Changes In Leaf Anatomy Research Articles

Alterações morfofisiológicas em folhas de Coffea arabica L. cv. "Oeiras" sob influência do sombreamento por Acacia mangium Willd

Diferenças na disponibilidade de radiação podem causar modificações na estrutura e função das folhas do cafeeiro, que podem responder de maneira diferencial à radiação por alterações morfológicas, anatômicas, de crescimento e na taxa fotossintética. O objetivo deste trabalho foi avaliar características morfofisiológicas de cafeeiros (Coffea arabica L. cv. "Oeiras") sombreados por acácia (Acacia mangium Willd.) na época seca e chuvosa no sul de Minas Gerais. As maiores taxas fotossintéticas e maiores espessuras da epiderme adaxial foram observadas na estação chuvosa nas linhas de cafeeiros a pleno sol. O sombreamento influenciou em menor espessura das folhas e em espaços intercelulares maiores no tecido esponjoso. Foi também verificada mudança na forma dos cloroplastos, os quais apresentaram-se mais alongados em folhas de cafeeiros a pleno sol quando relacionados aos arborizados.

Leaf anatomical responses to periodical waterlogging in simulated semidiurnal tides in mangrove Bruguiera gymnorrhiza seedlings

Leaf anatomical changes of Bruguiera gymnorrhiza (L.) Lamk seedlings grown in experimental equipment that simulated semidiurnal tides with salinities of 15‰ under greenhouse conditions were studied. Compared with the 0 h treatments, leaf thickness, palisade parenchyma thickness, spongy parenchyma thickness, palisade–spongy thickness ratio, xylem length of the vascular system and number of vessels and vessel lines under the 12 h treatments declined 31.9%, 59.1%, 21.7%, 47.1%, 48.9%, 67.1% and 51.6%, respectively. However, the upper and lower epidermis to leaf thickness ratio, upper and lower hypodermis to leaf thickness ratio and stomatal density of 12 h treatments showed increases of 47.9%, 50.9%, 14.3%, 21.4% and 104.3% over those of 0 h treatments, respectively. The cuticle to leaf thickness ratio (inundated for 0–6 h) decreased significantly with waterlogging duration at first and then increased. Moreover, the percentage of intercellular spaces in spongy tissue decreased from 4 to 10 h treatment and then tended to increase by nearly 20% in the 12 h treatment. Tannin cells that were distributed in the vascular tissue, crystalliferous cells and phloem fibers were more abundant in the short-duration waterlogging treatments than in the long-duration waterlogging treatment. It was concluded that significant changes in the leaf anatomical features as a result of periods of immersion would have come at the cost of reduction of photosynthesis and water transport when waterlogging duration was longer than 2 h. These anatomical characteristics further proved that B. gymnorrhiza had a relatively low tolerance to waterlogging at the seedling stage.

Ontogenetic transition of leaf physiology and anatomy from seedlings to mature trees of a rain forest pioneer tree, Macaranga gigantea

In a field study, we compared anatomy and diurnal gas exchange and chlorophyll fluorescence in sunlit mature leaves of Macaranga gigantea (Reichb. f. and Zoll.) Muell. seedlings, saplings, an adult tree and suckers originating from stumps. We tested the hypothesis that the pattern of resource use shifts across various life stages with ontogenetic changes in leaf anatomy and physiology. Among leaves of different developmental stages, seedling leaves were the smallest and thinnest, whereas adult tree leaves were the largest and thickest, and the air space within the lamina was largest in seedling leaves and smallest in adult tree leaves. Photosynthetic nitrogen-use efficiency (PNUE) was higher in seedling and sapling leaves than in adult tree leaves. Mean PNUE in seedling leaves was 1.6 times that in adult tree leaves. Nevertheless, among the developmental stages, net photosynthetic rate (Pn) per unit leaf area was lowest in seedling leaves because they have the lowest nitrogen (N) content per unit leaf area. In situ water vapor stomatal conductance (g(s) at a given leaf-to-air vapor pressure deficit was highest in sapling leaves, suggesting that they have a high hydraulic efficiency per unit leaf area. Among developmental stages, intrinsic water-use efficiency (Pn/g(s)) and photochemical capacity of photosystem II were lowest in seedling leaves. Sapling leaves had the highest N concentration and Pn per unit dry mass and the highest g(s), indicating that the gradual transition from the seedling stage to the sapling stage is accompanied by an accumulation of N in plant bodies and the development of hydraulic systems to counteract unfavorable environmental stresses. The properties of adult tree leaves (low PNUE, high carbon:N ratio, small and dense cells and thick lamina) indicate that, during the transition from the sapling stage to the adult tree stage, the priority of resource use in leaves gradually shifts from enhancement of photosynthetic performance to defense against herbivory and mechanical damage. Leaf morphology and physiology were coordinated with the differences in resource use at each life stage.

NUTRITIONAL STATUS, GROWTH, CO2 ASSIMILATION, AND LEAF ANATOMICAL RESPONSES IN TWO KIWIFRUIT SPECIES UNDER BORON TOXICITY

ABSTRACT Two Actinidia species [Actinidia deliciosa (A.Chev.) C.F. Liang et A.R. Ferguson var. deliciosa cv. Hayward] and (Actinidia arguta Planch.) were grown in a sand-perlite mixture (1 : 1) in a growth room and irrigated with Hoagland's nutrient solutions containing 20, 50, 100, 200, and 500 μM boron (B). Growth, leaf ion concentration, photosynthetic rate and B induced changes in leaf anatomy were investigated. The greatest shoot length of the two species was recorded with 50 μM B. Boron concentration was highest in the leaf margin, intermediate in the remaining leaf blade and minimum in petioles. Boron toxicity induced a decrease of photosynthetic rate (Pn), of the volume of mesophyll cells, an increase of the volume of intercellular spaces and cell damage. Inclusion of 500 μM of B in the nutrient solution decreased calcium (Ca) and manganese (Mn) concentrations in kiwifruit leaves but did not affect the concentration of other mineral nutrients. Finally, the two species did not differ significantly in leaf B accumulation and sensitivity to B.

Effects of relative irradiance on the leaf structure of Fagus sylvatica L. seedlings planted in the understory of a Pinus sylvestris L. stand after thinning

Beech seedlings were established in the understory of a Pinus sylvestris plantation close to one of the southernmost popula- tions of beech in Europe, the beech-oak forest of Montejo de la Sierra. Four years later, the overstory was partially reduced by removing pine trees. Solar radiation in the understory was evaluated by hemispherical canopy photographic technique and the effects of relative ir- radiance increment on the leaf anatomy of beech seedlings were analyzed during the two years after opening the stand. The increase in specific leaf mass (SLM) in seedlings during both years runs in parallel with the increase in relative irradiance estimated by the global light factor (GLF) which expresses the proportion of global radiation relative to that in the open. There were significant relationships bet- ween the light index as a surrogate of light environment and the morphological and anatomical characteristics of the leaves. In the first year, SLM increase was more related to total blade thickness. In the second year, thickness of palisade parenchyma (PP) appears more re- levant than that of spongy tissue (SP) as indicated by the absence of significance in the relationship between SP and SLM. Moreover, sto- matal density was also higher according to increasing relative irradiance. The shift response of beech seedlings to the overstory opening makes evident their capability of acclimatization to light increase through changes in leaf anatomy. Fagus sylvatica / morphology leaf / hemispherical photography / regeneration / shelterwood

Effects of leaf age on internal CO2 transfer conductance and photosynthesis in tree species having different types of shoot phenology

We examined the changes in leaf anatomy and some physiological characteristics during leaf expansion and maturation. Three deciduous tree species having different types of shoot phenology, maple (Acer mono Maxim.; ‘flush’ type), alder (Alnus japonica(Thunb.) Steud.; ‘successive’ type), and Japanese poplar (Populus maximowiczii A. Henry; ‘successive’ type), were studied. Leaf CO 2 assimilation rate at high irradiance (P max) and CO 2 transfer conductance inside the leaf (g i) varied significantly with leaf development. There were strong positive relationships between P max) and g i for all of the species. The variations in g i were partly related to those in the surface area of chloroplasts facing the intercellular airspaces, while some other factors that related to liquid phase conductance may also contribute to the variation in g i . The developments of mesophyll cells were accompanied by the concomitant increase in chloroplast and Rubisco content in Alnus and Populus (successive types).

CHANGES IN LEAF MORPHOLOGY AND ANATOMY OF IN VITRO-CULTURED CHESTNUT PLANTLETS DURING ACCLIMATIZATION

CHANGES IN LEAF MORPHOLOGY AND ANATOMY OF IN VITRO-CULTURED CHESTNUT PLANTLETS DURING ACCLIMATIZATION

Changes in leaf structure in relation to crown position and tree size of Betula papyrifera within fire-origin stands of interior cedar-hemlock

Dimensions of anatomical and morphological attributes of leaves can influence physiological response to changes in environment over time. Linking structural attributes of leaves to crown position and tree size within naturally developing cohorts of trees can provide a clearer understanding of changes in crown morphology for a species. This study examined leaf anatomy and morphology of Betula papyrifera Marsh. growing in two stages of stand development of interior cedar-hemlock forest, northern British Columbia. Anatomical and morphological measurements of leaves located at six different positions within the crown were made on trees selected from stands that originated 15 years (small tree size) and 145 years (large tree size) after catastrophic fire. Leaf area and mass were measured in the field. Microscopic measures were made in the laboratory of thicknesses of leaves, cuticle, upper and lower epidermis, palisade, and spongy mesophyll. Stomatal density and stomatal aperture lengths were also determined. Leaf anatomy and morphology varied significantly with position in the crown and among size-classes of trees. Changes in leaf anatomy observed among positions within the crown reflect the changing availability of light and moisture experienced during crown development. For both size-classes of tree the largest anatomical dimensions of leaves were at the outer and uppermost parts of the crown, whilst the smallest were at the lower and innermost parts. Foliage of large trees (145 years) exhibit leaf attributes characteristic of the sun-shade dichotomy reported in the literature, but this was not shown for foliage of young saplings (15 years). For the small trees (15 years) the largest leaves were located at the top of the crown while the smallest were located at the bottom. This has been reported for many tropical pioneers but is in contrast to the usual sun-shade dichotomy of temperate pioneers. The large trees (145 years) followed the typical pattern with the smallest leaves at the top of the crown. Information from this study contributes to our understanding of foliar development of tree crowns by demonstrating how leaf structure changes with crown position and tree size under stand competition.Key words: Betula papyrifera, British Columbia, crown development, cuticle thickness, leaf area, palisade mesophyll, paper birch, stand dynamics.

Estimation of the anatomical, stomatal and biochemical components of differences in photosynthesis and transpiration of wild‐type and transgenic (expressing yeast‐derived invertase targeted to the vacuole) tobacco leaves

ABSTRACTPhotosynthesis and transpiration rates of transgenic (expressing yeast‐derived invertase targeted to the vacuole) tobacco (Nicotiana tabacum L.) leaves were, respectively, 50 and 70% of those of a wild type at 20°C, 350 cm3 m−3 CO2 concentration, 450 μmol (photons) m−2 s−1 of light intensity, and 70% relative air humidity. These differences could be attributed: (a) to changes in leaf anatomy and, consequently, to changes in gases diffusion between the cells' surfaces and the atmosphere; (b) to different stomatal apertures, and, for the photosynthesis rate, (c) to the altered CO2 assimilation rate. Our objective was to estimate the relative contributions of these three sources of difference. Measurements on the wild‐type and the transgenic leaf cross‐sections gave values for the cell area index (CAI, cell area surface per unit of leaf area surface) of 15.91 and 13.97, respectively.The two‐dimensional model 2DLEAF for leaf gas exchange was used to estimate quantitatively anatomical, stomatal and biochemical components of these differences. Transpiration rate was equal to 0.9 for the wild‐type and to 0.63 mmol m−2 s−1 for the transgenic leaf: 24.0% of the difference (0.066 mmol m−2 s−1 was caused by the greater cell area surface in the wild‐type leaf, and 66.0% was caused by a smaller stomatal aperture in the transgenic leaf. Photosynthetic rate was 3.10 and 1.55 μmol m−2 s−1 for the wild‐type and transgenic leaves, respectively. Only 10.3% of this difference (0.16 μmol m−2 s−1) was caused by the difference in CAI, and the remaining 89.7% was caused by altered CO2 assimilation rate.

Impact of Water Stress on Leaf Anatomy and Ultrastructure in Cynodon dactylon (L.) Pers. under Natural Conditions

Seasonal changes in leaf anatomy and ultrastructure were studied in a sward of Cynodon dactylon (L.) Pers. grown under Mediterranean field conditions and under water stress. Relative water content (RWC), leaf water potential (ψ), and specific leaf weight were determined. Anatomical measurements included leaf thickness and number and area of bundle sheath and mesophyll cells. Quantitative measurements from electron micrographs were used to evaluate the subcellular structure of mesophyll and bundle sheath cells from well-watered and water-stressed plants. In well-watered swards leaf mesophyll cell number was highest at the beginning of June, but in stressed plants the maximum was reached in the middle of July. Water stress decreased both mesophyll and bundle sheath cell areas during the experimental period, although there were no significant differences in plant water relations in autumn. Ultrastructural changes in bundle sheath cells under stress included increase in starch deposition in chloroplasts, changes in the orientation of thylakoids, and reduction in chloroplast area. Furthermore, water stress increased the cell wall thickness by 20%. Stressed mesophyll chloroplasts were characterized by an increase in the peripheral reticulum and in starch granules and a decrease in the amount of grana stacking related to a decrease in leaf sodium concentration. The number of mitochondria per mesophyll cell was increased by water stress. Longer periods of stress induced folds in the outer tangential walls in bundle sheath cells. In mesophyll cells alterations in cellular shape and in plasmalemma were observed, and the cytosol appeared to be markedly heterogeneous. In late summer the chloroplast envelope was swollen and distorted; undulating dilated thylakoids were observed and the stroma appeared less electron dense. In mitochondria, the matrix became progressively clearer. Environmental stresses influenced leaf anatomy and cell ultrastructure. Furthermore, ultrastructural changes could be related to variations in potassium and sodium contents.

Effects of soil resistance to root penetration on leaf expansion in wheat (Triticum aestivumL.): composition, number and size of epidermal cells in mature blades

Wheat seedlings (Triticum aestivum L.) were grown on soils with contrasted resistances to root penetration (measured as penetrometer resistance, R s ). High R s reduced the rates of leaf appearance and expansion. Although the duration of expansion was increased, mature leaves were smaller. Underlying changes in leaf anatomy were investigated on cleared mature leaves, focusing on the epidermes. Three leaves were analysed: leaves 1 and 3 which started their development in the embryo, and leaf 5 which was initiated on the seedling, after imposition of contrasted soil conditions. In all leaves, high R s caused a reduction in mature cell sizes, lengths and widths, and a shift in the relative proportions of functionally different cell types, with a decrease in the relative proportions of stomata and associated cell types (interstomatal and sister cells) and an increase in the proportions of unspecialized elongated epidermal cells and of trichomes. In leaves 3 and 5 the number of cellular files across the blade was also reduced, while in leaf 1 it was similar at the two R s . These differences between leaves are attributed to differences in their developmental stage when root stress was first perceived. Remarkably, R s had no effect (leaf 1) or relatively small effects (leaves 3 and 5) on the total number of cells per file, suggesting that this parameter is either largely insensitive to variation in root environment, or is programmed at the outset before stress was perceived at the apex.

Significant Changes in Cell and Chloroplast Development in Young Wheat Leaves (Triticum aestivum cv Hereward) Grown in Elevated CO2.

Cell and chloroplast development were characterized in young Triticum aestivum cv Hereward leaves grown at ambient (350 [mu]L L-1) or at elevated (650 [mu]L L-1) CO2. In elevated CO2, cell and chloroplast expansion was accelerated by 10 and 25%, respectively, in the first leaf of 7-d-old wheat plants without disruption to the leaf developmental pattern. Elevated CO2 did not affect the number of chloroplasts in relation to mesophyll cell size or the linear relationship between chloroplast number or size and mesophyll cell size. No major changes in leaf anatomy or in chloroplast ultrastructure were detected as a result of growth in elevated CO2, but there was a marked reduction in starch accumulation. In leaf sections fluorescently tagged antisera were used to visualize and quantitate the amount of cytochrome f, the [alpha]- and [beta]-subunits of the coupling factor 1 in ATP synthase, D1 protein of the photosystem II reaction center, the 33-kD protein of the extrinsic oxygen-evolving complex, subunit II of photosystem I, and ribulose-1,5-bisphosphate carboxylase/oxygenase. A significant finding was that in 10 to 20% of the mesophyll cells grown in elevated CO2 the 33-kD protein of the extrinsic oxygen-evolving complex of photosystem II and cytochrome f were deficient by 75%, but the other proteins accumulated normally.

Nitrogen and irradiance levels affecting net photosynthesis and growth of young coffee plants (Coffea arabicaL.)

SummaryShoot growth attributes, leaf anatomical changes and net photosynthetic rates were determined in young coffee plants fertilized at high and low levels and subjected to shade or full sunlight. High nitrogen supply increased plant height irrespective of the light level imposed during growth, and promoted a larger leaf area in plants of the shade treatment. Specific leaf weight was 15% higher in plants grown in full sunlight than in shaded plants, at both nitrogen levels. The number of leaves developed on the orthotropic stem increased significantly under full irradiance and high nitrogen availability. Leaves were 11% thicker ih unshaded plants than in shaded ones, because of the increased size of the palisade and spongy parenchyma tissues. More thylakoids per granum and more grana per chloroplast were observed in shade-grown plants, mainly in those given high nitrogen treatment. Total nitrogen content expressed on a leaf-area basis was higher in sun plants supplemented with nitrogen, whereas chlorophyll a and b and protochlorophyll contents increased in shade-grown plants. Net photosynthetic rate in high nitrogen plants reached 7.19 µmol CO2 m−2 s−1 in the full irradiance treatment, and 5.46 µmol CO2 m−2 s−1 in shaded plants. Maximum net photosynthetic rates in the low nitrogen plants were 5.28 and 4.90 µmol CO2 m−2 s−1 in sun and shade plants, respectively. Increased photosynthetic light saturation was observed in high irradiance plants (628 µmol m−2 s−1) relative to shade plants (359 µmol m−2 s−1) in the high nitrogen treatment. The same pattern was observed in low nitrogen plants. The apparent quantum yield of shade acclimated plants was 14% higher than in those grown in full sunlight. Our results indicate that coffee may be classified as a facultative shade species, exhibiting features of sun adapted plants coupled with shade acclimation attributes, this fairly high adaptive capacity to shade:sun transition being strongly influenced by the level of nitrogen nutrition given to the plants.

EFFECTS OF PEG-INDUCED WATER STRESS ON MICROPROPAGATED GRAPES: EPICUTICULAR WAX, STOMATAL NUMBER, AND SURVIVAL EX VITRO

Micropropagated grapes (Vitis sp. `Valiant') were subjected to water stress while rooting with the addition of 2% (w/v) PEG 8000. PEG-treated plantlets exhibited reduced growth, as compared to control (in vitro, no PEG), but developed greater leaf epicuticular wax. PEG-treated plantlets had three times the wax level of control. Although treated plantlets showed changes in leaf anatomy, no effect on stomatal frequency or stomatal index was evident. Differences in epidermal cell configuration were also observed among leaves from different treatments. PEG-treated plantlets resembled those grown in the greenhouse, morphologically and anatomically, and exhibited a higher survival rate than control upon transfer to the greenhouse.

The Effect of Neutral Shade on the Structure of Mature, Sun-Acclimated Palisade Cells of Helianthus annuus L. II. 3-Day Exposure

Portions of mature leaves of Helianthus annuus var. Mammoth, field grown under full sunlight (FS), were shaded for 3 days at 40%, 25%, or 0% FS. Nonshaded leaf areas served as FS controls. The palisade cell structure of shaded and control leaf areas was analzyed stereologically. There were no detectable shade effects on the volume density values of the major organelle compartments except for a decrease in the starch compartment. The surface density values of chloroplast thylakoid membranes were significantly greater in shaded than in nonshaded chloroplasts at all levels of shading. The proportion of granal thylakoid surface area was greater than that for stromal thylakoids in shaded compared to nonshaded chloroplasts. There were no discernible shade effects on mean palisade cell volume. The total chlorophyll (chl)/mm3 of leaf and chl a/b ratios were lower in the shaded leaves than in the controls except for the full-shade sample (0%), which had higher chl a/b ratios than the control. Thylakoid packing ratios for palisade cells (mm2 thylakoid/mm2 leaf surface) were greater in the shade than in sun areas for 25% and 0% FS. Considering the overall response of thylakoid surface areas and changes in leaf anatomy, there was a greater light-gathering potential in the shaded half of the leaf. Although some shade response was observed in 40% FS (688 μmol s-1 m-2), the most pronounced responses occurred at 25% FS (347 μmol s-1 m-2). Thus, the threshold light level for shade response in sunflowers is near the latter value. These studies also show that cells within the same leaf respond independently to local irradiance levels.

Effects of CO2Enrichment on Internal Leaf Surface Area in Soybeans

Internal cell surface areas were measured on fully expanded leaves of soybean seedlings that had been continuously exposed to 348 or 645 ppm CO2 environments. Plants grown in the high CO2 treatment had thicker leaves but less palisade cell surface area per unit leaf area. Surface area of the mesophyll per unit leaf area was unaffected by CO2. The potential ramifications of these CO2-induced changes in leaf anatomy on photosynthesis and water-use efficiency are explored.

Light Acclimation in Citrus Leaves. I. Changes in Physical Characteristics, Chlorophyll, and Nitrogen Content

Abstract ‘Duncan’ grapefruit (C. paradisi Macf.) and ‘Pineapple’ sweet orange (Citrus sinensis L.) seedlings were grown in full sunlight, 50% and 90% shade; maximum photosynthetic photon flux densities (PPFD) of 2300, 1100 and 200 μmol s−1m−2, respectively. In fully expanded matured (hardened) leaves, leaf thickness, specific leaf weight (SLW), tissue density, and nitrogen content were highest in full sun leaves and lowest in 90% shade leaves. Leaf chlorophyll content was highest in 90% shade leaves. Half of the seedlings which were grown in full sunlight were transferred into 50% shade to simulate normal canopy development; half of the seedlings from 50% and 90% shade were moved into full sunlight to simulate changes that occur after hedging. Specific leaf weight and tissue density changed in the same direction as PPFD. Leaf nitrogen content decreased temporarily when leaves were exposed to new PPFD conditions regardless of the PPFD levels. Total leaf chlorophyll content initially decreased when seedlings were transferred into full sunlight but began to increase after 4–6 weeks. Chlorophyll content increased in seedlings transferred from full sun to 50% shade. Percentage of air space within leaf tissues did not change during acclimation to new PPFD levels. Changes in leaf anatomy, physical characteristics, and chemical components are mechanisms that enable citrus leaves to acclimate to a wide range of changing light environments, even after leaves are fully mature.

Release od Soluble Protein and Nitrogen in Alfalfa. III. Influence of Shading1

Changes in leaf anatomy, morphology, and chemical composition due to different amounts of solar radiation could affect the apparent bloat potential of alfalfa (Medicago sativa L.) mediated via release of soluble protein into rumen fluid in the presence of ample available carbohydrates. Our objectives were to determine whether shading during growth would influence production characteristics as well as concentration of several N fractions and total nonstructural carbohydrates (TNC) released from alfalfa.Alfalfa was shaded with saran shades [0 (control), 47, and 73% shade] from 12 May to 1 June [Period (P)l], 30 June to 7 July (P2), and 9 to 16 August 1978 (P3). Total nitrogen (TN), total soluble N (TSN), soluble nonprotein N (SNPN), soluble protein N (SPN), total nonstructural carbohydrates (TNC) were determined.The 73% shade reduced yields 42% for P1, 30% for P2, and 24% for P3, while the 47% shade reduced yields 29% for P1, 22% for P2, and 10% for P3 compared to the respective control yields. In P1 (20 days of shade), concentration of SNPN in herbage was 10.1 mg/g dry wt for control, 10.5 mg for 47% shade, and 11.5 mg for 73% shade, while that of SPN was 14.8 mg for control, 13.8 mg for 47% shade, and 12.6 mg for 73% shade.In P2 (7 days of shade), concentrations of TN, TSN, and SNPN in herbage increased as shade increased. Shade did not significantly influence the herbage concentrations of SPN in P2. In P3 (7 days of shade), shade did not significantly affect herbage concentrations of any of the N fractions. Shade of 73% reduced the TNC concentrations in herbage 16% for P1, 44% for P2, and 19% for P3, while shade of 47% recuded TNC 8% for P1, 28% for P2, and 10% for P3, compared to that in control herbage.Reduced apparent bloat potential of alfalfa, as estimated by SPN release, occurred only in plants grown under low solar radiation for a continuous 20‐day period. However, both short and long periods of decreased solar radiation caused decreased TNC concentrations in herbage. Because an abundant supply of TNCis essential for the microbial activity that leads to gas production in the rumen, even relatively short periods of shade could be considered as potentially bloat suppressing. We conclude that the usual incidence of cloudy weather is not likely to be a major mediator of bloat incidence in ruminants that graze alfalfa, although periods of high solar radiation maybe considered bloat‐promoting rather than bloatsuppressing.

Comparative photosynthesis of three gap phase successional tree species.

Photosynthesis was measured in situ on trees growing in an open, gap-like site and under a closed canopy. Photosynthetic responses also were monitored on trees grown in the laboratory under either a high or low light regime or on those trees transferred from a low to a high light regime. All three species studied, Liriodendron tulipifera, Acer rubrum and Cornus florida, were able to acclimate to a high light environment as evidenced by their higher photosynthetic rates. This acclimation was achieved by an increase in transfer conductance and was ultimately due to changes in leaf anatomy. Species avoided photo-bleaching primarily by changes in leaf orientation and canopy structure. Species-specific physiological responses to high light intensity may account for each species' known growth response following canopy opening.