PHOTOSYNTHETIC PATHWAYS OF HESPERALOE FUNIFERA AND H. NOCTURNA (AGAYACEAE): NOVEL SOURCES OF SPECIALTY FIBERS

Hesperaloe funifera and H. nocturna are currently being studied as potential new sources of fibers for specialty papers. This study investigated canopy architecture and light interception in H. funifera, and gas exchange in both species. H. funifera is an acaulescent rosette species with stiff, upright leaves. Mean leaf angle for 3-year-old plants was 70° from horizontal, and more than 90% of the leaf surface was at angles greater than 50°. Vertical orientation of leaves reduced seasonal variation in light interception and midday light interception during summer months. High leaf angles are interpreted as an adaptation to arid habitats that could reduce this species' suitability for cultivation in more humid areas. Both H. funifera and H. nocturna had leaf-tissue water contents and mesophyll-succulence values intermediate between previously investigated Agavaceae known to be either C₃ or Crassulacean acid metabolism (CAM) plants. Both species proved to have CAM, however. Gas exchange characteristics varied with leaf age, with older leaves having higher assimilation rates, greater water-use efficiency, and a higher proportion of nighttime CO₂ uptake. Interestingly, these older leaves had mesophyll succulence values closer to those of typical C₃ species. These Hesperaloe species can thus be characterized as nonsucculent CAM plants. Both species showed CO₂ uptake rates of 5–8 μmol m^-2 sec^-1 expressed on a total-surface-area basis and 10–18 μmol m^-2 sec^-1 expressed on a projected-leaf-area basis. Expanded cultivation of species possessing CAM in marginal areas has been recommended recently; the physiological studies reported here along with previous studies of their economic botany identify these Hesperaloe species as good crop candidates for dry regions.     

The nocturnal larvae of a specialist folivore prefer <Emphasis Type="Italic">Chromolaena odorata</Emphasis> (L.) foliage from a sunny environment,but does it matter?

Increasing evidence suggests that the responses of insect herbivores to environment-mediated changes in the phenotypic and phytochemical traits of their host plants are more complex than previously thought. Here, we examined the effects of habitat conditions (shaded versus full-sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) (Asteraceae). We also determined neonate larval preference of the specialist herbivore, Pareuchaetes pseudoinsulata Rego Barros (Lepidoptera: Erebidae) (a biological control agent) for shaded versus full-sun leaves. The study further evaluated the performance of the moth on C. odorata leaves obtained from both shaded and full-sun habitats. Leaves of C. odorata plants growing in the shaded habitat had higher water and nitrogen contents compared with full-sun leaves. Plants growing in shade had longer leaves but full-sun plants were taller and had greater aboveground biomass compared with shaded plants. Although neonate larvae of P. pseudoinsulata preferred to feed on full-sun foliage, development was faster when reared on shaded foliage. However, survival, pupal mass, growth rate, and Maw’s host suitability index of the moth did not significantly differ between full-sun and shaded foliage. Our inability to demonstrate significant differences in key insect performance metrics in P. pseudoinsulata between shaded and full-sun foliage, despite neonate larval preference for one of the foliage types, suggests that neither of the foliage types can be considered a superior host, and reiterate the fact that relationships between host plant quality (modulated by light intensity) and phytophagous insect performance are not simple.     

Leaf orientation and light interception by juvenile Pseudopanax crassifolius(Cunn.) C. Koch in a partially shaded forest environment

Leaf orientations and light environments were recorded for 40 juvenile Pseudopanax crassifolius trees growing in New Zealand in a partially shaded, secondary forest environment. Efficiencies of interception of diffuse and direct light by the observed leaf arrangments were calculated relative to those of three hypothetical leaf arrangements. Canopy gaps above the study plants were unevenly distributed with respect to azimuth and elevation above the horizon. Our results indicate that photosynthetically active radiation (PAR) received from the sides is more important than that received from directly above. In 33 of the plants leaf orientation was found to be significantly clustered towards one azimuth. The mean azimuth and the mean angle of declination were different for each plant. Leaves were steeply declined, and oriented towards the largest canopy gap at each site. Steep leaf angles reduced interception of direct and diffuse PAR when compared to interception by plant with a hypothetical horizontal leaf arrangement. When compared to a hypothetical arrangement with steep leaf declination and a uniform azimuth distribution, the observed leaf arrangement increased the efficiency of interception of diffuse PAR, but had a variable effect on the interception of direct PAR. Results indicate that the developing leaves of juvenile P. crassifolius orient towards the strongest sources of diffuse light, regardless of their value as a source of direct light. By maximising diffuse light interception while reducing direct light interception, leaf orientation may be a partial determinant of the types of habitats exploited by this species. This study emphasises the importance of considering diffuse light interception for plants growing in partially shaded environments.     

Acclimation of photosynthesis to lightflecks in tomato leaves: interaction with progressive shading in a growing canopy

Plants in natural environments are often exposed to fluctuations in light intensity, and leaf‐level acclimation to light may be affected by those fluctuations. Concurrently, leaves acclimated to a given light climate can become progressively shaded as new leaves emerge and grow above them. Acclimation to shade alters characteristics such as photosynthetic capacity. To investigate the interaction of fluctuating light and progressive shading, we exposed three‐week old tomato (Solanum lycopersicum ) plants to either lightflecks or constant light intensities. Lightflecks of 20 s length and 1000 μmol m^?2 s^?1 peak intensity were applied every 5 min for 16 h per day, for 3 weeks. Lightfleck and constant light treatments received identical daily light sums (15.2 mol m^?2 day^?1). Photosynthesis was monitored in leaves 2 and 4 (counting from the bottom) during canopy development throughout the experiment. Several dynamic and steady‐state characteristics of photosynthesis became enhanced by fluctuating light when leaves were partially shaded by the upper canopy, but much less so when they were fully exposed to lightflecks. This was the case for CO₂‐saturated photosynthesis rates in leaves 2 and 4 growing under lightflecks 14 days into the treatment period. Also, leaf 2 of plants in the lightfleck treatment showed significantly faster rates of photosynthetic induction when exposed to a stepwise change in light intensity on day 15. As the plants grew larger and these leaves became increasingly shaded, acclimation of leaf‐level photosynthesis to lightflecks disappeared. These results highlight continuous acclimation of leaf photosynthesis to changing light conditions inside developing canopies.     

LEAF DISPLAY,CANOPY STRUCTURE,AND LIGHT INTERCEPTION OF TWO UNDERSTORY PALM SPECIES

The implications of leaf size, leaf display, and crown size for whole-plant light interception were investigated in Geonoma cuneata and Asterogyne martiana, two understory palm species native to Central American rain forests. Adults of A. martiana had longer leaves, more leaves per plant, and greater total leaf area than G. cuneata adults. Geometric measurements within whole crowns were used to calculate light interception efficiency, a leaf-based measure of the proportion of total incident light that is intercepted by a crown. Light interception efficiency was higher in adult G. cuneata than in adult A. martiana; seedlings of the two species did not differ significantly in light interception efficiency. Decreased efficiency of adult A. martiana crowns was largely due to an increased proportion of pendent leaves. Compared to G. cuneata, adults of A. martiana had greater light interception capacity (the product of light interception efficiency and total leaf area), but they also had higher biomass costs of light interception. Lower biomass costs of light interception in adult G. cuneata enable this species to exploit successfully the most deeply shaded microsites in the rain forest understory.     

Leaf physiology of shade-grownCycas micronesica leaves following removal of shade

The photosynthetic characteristics ofCycas micronesica K.D. Hill were studied from August 1998 until February 1999 using chlorophyll fluorescence and gas-exchange techniques to determine the responses to long-term shade of 35% ambient light transmission, followed by the transfer of shade-grown leaves into full-sun conditions. The shade-grown leaves exhibited increased photosynthetic light use efficiency and effective quantum efficiency of photosystem II (PS II) and decreased photosynthetic light saturation point and dark respiration when compared with leaves grown in full sun. Shade was removed from shade-grownC. micronesica leaves during midday on December 14, 1998, when effective quantum efficiency of shaded leaves was 45% greater than that of sun leaves. Following one hour in full sun, effective quantum efficiency of the shade-grown leaves declined to below that of the sun-grown leaves. After receiving full sunlight for the rest of the photoperiod, maximum quantum efficiency of PS II photochemistry for shade-grown leaves was below that of sun-grown leaves throughout the night. The damage caused by excessive light to shade-grown leaves progressed for the first three days after shade removal. On day 3, effective quantum efficiency during midday was 30%, net photosynthesis was 47%, apparent quantum yield was 65%, and light compensation point was 136% of that for sun-grown leaves. After day 3, the relationship between full-sun leaves and the previously shaded leaves for these response variables was relatively stable. Two months following removal of shade, the previously shaded leaves continued to exhibit damage from high light. These results have application to transplanting cycad plants from a shaded nursery to a field site or, after tropical cyclones, where protective forest canopy cover has been destroyed and cycad plants in the forest subcanopy are abruptly exposed to full-sun conditions.     

LEAF ORIENTATION,RADIATION INTERCEPTION,AND NOCTURNAL ACIDITY INCREASES BY THE CAM PLANT AGAVE DESERTI (AGAVACEAE)

Both field measurements and a computer model were used to study the interception of photosynthetically active radiation (PAR) by Agave deserti (Engelm.), a desert CAM plant with a basal rosette of massive opaque leaves. PAR interception was determined in the winter and the summer for upper and lower leaf surfaces on a plant with about 60 leaves. Total daily PAR on the leaf surfaces was approximately 10 mol m^-2 for a winter day and 20 mol m^-2 for a summer day. For a PAR of 15 mol m^-2, the nocturnal increase in acidity was about 0.6 mol m^-2 for both leaf surfaces and various leaf orientations, except for the oldest most horizontal leaves where the increase was less than half as large. The acidity increase measured in the field was 90% saturated at 25 mol m^-2. Thus, daytime PAR in the desert is often limiting for the nocturnal acidity increase, especially for the lower leaf surfaces. Simulated tilting of the plant by 55° so that the vertical axis pointed to the sun at solar noon on a winter day increased the PAR incident on the upper surfaces of the leaves, but did not affect the total nocturnal increase in acidity by the whole plant. Although simulated removal of alternate leaves increased the PAR per unit leaf area for the remaining leaves, it reduced the total increase in nocturnal acidity of the whole plant by 31%. PAR interception by plants on slopes facing steeply north, east, or west was substantially reduced compared to the horizontal. Thus, the model proved to be quite useful for quantifying the relation between leaf orientation, PAR interception, and nocturnal increases in acidity by A. deserti, and it indicated that the lower frequency of plants on north- compared to south-facing slopes was due to PAR limitations.     

The narrow-leaf syndrome: a functional and evolutionary approach to the form of fog-harvesting rosette plants

Plants that use fog as an important water-source frequently have a rosette growth habit. The performance of this morphology in relation to fog interception has not been studied. Some first-principles from physics predict that narrow leaves, together with other ancillary traits (large number and high flexibility of leaves, caudices, and/or epiphytism) which constitute the “narrow-leaf syndrome” should increase fog-interception efficiency. This was tested using aluminum models of rosettes that differed in leaf length, width and number and were exposed to artificial fog. The results were validated using seven species of Tillandsia and four species of xerophytic rosettes. The total amount of fog intercepted in rosette plants increased with total leaf area, while narrow leaves maximized interception efficiency (measured as interception per unit area). The number of leaves in the rosettes is physically constrained because wide-leafed plants can only have a few blades. At the limits of this constraint, net fog interception was independent of leaf form, but interception efficiency was maximized by large numbers of narrow leaves. Atmospheric Tillandsia species show the narrow-leaf syndrome. Their fog interception efficiencies were correlated to the ones predicted from aluminum-model data. In the larger xerophytic rosette species, the interception efficiency was greatest in plants showing the narrow-leaf syndrome. The adaptation to fog-harvesting in several narrow-leaved rosettes was tested for evolutionary convergence in 30 xerophytic rosette species using a comparative method. There was a significant evolutionary tendency towards the development of the narrow-leaf syndrome the closer the species grew to areas where fog is frequently available. This study establishes convergence in a very wide group of plants encompassing genera as contrasting as Tillandsia and Agave as a result of their dependence on fog. Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users.     

Improved growth and water use efficiency of cherry saplings under reduced light intensity

Cherry (Prunus avium L.) saplings were grown under natural sunlight (controls) or moderate shading (up to 30%, depending on the incident light intensity and the hour of the day). Reduced light intensity increased the dry mass of each of the plant components studied. Consequently, the total dry mass of shaded plants was significantly greater than that of controls at the end of the growing season. However, the diurnal trend in the level of photosynthesis (per unit of leaf area) of shaded plants was similar to the controls in August, but lower in September. As the growing season proceeded, reduced photosynthetic rates, thinner mesophyll and larger specific leaf area in the shaded plants indicated that leaf development had adapted to shaded conditions throughout the growing season. It is suggested that increased growth of shaded plants was caused by a higher initial relative growth rate and a greater whole-plant photosynthesis. Shading consistently reduced transpiration over the season, therefore improving water use efficiency of shaded leaves. Our results suggest that a moderate reduction in light intensity can be a useful method for improving growth and saving water in hot and dry environments.     

PHOTOSYNTHETIC RATE AND MESOPHYLL SURFACE AREA IN EXPANDING LEAVES OF ALTERNANTHERA PHILOXEROIDES GROWN AT TWO LIGHT LEVELS

Leaves of Alternanthera philoxeroides, alligator weed, developed at a photosynthetic photon flux density (PPFD, light energy at wavelengths of 400 to 700 nm) of 790 μmol sec⁻¹ m⁻² (High Light) had less surface area, were thicker, had a higher maximum P_n (net rate of CO₂ uptake), and required a higher PPFD for saturation of P_n, than leaves developed at 160 μmol sec⁻¹ m⁻² (Low Light). Mesophyll thickness at Low Light was within 19% of maximum 2 days after emergence but at High Light, thickness increased 79% between 2 and 16 days after leaf emergence. The ratio of mesophyll surface area to leaf surface area decreased during development in both light treatments; the ratio, however, was over 70% greater in fully expanded High Light leaves than in Low Light leaves. Maximum P_n expressed on a leaf surface area basis was 158% greater in High Light leaves than in Low Light leaves, but P_n was only 58% greater when expressed on a mesophyll surface area basis. It was estimated that fully expanded High Light leaves fixed 72% more CO₂ per leaf (P_n expressed per unit surface area times the total surface area per leaf) than fully expanded Low Light leaves when P_n was measured at the PPFD leaves expanded under. Both High and Low Light leaves would fix about the same amount of CO₂ per leaf when P_n was measured at 160 μmol sec⁻¹ m⁻² because the larger surface area of the Low Light leaves offset small differences in P_n.     

Effects of irradiance on growth,photosynthesis, and water use efficiency of seedlings of the chaparral shrub,Ceanothus megacarpus

Summary The effects of irradiance during growth on biomass allocation, growth rates, leaf chlorophyll and protein contents, and on gas exchange responses to irradiance and CO₂ partial pressures of the evergreen, sclerophyllous, chaparral shrub, Ceanothus megacarpus were determined. Plants were grown at 4 irradiances for the growth experiments, 8, 17, 25, 41 nE cm^-2 sec^-1, and at 2 irradiances, 9 and 50 nE cm^-2 sec^-1, for the other comparisons.At higher irradiances root/shoot ratios were somewhat greater and specific leaf weights were much greater, while leaf area ratios were much lower and leaf weight ratios were slightly lower than at lower irradiances. Relative growth rates increased with increasing irradiance up to 25 nE cm^-2 sec^-1 and then leveled off, while unit leaf area rates increased steeply and unit leaf weight rates increased more gradually up to the highest growth irradiance.Leaves grown at 9 nE cm^-2 sec^-1 had less total chlorophyll per unit leaf area and more per unit leaf weight than those grown at 50 nE cm^-2 sec^-1. In a reverse of what is commonly found, low irradiance grown leaves had significantly higher chlorophyll a/b than high irradiance grown leaves. High irradiance grown leaves had much more total soluble protein per unit leaf area and per unit dry weight, and they had much higher soluble protein/chlorophyll than low irradiance grown leaves.High irradiance grown leaves had higher rates of respiration in very dim light, required higher irradiances for photosynthetic saturation and had higher irradiance saturated rates of photosynthesis than low irradiance grown leaves. CO₂ compensation irradiances for leaves of both treatments were very low, <5 nE cm^-2 sec^-1. Leaves grown under low and those grown under high irradiances reached 95% of their saturated photosynthetic rates at 65 and 85 nE cm^-2 sec^-1, respectively. Irradiance saturated rates of photosynthesis were high compared to other chaparral shrubs, 1.3 for low and 1.9 nmol CO₂ cm^-2 sec^-1 for high irradiance grown leaves. A very unusual finding was that leaf conductances to H₂O were significantly lower in the high irradiance grown leaves than in the low irradiance grown leaves. This, plus the differences in photosynthetic rates, resulted in higher water use efficiencies by the high irradiance grown leaves. High irradiance grown leaves had higher rates of photosynthesis at any particular intercellular CO₂ partial pressure and also responded more steeply to increasing CO₂ partial pressure than did low irradiance grown leaves. Leaves from both treatments showed reduced photosynthetic capability after being subjected to low CO₂ partial pressures (100 bars) under high irradiances. This treatment was more detrimental to leaves grown under low irradiances.The ecological implications of these findings are discussed in terms of chaparral shrub community structure. We suggest that light availability may be an important determinant of chaparral community structure through its effects on water use efficiencies rather than on net carbon gain.     

Field photosynthesis,microclimate and water relations of an exotic temperate liana,Pueraria lobata,kudzu

Summary Kudzu occurs in a variety of habitats in the southeastern United States. It is most common in exposed, forest edge sites and road cuts where it forms an extensive ground canopy as well as a canopy overtopping nearby trees, but it can also be found in completely open fields and deeply shaded sites within a forest. Microclimate, stomatal conductance, leaf water potential and photosynthetic responses to light, temperature and humidity were measured in two contrasting microhabitats on Pueraria lobata, kudzu. Midsummer leaf temperatures and leaf-to-air water vapor deficits for plants growing in an exposed site were significantly greater than for those in a shaded site, exceeding 35° C and 50 mmol mol^-1, respectively. Maximum stomatal conductance exceeded 400 mmol m^-2 s^-1 in exposed leaves during peak vegetative growth. Stomatal conductance in shaded leaves was approximately half the value measured in exposed leaves on any particular dya. Maximum photosynthetic carbon uptake was also higher in leaves growing in exposed sites compared to leaves in shaded sites, exceeding 18.7 and 14.0 mol m^-2 s^-1, respectively. Photosynthesis, stomatal conductance and intercellular CO₂ concentration decreased dramatically in response to increasing water vapor deficit for leaves from both sites. However, transpiration showed an initial increase at intermediate water vapor deficits, leveling off or even decreasing at higher values. Leaf water potential demonstrated marked diurnal variation, but remained constant over a wide range of transpirational water fluxes. This latter feature, combined with microenvironmental modification through rapid leaf orientation and pronounced stomatal responses to water vapor deficits may represent important adaptive responses in the exploitation of a diverse array of habitats by kudzu.     

The photosynthesis of ryegrass leaves grown in a simulated sward

Plants were taken from simulated swards of perennial ryegrass (Lolium perenne) grown in a controlled environment and the rates of photosynthesis of the youngest fully expanded leaves, and the second and third youngest leaves on the same tillers were measured. The youngest leaves had the highest rates and the third the lowest, with the second leaves intermediate. The rate of photosynthesis in bright light of successive youngest expanded leaves decreased as the swards increased in leaf area, but did not when plants were grown so that the main stem was not shaded. When plants were grown at different densities and the photosynthetic rates of leaves of a particular ontogenetic rank were measured, it was found that leaves on plants from higher densities had lower rates of photosynthesis. Also leaves on plants grown in bright light had higher photosynthetic rates than those on plants grown in dim light. It is concluded that the decline in the photosynthetic capacity of successive leaves in a rapidly growing simulated sward is due to the intense shading to which they are subjected during their expansion.     

Two Steps of Gas Exchange in Leaf Photosynthesis

Photosynthesis and transpiration of excised leaves of Taraxacum officinale L. and a few other species of plants were measured, using an open gas analysis system. The rates of CO₂ uptake and transpiration increased in two steps upon illumination of stomata-bearing epidermis of these leaves at a light intensity of 50 mW × cm⁻². Abscisic acid inhibited only the second step of gas exchange. Illumination of the astomatous epidermis of hypostomatous leaves caused only the first step of gas exchange. These data indicate that the first and second steps arise from cuticular and stomatal gas exchange, respectively. The rate of the cuticular photosynthesis in a Taraxacum leaf reached saturation at a light intensity of 5 mW × cm⁻², and the rates of the stomatal photosynthesis and transpiration reached saturation at a higher intensity of 35 mW × cm⁻². The cuticular photosynthesis of a Taraxacum leaf was 18% of the stomatal photosynthesis at 50 mW × cm⁻² and 270% at 5 mW × cm⁻². The other species of leaves showed the same trend. The importance of cuticular CO₂ uptake in leaf photosynthesis, especially under low light intensity was stressed from these data.     

Carbon dioxide assimilation and stomatal response of afroalpine giant rosette plants

Maximal rates of CO₂ assimilation of 8–11 mol m^-2 s^-1 at ambient CO₂ concentration were measured for Dendrosenecio keniodendron, D. brassica, Lobelia telekii and L. keniensis during the day in the natural habitat of these plants at 4,200 m elevation on Mt. Kenya. Even at these maximal rates, the CO₂ uptake of all species was found to correspond to the linear portion of the CO₂ response curve, with a calculated stomatal limitation for CO₂ diffusion of 42%. Photosynthesis was strongly reduced at temperatures above 15° C. In contrast to this sensitivity to high temperatures, frozen leaves regained full photosynthetic capacity immediately after thawing. Stomata responded to dry air, but not to low leaf water potentials which occurred in cold leaves and at high transpiration rates. During the day reduced rates of CO₂ uptake were associated with reduced light interception due to the erect posture of the rosette leaves and with high temperatures. Stomata closed at vapour pressure deficits which were comparable in magnitude to those characteristic of many lowland habitats (40 mPa Pa^-1).     

The nocturnal larvae of a specialist folivore perform better on Chromolaena odorata leaves from a shaded environment

Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment‐mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full‐sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) King & Robinson (Asteraceae). In addition, the performance was evaluated in two generations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full‐sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non‐structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but above‐ground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full‐sun foliage. Although immature survival and adult longevity did not differ between habitats, Maw's host suitability index indicated that shaded leaves were more suitable for the growth and reproduction of P. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light‐mediated changes in plant traits and leaf characteristics can affect insect folivore performance.     

Leaf position,light levels,and nitrogen allocation in five species of rain forest pioneer trees

We used path analysis to ask whether leaf position or leaf light level was a better predictor of within-plant variation in leaf nitrogen concentration in five species of rain forest pioneer trees (Cecropia obtusifolia, Ficus insipida, Heliocarpus appendiculatus, Piper auritum, and Urera caracasana) from the Los Tuxtlas Biological Station, Veracruz, Mexico. Three hundred seventy-five leaves on 28 plants of the five species were analyzed for leaf nitrogen concentration, leaf mass per area, and leaf light interception at different positions (= nodes) along a shoot. Mean values of leaf nitrogen concentration ranged from 0.697 to 0.993 g/m² in the five species, and varied by as much as 2.24 g/m² among leaves on individual plants. Leaf position on the shoot explained significantly more of the within-plant variation in leaf nitrogen concentration than did leaf light level in four of the five species: Cecropia obtusifolia, Heliocarpus appendiculatus, Piper auritum (branch leaves only), and Urera caracasana. However, individual species differed considerably in the patterns of nitrogen allocation and leaf mass per area among leaves on a shoot. These results suggest that leaf nitrogen deployment in these plants is, in part, developmentally constrained and related to the predictability of canopy light distribution associated with plant growth form.     

A tropical epiphytic orchid uses a low‐light interception strategy in a spatially heterogeneous light environment

Light is considered a non‐limiting factor for vascular epiphytes. Nevertheless, an epiphyte's access to light may be limited by phorophyte shading and the spatio‐temporal environmental patchiness characteristic of epiphytic habitats. We assessed the extent to which potential light interception in Rodriguezia granadensis, an epiphytic orchid, is determined by individual factors (plant size traits and leaf traits), or environmental heterogeneity (light patchiness) within the crown of the phorophyte, or both. We studied 104 adult plants growing on Psidium guajava trees in two habitats with contrasting canopy cover: a dry tropical forest edge, and isolated trees in a pasture. We recorded the number of leaves and the leaf area, the leaf position angles, and the potential exposure of the leaf surface to direct irradiance (silhouette area of the leaf blade), and the potential irradiance incident on each plant. We found the epiphytes experience a highly heterogeneous light environment in the crowns of P. guajava. Nonetheless, R. granadensis plants displayed a common light interception strategy typical of low‐light environments, resembling terrestrial, forest understory plants. Potential exposure of the total leaf surface to direct irradiance correlated positively with plant size and within‐plant variation in leaf orientation. In many‐leaved individuals, within‐plant variation in leaf angles produced complementary leaf positions that enhanced potential light interception. This light interception strategy suggests that, in contrast to current wisdom, enhancing light capture is important for vascular epiphytes in canopies with high spatio‐temporal heterogeneity in light environments.     

EFFECTS OF CANOPY POSITION AND IRRADIANCE ON THE LEAF PHYSIOLOGY AND MORPHOLOGY OF PENTACLETHRA MACROLOBA (MIMOSACEAE)

Pentaclethra macroloba (Willd.) Kuntze (Mimosaceae) is a dominant late-successional tree species in the Atlantic lowland forests of Costa Rica. Leaves of P. macroloba from three heights in the forest canopy were compared with leaves of seedlings grown in controlled environment chambers under four different irradiance levels. Changes in leaf characteristics along the canopy gradient paralleled changes resulting from the light gradient under controlled conditions. The effect of light or canopy position on light-saturated photosynthesis was small, with maximum photosynthesis increasing from 5 to 6.5 μmol m^−-2 s^−-1 from understory to canopy. Both chamber grown and field leaves showed large adjustments in photosynthetic efficiency at low light via reductions in dark respiration rates and increases in apparent quantum yields. Light saturation of all leaves occurred at or below 500 μmol m^−-2 s^−-1. Leaf thickness, specific leaf weight, and stomatal density increased to a greater extent than saturated photosynthesis with higher irradiance during growth or height in the canopy. As a result, there was a poor correspondence between leaf thickness and light-saturated photosynthesis on an area basis. It is concluded that Pentaclethra macroloba possesses the characteristics of a typical shade-tolerant species.