Photosynthetic acclimation in eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings following transfer of shade-grown seedlings to high light

 We studied photosynthetic acclimation of eastern hemlock [Tsuga canadensis (L.) Carr.] seedlings in the first month after sudden exposure of shade-grown seedlings to full sunlight. In a greenhouse experiment, seedlings were grown under full sun or 80% shade, and after 7 months, a sample of the shaded trees was transferred to full sun in the greenhouse. Photosynthetic responses of shaded, transferred, and sun trees were followed over the course of 26 days to track short to medium-term acclimation responses. A partial acclimation of photosynthesis at high light occurred in pre-existing (formed in the previous environment) and new foliage of transferred seedlings. This was associated with non-stomatal limitations to photosynthesis. Pre-existing foliage of transferred plants had a prolonged reduction in the ratio of variable to maximal fluorescence, and a limited capacity to adjust photochemical quenching or photosystem II quantum yield in the light to increasing light intensity compared to sun foliage, and apparently had some difficulty sustaining non-photochemical quenching. Seedling survival was only 58% among transferred seedlings, compared to 80% and 100% in the shade or sun groups, respectively. Photosystem II quantum yield in the light, and photochemical and non-photochemical quenching were similar between newly formed foliage of transferred and sun plants. These findings indicate that eastern hemlock depends strongly on the production of new foliage for photosynthetic adjustments to high light, and that development of photosynthetic competence may be a gradual process that occurs over successive foliar production cycles. Received: 12 May 1998 / Accepted: 27 July 1998     

Growth responses to arbuscular mycorrhizae by rain forest seedlings vary with light intensity and tree species

Light intensity and root colonization by arbuscular mycorrhizal (AM) fungi are considered important factors affecting the performance of rain forest plants, yet few studies have examined how these two factors interact. Whether AM colonization promoted growth or caused shifts in biomass allocation in seedlings of four species of Australian rain forest tree (Flindersia brayleana, Acmena resa, Cryptocarya mackinnoniana and Cryptocarya angulata), grown in a glasshouse under light conditions that mimicked the shaded understory (3% PAR) and small light gaps (10% PAR), was examined. Seedlings were grown in sterilized field soil and either inoculated with AM fungi or provided sterile inoculum. Four major findings emerged. First, in all species, seedlings grown in small gap light intensities were larger than seedlings grown in understory light intensities. Second, when seedling biomass was included as a covariate, variation in light intensity was associated with significant shifts in biomass allocation. In all species, leaf area ratio was lower at 10% PAR than at 3% PAR, while root-to-shoot ratio showed the opposite pattern in one of the four species (C. mackinonniana). Third, although percentage root length colonized by AM fungi was greater at 10% PAR than 3% PAR in all species, this difference could be accounted for by variation in seedling size in all species except C. angulata. Fourth, growth and biomass allocation responses to AM colonization varied with light intensity and plant species. AM colonization promoted growth in both light regimes only in F. brayleana, while it had no effect on growth in C. mackinnoniana and C. angulata in either light regime and promoted growth only under high light in A. resa. AM colonization had no effect on leaf area ratio or root-to-shoot ratio in any of the species, and significantly altered specific root length in only one of the four species (C. mackinnoniana). These findings suggest that rain forest seedlings are highly variable in their growth responses to AM colonization and that some of this variability is related to the light intensity of the environment. Given that seedlings may spend many years in the shaded understory, these differences among species could have important effects on long-term seedling performance and seedling community dynamics.     

The responses to shade of seedlings of very small-seeded tree and shrub species from tropical rain forest in Singapore

1. Newly germinated seedlings of six tree and shrub species with very small seeds (31–460 μg dry mass), one light-demanding and five shade-tolerant at the stage of establishment in the wild, were grown for 5 months in neutral shade houses with 0·5, 1, 3·5 and 7·5% daylight.
2. The ratio of yield in 7·5% to that in 1% was 8:1 for the light-demanding Melastoma malabathricum but only 2:1 for the confamilial shade-tolerator Pternandra echinata. The Pternandra, Urophyllum hirsutum, Ficus chartacea, Ficus grossularioides and Pellacalyx saccardianus showed a graded series of responses to irradiance, generally consistent with their apparent demands for light in the wild. In contrast, survival of very deep shade was not clearly related to light demand in nature.
3. The results support the conclusion drawn from observational studies that large seed size is not primarily adaptive in resisting shade but in resisting the associated risks of burial by litter, desiccation during dry spells, uprooting by birds and other kinds of damage by animals or falling debris.     

The influence of nitrogen on rain forest dipterocarp seedlings exposed to a large increase in irradiance

Dipterocarps dominate the canopy of lowland tropical rain forest in South‐east Asia. Seedlings of these species form diverse assemblages on the forest floor where low irradiance severely limits their growth. Further growth depends largely upon the increased irradiance that can occur with the creation of canopy gaps. However, the response of dipterocarp seedlings to increased irradiance and their subsequent establishment in the canopy may be influenced by the availability of other resources, such as nutrient availability. We investigated the influence of nitrogen supply on aspects of the photosynthetic physiology and growth of seedlings of four dipterocarp species (Shorea leprosula, Shorea johorensis, Shorea oleosa and Dryobalanops lanceolata) growing under low irradiance, during transfer from low to high irradiance, and during subsequent growth at high irradiance. All four species increased growth and photosynthetic capacity in response to N‐supply at high irradiances but not at low irradiance approximating that which can be expected to occur in the forest understorey. When seedlings grown at low irradiances and varying N‐supply were exposed to a large increase in irradiance, all species showed some degree of initial photodamage (measured through chlorophyll fluorescence), the extent of which was similar between species but differed markedly depending on the pre‐exposure growth irradiance and N‐supply. Greater photodamage occurred in seedlings grown at lower compared with higher N‐supply and irradiance. Despite these initial difference in the extent of this photodamage, all seedlings demonstrated a similar capacity to recover from damage. However, the alterations in the photosynthetic physiology of leaves during this recovery differed between species and depended on N‐supply. Under high N‐supply all species apart from S. oleosa increased photosynthetic capacity per unit chlorophyll following exposure to high irradiance by increasing photosynthetic capacity per unit leaf area while, under low N‐supply, an increase in photosynthetic capacity per unit leaf only occurred in D. lanceolata. Our results suggest that variations in N‐availability may have a much greater impact on the relative competitiveness of dipterocarp seedlings during the regenerative phase following canopy gap formation than physiological differences between seedlings. Our results demonstrate a potentially significant role for N‐availability in the regeneration dynamics and distribution of canopy‐dominating dipterocarp species.     

Fine root biomass under light gap openings in an Amazon rain forest

Summary Belowground processes in light gap openings are poorly understood, particularly in tropical forests. Fine roots in three zones of light gap openings and adjacent intact forest were regularly measured in buried bags and surface litter envelopes for 2 years. Fine root biomass does not vary significantly within gaps for either buried bags or for surface litter envelopes. When entire gaps are compared without regard for within gap zones, root growth into both surface litter and buried bags is significantly different between gaps, with highest rates of fine root biomass accumulation in the smallest gap. These results suggest that the aboveground within-gap zones do not result in a congruent pattern of below-ground zonation. Gap size, decomposition of the fallen tree, and pre-gap fine root growth rates should be considered to determine fine root growth patterns following the formation of light gap openings.     

Photosynthetic induction responses of two rainforest tree species in relation to light environment

Photosynthetic induction of in situ saplings of two Costa Rican rainforest tree species wre compared in relation to their light environment, using infrared gas analysis and hemispherical photography. The species studied were Dipteryx panamensis, a climax species found in bright microsites, and Cecropia obtusifolia, a pioneer species. In the morning, when leaves were most responsive, induction time necessary to reach 90% of the lightsaturated rate of photosynthesis was on average 16 min for Dipteryx and 10 min for Cecropia. However, induction times for both species increased in the afternoon resulting in shorter daily average induction times for Dipteryx than for Cecropia. Dipteryx also maintained higher levels of induction for a longer period under low light conditions than did Cecropia. The two species differed in the way they adjusted to light availability. Dipteryx saplings growing in shady sites had faster rates of induction than saplings growing in bright sites, with no difference in light-saturated photosynthetic rate. In contrast, Cecropia saplings growing in bright sites had higher light-saturated photosynthetic rates than saplings growing in shady sites, with no difference in rates of induction. Dipteryx appears to exploit temporal variation in light availability by refining the quickness of the induction response to the light environment, while Cecropia adjusts its scale of exploitation by realizing a higher lightsaturated photosynthetic rate in sites of higher light.     

Physiological characteristics of Japanese red pine, Pinus densiflora Sieb. et Zucc., in declined forests at Mt. Gokurakuji in Hiroshima Prefecture, Japan

The decline of Japanese red pine trees (Pinus densiflora Sieb. et Zucc.) at Mt. Gokurakuji (693 m a.s.l.), 30 km west of Hiroshima city, west Japan, was studied. The effects of air pollution and acid deposition on the physiological characteristics of the trees, especially those of the needles, were investigated. Ozone concentration was not correlated with the physiological status of the needles and SO₂ concentration was not high in the declined area. NO₂ concentration correlated negatively with needle longevity while it correlated positively with ethylene emission from 1-year-old needles. Average needle longevity was about 2.8 years in non-declined areas; however the longevity was 1.3 years in the most polluted area. The minimal fluorescence at night (F ₀)of 1-year-old needles decreased with increasing NO₂ concentration. The maximum stomatal conductance (gl), net photosynthesis (P _n)and intercellular CO₂ concentration (C _i) in the declined areas were lower than in the non- declined areas (about 50%, 30% and 20% lower, respectively). The lower C _isuggested that the major part of the decrease in P _ncan be explained by stomatal restriction. The soil pH, N content and C/N ratio showed no significant difference between the declined and non-declined areas. The physiological disorders of needles were due to the damage by air pollutants, and important roles of NO₂ are suggested. Lowering of P _n and the shortening of needle longevity appear to be the main causes of the decline in pines in the forest decline area. Received: 16 December 1998 / Accepted: 7 January 2000     

Influence of leaf size on tree architecture: first branch height and crown dimensions in tropical rain forest trees

 First branch height is an important attribute of sapling architecture, as it defines the height at which prolonged lateral growth is possible. First branch height, measured on saplings of 70 species in tropical rain forests of Australia, Costa Rica, Panama, and Sabah, Malaysia, was highly correlated with leaf blade and petiole length. The observed relationship, first branch height ∝ blade length × (petiole length)^0.5, implies that the ratio of first branch height to blade length increases somewhat with increasing leaf size, among species with a given ratio of petiole to blade length. Orthotropic species, with more or less radially symmetric arrangements of leaves on ascending axes, had a mean first branch height of 7x that observed for plagiotropic species, with planar leaf arrangements. The greater first branch height of orthotropic species was associated with their larger leaves and longer petioles. Plagiotropic species had wider crowns than orthotropic species in the sapling stage, as assessed at the Costa Rican site. Thus, leaf dimensions influence the dynamics of crown construction (or visa versa), as well as affecting leaf energy balance and gas exchange. Received: 5 September 1997 / Accepted: 3 March 1998     

Photosynthesis rates of selected tree species in lowland dipterocarp rainforest of Sabah, Malaysia

 Diurnal courses of net photosynthesis, transpiration and water potential of leaves of ten woody species from the natural lowland dipterocarp forests in Sabah (North Borneo, Malaysia) and one exotic tree species were studied in the field. The indigenous species represent different ecological niches and successional stages in the various layers of the dipterocarp forest, such as pioneers, trees of the understorey or main canopy and emergents. Diurnal changes in CO₂ exchange and transpiration reflected primarily differences in irradiance. The diurnal courses of water potential mainly tracked the rate of transpiratory water loss. Light-dependency describes most of the diurnal variations of leaves’ gas exchange. Light response curves of net photosynthesis of the investigated species of the Dipterocapaceae were almost equal (light saturated assimilation rate, A_max: 5.0–7.2 μmol CO₂ m^–2 s^–1), while those of the other species exhibited remarkable differences (A_max: 5.5–14.2 μmol CO₂ m^–2 s^–1). Leaf area, chlorophyll content and specific leaf dry weight as the reference parameters for assimilation gave a general ranking of the A_max, which is highest for the pioneering species, less for the understorey trees and lowest for emergents. Light compensation points and light saturation of net photosynthesis were attained mainly between 6 and 9 μmol photons m^–2 s^–1 and between 230 and 534 μmol photons m^–2 s^–1, respectively, but were higher for pioneering species. Photosynthetic performance may be a diagnostic feature of the successional and ecological status of species, i.e. to characterize pioneering species from understorey species or from emergents of the dipterocarp forest. Received: 3 March 1997 / Accepted: 15 December 1997     

Stomatal behavior and photosynthetic performance under dynamic light regimes in a seasonally dry tropical rain forest

Rates of photosynthetic induction upon exposure to high light and rates of induction loss after darkening the leaf were measured in the field for four species of tropical shrubs in the family Rubiaceae. During wet season mornings, stomatal conductance (g _s) in the shade prior to induction was generally high enough so that the time course of induction was determined primarily by rates of activation of biochemical processes. During wet season afternoons, however, g _s values in the shade tended to be considerably lower and photosynthetic induction following a light increase exhibited a slower time course. In the afternoon, the time course of induction was determined by a combination of stomatal opening time and the rates of activation of light regulated enzymes. Stomatal behavior was also correlated with patterns of induction loss following a transfer from high light to darkness. In the afternoon, maximum g _s was lower for all species, and for a given time in the darkness, leaves showed a greater loss of induction in the afternoon than in the morning. During the dry season, maximum g _s and average values for g _s in the shade were reduced in all species. Along with these shifts in stomatal behavior, reduced rates of photosynthetic induction were observed. In the high-light species, the lower maximum g _s values observed during the dry season were also correlated with increased induction loss for a given time in the darkness. For all species, stomatal behavior was affected by season and time of day and, with the exception of wet season mornings, stomata appeared to exert significant control over rates of induction and patterns of induction loss. The results of simulation modeling suggest that the observed seasonal and diurnal changes in rates of induction and induction loss can have significant consequences on sunfleck carbon gain under a dynamic light regime. Received: 17 March 1999 / Accepted: 26 October 1999     

Buttress form of the central African rain forest tree Microberlinia bisulcata, and its possible role in nutrient acquisition

Buttressing is a trait special to tropical trees but explanations for its occurrence remain inconclusive. The two main hypotheses are that they provide structural support and/or promote nutrient acquisition. Studies of the first are common but the second has received much less attention. Architectural measurements were made on adult and juvenile trees of the ectomycorrhizal species Microberlinia bisulcata, in Korup (Cameroon). Buttressing on this species is highly distinctive with strong lateral extension of surface roots of the juveniles leading to a mature buttress system of a shallow spreading form on adults. This contrasts with more vertical buttresses, closer to the stem, found on many other tropical tree species. No clear relationship between main buttress and large branch distribution was found. Whilst this does not argue against the essential structural role of buttresses for these very large tropical trees, the form on M. bisulcata does suggest a likely second role, that of aiding nutrient acquisition. At the Korup site, with its deep sandy soils of very low phosphorus status, and where most nutrient cycling takes place in a thin surface layer of fine roots and mycorrhizas, it appears that buttress form could develop from soil-surface root exploration for nutrients by juvenile trees. It may accordingly allow M. bisulcata to attain the higher greater competitive ability, faster growth rate, and maximum tree size that it does compared with other co-occurring tree species. For sites across the tropics in general, the degree of shallowness and spatial extension of buttresses of the dominant species is hypothesized to increase with decreasing nutrient availability.     

Acclimation to changing light conditions of long-term shade-grown beech (Fagus sylvatica L.) seedlings of different geographic origins

 Effects of changing light conditions on the ecophysiological condition behind survival were examined on beech from two different populations. Plants were grown in a greenhouse under simulated understorey and canopy gap light conditions. Upon exposure to high light maximum photosynthesis of shade-acclimated leaves increased followed by a reduction over several days to between high- and low-light control rates. In the reciprocal transfer, the decrease in maximum photosynthesis was rapid during the first 2–3 days and then levelled off to values comparable to low-light controls. Seedlings from Sicily (Madonie) showed generally higher maximum photosynthetic rates than those from Abetone. Leaf conductance varied in the same direction as photosynthesis in high- to low-light seedlings but to a lesser degree. Leaves grown under low light and exposed to high light experienced photoinhibition. The Abetone population was more susceptible to photoinhibitory damage than the seedlings from Sicily. Exposure to high light of shade-acclimated seedlings resulted in intermediate chlorophyll concentrations between levels of the high-light and low-light seedlings. Carotenoid concentration was unaffected by treatments. Seedlings grew more in high light, but had a lower leaf area ratio. Light-limited seedlings showed a shift in carbon allocation to foliage. Leaves formed in the new light regime maintained the same anatomy that had been developed before transfer. Seedlings from Sicily had thicker leaves than those of seedlings from Abetone. Seedlings from Abetone were found to be more susceptible to changing light conditions than seedlings from Sicily. We conclude that small forest gaps may represent a favorable environment for photosynthesis and growth of beech regeneration as a result of the limited ability of seedlings to acclimate to sudden increases in high irradiance and because of the moderate levels of light stress in small gaps. Received: 11 April 1997 / Accepted: 11 December 1997     

Arbuscular mycorrhizal fungi in the tree seedlings of two Australian rain forests: occurrence, colonization, and relationships with plant performance

The roots of rain forest plants are frequently colonized by arbuscular mycorrhizal fungi (AMF) that can promote plant growth in the nutrient poor soils characteristic of these forests. However, recent studies suggest that both the occurrence of AMF on rain forest plants and the dependence of rain forest plants on AMF can be highly variable. We examined the occurrence and levels of AMF colonization of some common seedling species in a tropical and a subtropical rain forest site in Queensland, Australia. We also used a long-term database to compare the growth and mortality rates of seedling species that rarely formed AMF with those that regularly formed AMF. In both forests, more than one-third of the seedling species rarely formed AMF associations, while 40% of species consistently formed AMF in the tropical site compared to 27% in the subtropical site. Consistent patterns of AMF occurrence were observed among plant families at the two sites. Variation among seedling species in AMF occurrence or colonization was not associated with differences in seed mass among species, variation in seedling size and putative age within a species, or lack of AMF inoculum in the soil. Comparisons of four seedling species growing both in the shaded understory and in small canopy gaps revealed an increase in AMF colonization in two of the four species in gaps, suggesting that light limitation partially explains the low occurrence of AMF. Seedling survival was significantly positively associated with seed biomass but not with AMF colonization. Furthermore, seedling species that regularly formed AMF and those that did not had similar rates of growth and survival, suggesting that mycorrhizal and nonmycorrhizal strategies were equivalent in these forests. Furthermore, the high numbers of seedlings that lacked AMF and the overall low rate of seedling growth (the average seedling required 6 years to double its height) suggest that most seedlings did not receive significant indirect benefits from AMF through connection to canopy trees via a common mycorrhizal network.     

Effects of blue light deficiency on acclimation of light energy partitioning in PSII and CO2 assimilation capacity to high irradiance in spinach leaves

Blue light effects on the acclimation of energy partitioningcharacteristics in PSII and CO₂ assimilation capacity in spinachto high growth irradiance were investigated. Plants were grownhydroponically in different light treatments that were a combinationof two light qualities and two irradiances, i.e. white lightand blue-deficient light at photosynthetic photon flux densities(PPFDs) of 100 and 500 µmol m^–2 s^–1. The CO₂assimilation rate, the quantum efficiency of PSII (PSII) andthermal dissipation activity / in young, fully expanded leaves were measured under 1,600 µmol m^–2 s^–1white light. The CO₂ assimilation rate and PSII were higher,while / was lower in plants grown under high irradiancethan in plants grown under low irradiance. These responses wereobserved irrespective of the presence or absence of blue lightduring growth. The extent of the increase in the CO₂ assimilationrate and PSII and the decrease in / by high growth irradiance was smaller under blue light-deficient conditions. These resultsindicate that blue light helps to boost the acclimation responsesof energy partitioning in PSII and CO₂ assimilation to highirradiance. Similarly, leaf N, Cyt f and Chl contents per unitleaf area increased by high growth irradiance, and the extentof the increment in leaf N, Cyt f and Chl was smaller underblue light-deficient conditions. Regression analysis showedthat the differences in energy partitioning in PSII and CO₂assimilation between plants grown under high white light andhigh blue-deficient light were closely related to the differencein leaf N.     

Crown light environments of saplings of two species of rain forest emergent trees

Summary The crown light environments of saplings of two Costa Rican rain forest tree species were simultaneously compared. The species, Dipteryx panamensis (Pitt.) Record & Mell., a relatively shade-intolerant species, and Lecythis ampla Miers, a shade-tolerant species, have contrasting growth and branching patterns. Quantum sensors were placed throughout the crowns of saplings up to 2.5 m tall and quantum fluxes were recorded with microloggers for seven-day periods. The shade-intolerant species had total quantum flux densities 35% larger than those of the shade-tolerant species, but totals for both species were less than 2% of full sun. More than 90% of the quantum flux densities measured within the crowns of both species were less than 25 mol m^-2s^-1. Lateral light was an important component of daily quantum flux totals; for saplings of both species, the half-hour with the maximum average irradiance for the day frequently occurred in mid-morning or midafternoon. Despite dissimilar crown and leaf display, there was no difference in the overall variability of irradiance within the crowns of the two species. However, quantum fluxes received within the crowns differed substantially in both species. Within-crown locations differed significantly from day to day because of variation in weather conditions. Daily total quantum flux densities and totals expressed as a percent of full sun were significantly correlated with height growth over the previous 12 months.     

Diurnal patterns of chlorophyll a fluorescence and stomatal conductance in species of two types of coastal tree vegetation in southeastern Brazil

 Diurnal measurements of chlorophyll fluorescence parameters and stomatal conductance to water vapour were carried out in five woody species and a bromeliad from two coastal vegetation types in Brazil, the rock outcrop of Pao de Acúcar and the sand dunes of Macaé. The environmental conditions of both study sites are characterized by high temperatures and light levels. The studied species comprised C₃ plants with different degrees of stomatal closure during the day, overall daily stomatal conductance and a plant with a typical CAM pattern. Plants on Pao de Acúcar exhibited only a small decline in potential quantum yield throughout the day. The non-photochemical quenching and the approximate photosynthetic electron transport rates were maximal during the peak of irradiance. In Macaé, light response curves of fluorescence parameters in the CAM-tree Clusia hilariana showed a clear differentiation between phases III and IV of CAM. In phase III, decarboxylation of organic acids probably maintained high internal CO₂ levels and there was only a small decrease in photochemical quenching with saturating light levels. In phase IV, the depletion of the organic acid pool and low stomatal conductances resulted in much lower levels of effective quantum yield and a substantial increase in both 1-q_P and non-photochemical quenching with increasing light levels. This behaviour during phase IV in the CAM-tree C. hilariana was comparable to the performance of the C₃-Clusia C. aff parviflora at Pao de Acúcar. It is seen that both the C₃ species and the CAM tree occurring in the two coastal communities effectively adjust their photochemical efficiency to environmental conditions and especially to diurnal variations of stress. Received: 29 April 1996 / Accepted: 14 November 1996     

Artificially increased ascorbate content affects zeaxanthin formation but not thermal energy dissipation or degradation of antioxidants during cold-induced photooxidative stress in maize leaves

Infiltrating detached maize (Zeamays L.) leaves with L-galactono-1,4-lactone (L-GAL) resulted in a 4-fold increase in the content of leaf ascorbate. Upon exposure to high irradiance (1000 μmol photons m⁻² s⁻¹) at 5 °C, L-GAL leaves de-epoxidized the xanthophyll-cycle pigments faster than the control leaves; the maximal ratio of de-epoxidized xanthophyll-cycle pigments to the whole xanthophyll-cycle pool was the same in both leaf types. The elevated ascorbate content, together with the faster violaxanthin de-epoxidation, did not affect the degree of photoinhibition and the kinetics of the recovery from photoinhibition, assayed by monitoring the maximum quantum efficiency of photosystem II primary photochemistry (F_v/F_m). Under the experimental conditions, the thermal energy dissipation seems to be zeaxanthin-independent since, in contrast to the de-epoxidation, the decrease in the efficiency of excitation-energy capture by open photosystem II reaction centers (F_v′/F_m′) during the high-irradiance treatment at low temperature showed the same kinetic in both leaf types. This was also observed for the recovery of the maximal fluorescence after stress. Furthermore, the elevated ascorbate content did not diminish the degradation of pigments or α-tocopherol when leaves were exposed for up to 24 h to high irradiance at low temperature. Moreover, a higher content of ascorbate appeared to increase the requirement for reduced glutathione. Received: 20 May 1999 / Accepted: 29 October 1999