The observed patterns of shoot and leaf turnover were compared with data on leaf characteristics and nitrogen use efficiency indices of these species. There was no consistent difference in shoot production (number of shoots produced per unit ground area) between species with low production and those with high production: Carex diandra (low production) and C. lasiocarpa (high production) had high shoot production, while shoot production in c. rostrata (low production) and C. acutiformis (high production) was much lower. The rank order of the mean lifespan of shoots was: C. diandra. Thus, the lifespan of shoots increased with increasing maximum dry matter production of these Carex species. In all species, increased N supply led to a significant reduction in shoot lifespan. The reduction of shoot lifespans in response to enhanced N supply will result in increased nutrient turnover rates in these species. There was no consistent difference in the number of leaves produced per shoot between low-production and high-production species. C. diandra and C. lasiocarpa had relatively low leaf production, while C. rostrata and C. acutiformis had relatively high leaf production per shoot. Thus, this pattern is opposite to the pattern in shoot production. The rank order of the mean lifespan of leaves was: C. diandra. This implies that the high-production species had longer mean leaf lifespans than the low-production species. Mean leaf lifespan was not significantly affected by enhanced N supply, except in C. diandra, where leaf lifespan decreased in response to enhanced N supply. Shoot lifespans did not show any significant relation with the specific leaf area (SLA, leaf area per unit leaf mass) or the leaf area ratio (LAR, leaf area per unit plant mass) of the species under study. There was, however, a negative relation (r ²=0.71) with the nitrogen concentration in the leaves. Shoot lifespans were positively related (r ²=0.79) with whole-plant nitrogen use efficiency (NUE, dry matter production per unit N-loss) and with the mean residence time of nitrogen (MRT, the average time-span during which a unit of nitrogen is present in the plant) (r ²=0.78), but not with the nitrogen productivity (A, annual dry matter production per unit N in the plant). Leaf lifespan was positively related with the mean residence time of nitrogen in the plants (r ²–0.70). For all the other parameters, there were no significant relations with leaf lifespan. From these results we conclude that: (1) at the stand level, shoot and leaf lifespans are positively related with maximum dry matter production; and (2) shoot and leaf lifespan are important determinants of whole-plant nitrogen economy.     

Effects of light quantity and quality and soil nitrogen status on nitrate reductase activity in rainforest species of the genus Piper

Summary We studied nitrate reductase (NR) activity in six species of the genus Piper (Piperaceae) growing under a broad range of light availabilities. Field measurements were made on plants growing naturally in rainforest at the Los Tuxtlas Tropical Biological Preserve, Veracruz, Mexico at high- and lowlight extremes for each species. Foliar nitrogen on an area basis was positively related to the average daily photosynthetically active photon flux density (PFD) received by the leaf (r=0.76, p<0.01). In vivo NR activity was highly correlated with PFD (r=0.95, p<0.001) and less so with total leaf nitrogen (r=0.68, p<0.05). In vivo NR activity was always higher in high-light plants than in low-light plants within a species. Similarly, gap species such as P. auritum had much higher in vivo NR activities than shade species such as P. aequale. Soil NO ₃ ^– and NH ₄ ⁺ pools and nitrogen-mineralization rates at Los Tuxtlas were similar between high- and low-light sites, indicating that the elevated NR activities in high-light plants were not the result of higher NO ₃ ^– availabilities in high-light microsites. We performed additional experiments at Stanford, California, USA on Piper plants grown at high- and low-light. Foliar NR was highly inducible by nitrate in the gap species (auritum) but not in the generalist (hispidum) or shade (aequale) species. Root NR activities were, in general, an order of magnitude lower than foliar activities. In total, these studies suggest that Piper gap species are inherently more competent to assimilate NO ₃ ^– and are better able to respond to sudden increases in NO ₃ ^– availability than are shade species.CJWDPB publication # 1097     

DEVELOPMENTAL STABILITY IN LEAVES OF CLARKIA TEMBLORIENSIS (ONAGRACEAE) AS RELATED TO POPULATION OUTCROSSING RATES AND HETEROZYGOSITY

Four natural populations of Clarkia tembloriensis, whose levels of heterozygosity and rates of outcrossing were previously found to be correlated, are examined for developmental instability in their leaves. From the northern end of the species range, we compare a predominantly selfing population (t? = 0.26) with a more outcrossed population (t? = 0.84), which is genetically similar. From the southern end of the range, we compare a highly selfing population (t? = 0.03) with a more outcrossed population (t? = 0.58). We measured developmental stability in the populations using two measures of within-plant variation in leaf length as well as calculations of fluctuating asymmetry (FA) for several leaf traits. Growth-chamber experiments show that selfing populations are significantly more variable in leaf length than more outcrossed populations. Developmental instability can contribute to this difference in population-level variance. Plants from more homozygous populations tend to have greater within-plant variance over developmentally comparable nodes than plants from more heterozygous populations, but the difference is not significant. At the upper nodes of the plant, mature leaf length declines steadily with plant age, allowing for a regression of leaf length on node. On average, the plants from more homozygous populations showed higher variance about the regression (MSE) and lower R² values, suggesting that the decline in leaf length with plant age is less stable in plants from selfing populations than in plants from outcrossing populations. Fluctuating asymmetry (FA) was calculated for four traits within single leaves at up to five nodes per plant. At the early nodes of the plant where leaf arrangement is opposite, FA was also calculated for the same traits between opposite leaves at a node. Fluctuating asymmetry is significantly greater in the southern selfing population than in the neighboring outcrossed population. Northern populations do not differ in FA. Fluctuating asymmetry can vary significantly between nodes. The FA values of different leaf traits were not correlated. We show that developmental stability can be measured in plants using FA and within-plant variance. Our data suggest that large differences in breeding system are associated with differences in stability, with more inbred populations being the least stable.     

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.     

Leaf anatomical responses to light in five tropical moraceae of different successional status

We characterized the leaf anatomical characteristics and maximum assimilation rates of five neotropical Moraceae of different genera and successional positions. Plants were grown under different light levels and transferred to high light, simulating canopy openings. Total blade thickness increased with irradiance among all species, and thicker blades were developed when plants were switched. However, blade thickness, and the extent to which it was modified, was independent of the species’ successional position and did not predict photosynthetic performance. Palisade thickness was a good predictor of maximum photosynthetic rate, but only on a species-specific basis. Overall, leaf thickening with increasing irradiance was associated more with structural than with photosynthetic changes. The early successionals Cecropia obtusifolia and Ficus insipida exhibited similarly high photosynthetic plasticity and acclimation values, but differed in their leaf anatomical traits. The late successional Poulsenia armata produced the most anatomically plastic leaves, but failed to acclimate either anatomically or photosynthetically when transferred to higher light levels.     

Photosynthetic acclimation of plants to growth irradiance: the relative importance of specific leaf area and nitrogen partitioning in maximizing carbon gain

Changes in specific leaf area (SLA, projected leaf area per unit leaf dry mass) and nitrogen partitioning between proteins within leaves occur during the acclimation of plants to their growth irradiance. In this paper, the relative importance of both of these changes in maximizing carbon gain is quantified. Photosynthesis, SLA and nitrogen partitioning within leaves was determined from 10 dicotyledonous C₃ species grown in photon irradiances of 200 and 1000 µmol m^?2 s^?1. Photosynthetic rate per unit leaf area measured under the growth irradiance was, on average, three times higher for high‐light‐grown plants than for those grown under low light, and two times higher when measured near light saturation. However, light‐saturated photosynthetic rate per unit leaf dry mass was unaltered by growth irradiance because low‐light plants had double the SLA. Nitrogen concentrations per unit leaf mass were constant between the two light treatments, but plants grown in low light partitioned a larger fraction of leaf nitrogen into light harvesting. Leaf absorptance was curvilinearly related to chlorophyll content and independent of SLA. Daily photosynthesis per unit leaf dry mass under low‐light conditions was much more responsive to changes in SLA than to nitrogen partitioning. Under high light, sensitivity to nitrogen partitioning increased, but changes in SLA were still more important.     

Diets of Frugivorous Bats in Montane Rain Forest and Coffee Plantations in Southeastern Chiapas,Mexico

Frugivorous are one of the main diaspore dispersers in tropical ecosystems, particularly in open areas and sites in the early stages of ecological succession. Frugivorous bat species respond differentially to habitat modification, and in the context of their diaspore dispersal functions it is important to understand species' ecological requirements. We compared the diversity of diaspores, obtained from fecal samples and from fruits carried by frugivorous bats, among five shaded coffee plantations under different management regimes and a montane rain forest in southeastern Chiapas, Mexico. At each site, bats were captured every 2 mo from March 2004 to July 2005, using six mist‐nets, during two consecutive nights. We captured 2589 individuals from 18 frugivorous species, from which we collected 969 fecal samples, containing 42 diaspore species associated with early and late successional plants. Although, we captured more frugivorous bat species in montane rain forest, the number of diaspore species in this site (N=14) was not significantly different from the coffee plantations under different management regimes (16–24). In montane rain forest, Sturnira ludovici fed mainly on Piper auritum, but in coffee plantations ate Peperomia sp., Saurauia madrensis, Solanum chrysotrichum and Solanum diphyllum. Artibeus jamaicensis and Artibeus intermedius feed mostly Cecropia obtusifolia and Ficus cookii in all coffee plantations. We suggest that the presence of frugivorous bats in shaded coffee plantations is favored by trees and shrubs associated with secondary and introduced vegetation that farmers have allowed to grow within or around the plantations.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.