Response of the Succulent Leaves of Peperomia magnoliaefolia to Dehydration: Water Relations and Solute Movement in Chlorenchyma and Hydrenchyma

Relative water content, solute concentrations, and osmolality were determined in the water storage tissue (hydrenchyma) and the assimilatory tissue (chlorenchyma) of the succulent leaves of Peperomia magnoliaefolia (Jac) (Piperaceae) during slow desiccation. Relative water loss was significantly greater for the hydrenchyma than for the chlorenchyma. When whole leaves had lost 50% of their initial water content, the concomitant decrease of the relative water content of the hydrenchyma was 75 to 85%, but of the chlorenchyma only 15 to 25%. In spite of this differential water loss, the osmolality in both tissues increased to the same extent, indicating solute flow from the hydrenchyma to the chlorenchyma during desiccation. Solute translocation appeared to be unspecific, probably reflecting symplastic mass flow from one tissue to the other. The observed volume preservation of the chlorenchyma stabilized photosynthesis of Peperomia magnoliaefolia (Jac) leaves, which was less inhibited by a given decrease of the relative water content of the whole leaves than in nonsucculent leaves.     

Tissue Partitioning During Leaf Development in Ornamentally-grown Frithia pulchra (Mesembryanthemaceae), a 'Window Plant'

Young (i.e. 5-mm long) leaves of the window-plant Frithia pulchra(Mesembryanthemaceae) allocate approx, 21 % of their volumeto epidermis, 49 % to chlorenchyma, and 29 % to window tissue.By the time leaves are 25 mm long, the relative volumes of epidermisand chlorenchyma decrease to approx, 7 and 27 % respectively.During the same period, the relative volume of window tissueincreases from 29 to 66 %. The relative volumes of epidermis,window, and chlorenchyma tissues do not change as leaf lengthincreases from 25 to 57 mm. These results indicate that earlystages of leaf development in F. pulchra involve preferentialreallocations of volume to different tissues, whereas laterstages of leaf development involve uniform expansion of allof the leaf's tissues (i.e. the relative volumes of tissuesdo not change). The relative volumes of epidermis and windoware always largest in the lower third of a leaf. The relativevolume of chlorenchyma is largest in the upper third of youngleaves but becomes constant in the upper two thirds of leavesduring later stages of development. These results, indicatingthat leaves and tissues of F. pulchra are asymmetric and developpolarly, are discussed relative to corresponding studies ofcellular size and leaf structure. Frithia pulchra, leaf development, Mesembryanthemaceae, stereology, window plant     

Cellular Structure and Light Absorbtion in Leaves of Frithia pulchra (Mesembryanthemaceae)

In order to quantify the structural differences between celltypes of leaves from a ‘ window’ plant, an ultrastructuralmorphometric analysis was made of the epidermal, window andchlorenchyma tissues of Frithia pulchra. Epidermal cells arethe largest cells found in Frithia leaves and are characterizedby the presence of a thick outer tangential cell wall and numerousvacuolar inclusions. Epidermal tissue has an optical densityof 0.30. The transparent window tissue (i.e. optical density= 0.08) has a uniform ultrastructure throughout the length ofthe leaf. The vacuome comprises aproximately 97 per cent ofthe protoplasmic volume of window cells. Chlorenchyma cellspossess thin cell walls and are surrounded by numerous intercellularspaces. Cells of the apical chlorenchyma tissue possess approximately30 plastids per cell. These chloroplasts have an average individualvolume of 220 µm². Cells of the basal chlorenchyma tissuecontain chloroplasts that are five to six times smaller andmore numerous than those in cells of the apical chlorenchyma.The increased volume of chloroplasts in the apical comparedwith basal chlorenchyma cells (i.e. 31.4 and 20.2 per cent ofthe protoplasm, respectively) is positively correlated withtheir optical densities of 1.46 and 0.97, respectively. Frithia pulchra, stereology, leaf, light absorption, window plant     

Altered Synthesis and Composition of Cell Wall of Grape (Vitis vinifera L.) Leaves during Expansion and Growth-Inhibiting Water Deficits

The rate and composition of cell wall polysaccharide synthesisduring development and growth-inhibiting water deficits wereinvestigated in leaves of grape (Vitis vinifera L.). The rateof leaf expansion was monitored as plant water status was manipulatedby modulating the supply of irrigation water to potted plantsover several days. The corresponding wall synthesis was determinedby incubating leaf tissue with [¹⁴C]glucose and quantifyingincorporation into wall components. Samples were obtained fromrapidly expanding and mature leaves before, during, and following(recovery from) moderate water deficits. Uptake was approximately2-fold greater for mature leaf tissue than for rapidly expandingtissue at both high and low water status. In contrast, incorporationinto cell wall polysaccharides was 18 to 41% (under low andhigh water status) of uptake in expanding leaves but less than4% in mature tissue. Incorporation of precursor into wall polysaccharideswas insensitive to plant water status in mature leaves, butwas inhibited to less than 50% of well-watered controls in expandingleaves at low water potential. Incorporation of label into cellulose,uronic acid, and neutral sugar fractions was differentiallyaffected by water deficits, with cellulose synthesis apparentlyexhibiting the greatest sensitivity to low water status. Afterrewatering, growth, as well as uptake and incorporation of labelrecovered, although the latter did not attain prestress rates.The results indicate a high sensitivity of wall polysaccharide(particularly cellulose) synthesis to growth-inhibiting waterdeficits. ¹ Supported by United States Department of Agriculture, CompetitiveResearch grant GAM 8502539. (Received November 15, 1989; Accepted January 17, 1990)     

Water flow and water storage in Agave deserti: osmotic implications of crassulacean acid metabolism

Abstract Water flow and water storage were investigated for Agave deserti, a desert succulent showing crassulacean acid metabolism (CAM). The anatomy and water relations of the peripheral chlorenchyma, where CAM occurs, and the central water-storage parenchyma were investigated for its massive leaves so that these tissues could be incorporated as discrete elements into an electrical-circuit analogue of the whole plant. The daily cycling of osmotic pressure was represented by voltage sources in series with the storage capacitors. With soil water potential and leaf transpiration rate as input variables, axial water flow through the vascular bundles and radial flows into and out of storage during the day/night cycle were determined. The predominantly nocturnal transpiration was coincident with increases in cell osmotic pressure and in titratable acid of the leaf chlorenchyma. In the outer layers of the chlorenchyma, water potential was most negative at the beginning of the night when transpiration was maximum, while the water-storage parenchyma reached its minimal water potential 9 h later. The roots plus stem contributed 7% and the leaves contributed 50% to the total water flow during maximal transpiration; peak water flow from the soil to the roots occurred at dawn and was only 58% of the maximal transpiration rate. Over each 24-h period, 39% of the water lost from the plant was derived from storage, with flow into storage occurring mainly during the daytime. Simulations showed that the acid accumulation rhythm of CAM had little impact on water uptake from the soil under the conditions employed. In the outer chlorenchyma, water potential and water flows were more sensitive to the day/night changes in transpiration than in osmotic pressure. Nevertheless, cell osmotic pressure had a large influence on turgor pressure in this tissue and determined the extent to which storage was recharged during the latter part of the night.     

Development of biochemical specialization and organelle partitioning in the single-cell C4 system in leaves of Borszczowia aralocaspica (Chenopodiaceae)

The terrestrial plant Borszczowia aralocaspica (Chenopodiaceae) has recently been shown to contain the entire C(4) photosynthesis mechanism within individual, structurally and biochemically polarized chlorenchyma cells rather than in a dual cell system, as has been the paradigm for this type of carbon fixation (Nature 414: 543-546, 2001). Analysis of carbon isotope composition and (14)CO(2) fixation shows that photosynthesis and growth of B. aralocaspica occurs through carbon acquired by C(4) photosynthesis. The development of this unique single-cell C(4) system in chlorenchyma cells was studied by analysis of young (0.2-0.3 cm length), intermediate (ca. 0.5-0.6 cm length), and mature leaves (ca. 3 cm length). The length of chlorenchyma cells approximately doubles from young to intermediate and again from intermediate to the mature leaf stage. In young chlorenchyma cells, there is a single type of chloroplast; the chloroplasts are evenly distributed throughout the cytosol, and all contain starch and rubisco. During leaf development, the activities of phosphoenolpyruvate carboxylase (PEPC; which is cytosolic), rubisco, and pyruvate,Pi dikinase (PPDK) increase on a chlorophyll basis. As leaves mature, chloroplasts differentiate into two distinct structural and biochemical types that are spatially separated into the proximal and distal parts of the cell (the proximal end being closest to the center of the leaf). The early stages of this polarization are observed in intermediate leaves, and the polarization is fully developed in mature leaves. The chloroplasts in the distal ends of the cell have reduced grana and little starch, while those at the proximal ends have well-developed grana and abundant starch. In mature leaves, PPDK is expressed in chloroplasts at the distal end of the cells, while rubisco and adenosine diphosphate glucose (ADPG) pyrophosphorylase are selectively expressed in chloroplasts at the proximal end of the cell. Mitochondrial polarization also occurs during development as nicotinamide-adenine dinucleotide phosphate-malic enzyme (NAD-ME) and the photorespiratory enzyme glycine decarboxylase are expressed in mature but not young leaves and are localized in mitochondria at the proximal end of the cells. The data show that single-cell C(4) develops from a single pool of identical organelles that develop differential biochemical functions and spatial partitioning in the cell during maturation.     

Water Movement and Storage in a Desert Succulent: Anatomy and Rehydration Kinetics for Leaves of Agave deserti

Smith, J. A. C. and Nobel, P. S. 1986. Water movement and storagein a desert succulent: anatomy and rehydration kinetics forleaves of Agave deserti.—J. exp. Bot. 37: 1044–1053. Anatomic and kinetic aspects of water storage were investigatedfor the succulent leaves of the desert CAM plant, Agave deserti.An approximately linear relationship was found between the numberof vascular bundles and leaf surface area, both for leaves ofdifferent sizes and also along the length of a single leaf.The bundles, which were distributed throughout the leaf cross-section,were separated from each other by about eight water-storagecells. Even though the cell walls of the water-storage groundtissue made up only 2?5% of the cell volume, they provided about10% of the total cross-sectional area available for water transportradial to the xylem because cell-cell contact in such a directionaveraged 25% of the cell surface area. The rehydration kineticsof partially dehydrated leaf segments were resolved into threephases: (1) a relatively rapid movement into the vascular tissue(half-time of 2 min); (2) water movement into storage in theground tissue (half-time of 59 min); and (3) water movementinto the intercellular air spaces (half-time of about 10 h).Using the observed kinetics for water movement into the storagetissue and standard diffusion theory, the bulk-averaged diffusivityof water in the relatively homogeneous ground tissue (D₁) was2?0 ? 10^–10 m² s^–1 Using this (D₁) and pathway analysis,most of the water moving from the xylem into storage in themassive leaves of A. deserti apparently occurred from cell tocell across the cell membranes rather than through the cellwalls. Key words: Agave deserti, capacitance, diffusivity, leaf anatomy, succulence, water storage     

Responses of a CAM Plant to Drought and Rainfall: Capacitance and Osmotic Pressure Influences on Water Movement

Daily and seasonal patterns in water flow and water potentialwere investigated for the Crassulacean acid metabolism succulentAgave deserti during an extended summer drought and for a periodfollowing rainfall. Field measurements of transpiration andof osmotic pressure changes over selected 24 h periods wereused as input variables for a computer model of water flow thatwas based on an electrical circuit analog of the whole plant.Parameters such as root resistance and tissue capacitance werealso varied to reflect the effects of changing plant or soilwater status. The model predicted internal water flow and waterpotential during the drought cycle and was used to assess therole of tissue osmotic properties in water uptake from the soiland in internal water redistribution. For plants under wet soil conditions, 55% of the night-timetranspiration was derived from water storage, this storage beingrecharged during the day. As drought progressed, transpirationand the nocturnal increase in osmotic pressure declined, althoughthe osmotic pressure itself increased. The difference in osmoticpressure between the water storage tissue and the chlorenchymacaused a net flow of water into the chlorenchyma after 3 weeksof drought, thereby increasing chlorenchyma turgor pressure.Simulations also indicated that a large increase in root resistancemust occur to prevent substantial water loss from the plantto the dry soil. After rainfall, recharge of plant water storagewas complete within one week, although full recovery in theamplitude of daily osmotic pressure variations took longer. Key words: Agave deserti, transpiration, water potential, water storage     

Ochna pulchra Hook: Leaf Growth and Development Related to Photosynthetic Activity

Ochna pulchra Hook. is a deciduous broad-leaved tree in theMixed Bushveld vegetation of the Northern Transvaal. The growthand development of leaves taken from trees in the field werestudied from a stage shortly before bud break, in late spring,until they were fully expanded and at the peak of photosyntheticactivity. Leaf area was measured by photographing the leaf against a transparentmm² grid. Finally a constant relationship between leaf area(A) and the linear dimension of length (L) and breadth (B) wasestablished: A = b x LB, where coefficient b = 0.72. Transverse sections of the lamina of the youngest leaves showeda five-layered plate meristem with a few functional conductingelements in the midrib. During further leaf development, celldivision was followed by means of autoradiography using [³H]thymidine.It was most active during the week after bud break. Leaf cell increment following on cell division made the majorcontribution to leaf growth resulting in a lamina that was atleast 90% expanded 4 weeks after bud break. The histologicalchanges accompanying cell division were observed using lightand electron microscopy. Even in late stages of leaf development mature and differentiatingstomata occurred together, limited to the abaxial epidermisand the midrib. Scanning electron microscopy showed stomataldistribution, their increasing density and gradual opening.The structure of these sunken stomata could reduce the outwarddiffusion of water vapour and increase the diffusion resistanceto carbon dioxide. Carbon assimilation rates of the developing leaves were measuredusing an IRGA (infra-red gas analyser) and their chlorophyllvalues were calculated. Photosynthesis was first measured amonth after bud break when the leaves were fully expanded, over50 % of the stomata exposed and leaf mesophyll tissue differentiatedwith mature chloroplasts. Net photosynthetic rates and chlorophyllvalues peaked 1 month later. Ochna pulchra Hook., photosynthesis, leaf development, leaf area, stomata, chlorophyll, savanna     

Temperature and Water Relations for Sun and Shade Leaves of a Desert Broadleaf, Hyptis emoryi

The temperature and water relations of sun versus shade leavesof Hyptis emoryi Torr. were evaluated from field measurementsmade in late summer. Throughout most of the day sun leaves hadhigher temperatures and higher resistances to water vapour diffusion,but lower transpiration rates and lower stem water potentials,than did shade leaves. Leaf absorptivity to solar irradiationwas less for 1.5-cm-long sun leaves (0.44) than for 4.0-cm shadeleaves (0.56). For both leaf types the stomatal resistance increasedas the water vapour concentration drop from the leaf to theair increased. Energy balance equations were used together with the measuredtemperature dependence of photosynthesis to predict the effectof variations in leaf absorptivity, length, and resistance onnet photosynthesis. The influence of leaf dimorphism on wholeplants was determined by calculating daily photosynthesis andtranspiration for plants with various percentages of sun andshade leaves. A hypothetical plant with all sun leaves in thesun had about twice the photosynthesis and half the transpirationratio as did plants with sun leaves in the shade or shade leavesin the sun or shade. Plants with both sun and shade leaves hadthe highest predicted photosynthesis per unit ground area. Thepossible adaptive significance of the seasonal variation insun and shade leaf percentages observed for individual H. emoryibushes is discussed in terms of water economy and photosynthesi     

Properties of Internal Water Exchange in Leaves of Ilex opaca Ait. and Cornus florida L.

Resistances, capacitances, and time constants for internal watertransfer were estimated in leaves from naturally-occurring individualsof Ilex opaca Ait. and Cornus florida L., both common under-storytree species in the North Carolina piedmont deciduous forest.The water exchange properties depended strongly on the currentwater status of the leaf tissue. Resistance to water transferfrom leaf storage sites, capacitance of storage sites, and thetime constant for transfer of water out of storage sites allincreased as tissue water deficits developed. The exchange propertieswere determined over a range of leaf water potentials whichthe plants typically experienced in the field. The measuredresponses are viewed as properties integrally associated withother aspects of water transfer elsewhere in the plant and inthe soil and atmosphere, and may be significant in determiningleaf water deficits under a fluctuating environment.     

Effect of Verticillium dahliae on Photosynthesis, Leaf Expansion and Senescence of Field-grown Sunflower

On the basis of known sunflower (Helianthus annuus L.) responsesto soil water deficit, it is proposed that the effect of thefungus Verticillium dahliae Klebahn on plant leaf area precedesand is greater than its effect on leaf photosynthesis and stomatalconductance. To test this hypothesis, we measured shoot andleaf area growth, leaf photosynthetic rate, stomatal conductanceand disease symptoms in a field experiment including hybridsof high (Sankol) and low (Dekasol 3900) susceptibility to V.dahliae. Plants inoculated with V. dahliae and controls werecompared. We also investigated the effect of V. dahliae on keycomponents of plant leaf area, leaf expansion and senescence,in inoculated and control plants of Sankol and Toba, a hybridof intermediate susceptibility to V. dahliae. Reduction in plantleaf area caused by V. dahliae was first detected 31 d afterinoculation (DAI), when visual symptoms of disease in inoculatedplants were slight (Sankol) or absent (Dekasol 3900). Reductionin leaf photosynthesis was first observed 66 DAI; stomatal conductanceand leaf dark respiration were both unaffected by V. dahliaeduring the whole experiment. In comparison with controls, V.dahliae reduced seasonal duration of plant leaf area by 25%in Dekalb 3900 and by 55% in Sankol, whereas the average reductionin leaf photosynthetic rate was 9%. In correspondence with thereduction in leaf area duration, inoculation reduced shoot drymatter of mature Sankol by 50%. In both experiments, less leafexpansion accounted for most of the early reduction in plantleaf area; as the disease progressed, increasing senescencealso contributed to reduced plant leaf area. It is concludedthat the response of sunflower to V. dahliae resembled the responseof the plant to soil water deficit: (1) plant leaf area, ratherthan leaf photosynthetic rate, accounted for the reduction ingrowth in mass; and (2) reduced leaf expansion early in theseason and faster leaf senescence in older plants accountedfor the decrease in plant leaf area. Copyright 2000 Annals ofBotany Company Helianthus annuus, Verticillium dahliae, allometry, apical dominance, drought, leaf expansion, leaf senescence, photosynthesis, stomatal conductance, growth     

Growth Analysis of Soybean Seedlings During the Lifespan of the Cotyledons

Patterns of seedling growth of Glycine max in light and darknesswere compared during the period from germination to cotyledonabscission. Fitted growth curves and the derived functions,relative growth rate, unit leaf rate, leaf area ratio and specificleaf area, were used to assess the relative importance in seedlinggrowth of cotyledon storage reserves, cotyledon photosynthesisand leaf photosynthesis. The cotyledons are of an intermediatetype with a predominant storage and a minimal photosyntheticfunction. Cotyledon reserves support seedling growth until theprimary leaves expand, after which growth depends on leaf photosynthesis. Glycine max (L.) Merr., soybean, cotyledons, growth analysis, seedling development     

A New Model for Leaf Photosynthesis Incorporating the Gradients of Light Environment and of Photosynthetic Properties of Chloroplasts within a Leaf

A leaf photosynthesis model was constructed based upon the notionthat the leaf photosynthesis is a summation of photosynthesisof each chloroplast under in situ micro-environmental conditions.Intra-leaf light environment was calculated using the valuesof transmittance and reflectance of leaf tissues reported previously.Simulations of light response curves of whole leaf photosynthesiswere carried out for the model leaves with different patternsof gradients in light environment and/or in photosynthetic activities.The results indicate that the higher absorption coefficientof chlorophyll in spongy tissue than in palisade tissue andintra-leaf vertical gradient in photosynthetic activity of thechloroplasts as reported for real dorsiventral leaves are bothadvantageous to the productivity of the leaf because these propertiesraise the efficiency of the light utilization. Intra-leaf light environment, leaf photosynthesis, light utilization, palisade tissue, photosynthetic productivity, spongy tissue     

C3 and C4 Photosynthetic and Anatomical Forms of Alloteropsis semialata (R.Br.) Hitchcock: 2. A Comparative Investigation of Leaf Ultrastructure and Distribution of Chlorenchyma in the Two Forms

Alloteropsis semialata (R.Br.) Hitchcock is a widely distributedgrass species known to show marked morphological, anatomicaland physiological variation. An unusual feature of this grassis that it shows both C₃, and C₄ photosynthetic pathways withthe respective anatomy related to these pathways in one speciesand within a single environment. This study investigates ultrastructuraldifferences between C₃ and C₄ forms of A. semialata, and showsdistribution of chlorenchyma in the two forms, at light microscopelevel. Quantitative assessment shows marked differences in mesophylland Kranz sheath cells with regard to size and organelle content;also in distribution of starch grains, lipid droplets and organellesin the mesophyll cells of both forms. These differences arediscussed in relation to the C₃/C₄ syndrome. Measurements were made using a Digiplan electronic planimeteron transmission electron micrographs of mature leaves sectionedtransversely at mid-lamina, and on drawings of leaf sectionsshowing the distribution of chlorenchymatous tissue. Alloteropsis semialata(R.Br.) Hitchcock, leaf structure, chlorenchyma, ultrastructure, C₃-photosynthesis, C₄-photosynthesis     

Electrical Impedance Analysis in Plant Tissues8

Electrical impedance spectra (100 Hz–800 kHz) were measuredin leaves of Peperomia obtusifolia L. (a succulent) and Brassicaoleracea L. (cabbage). By measuring impedances at three or moreinter-electrode distances in a single leaf, electrode impedanceand specific tissue impedance were separated. Analysis of impedance data from B. oleracea leaves in relationto an equivalent circuit model showed that leaf developmentwas accompanied by increases in extracellular resistance, cytoplasmicresistance and vacuole interior resistance, together with decreasesin plasma membrane capacitance and tonoplast capacitance. AfterB. oleracea leaves were subjected to a –6 °C freeze-thawstress, extracellular resistance, cytoplasmic resistance andvacuole interior resistance decreased, but plasma membrane capacitanceand tonoplast capacitance did not change. These results indicatethat useful measurements of leaf parameters can be obtainedby this technique. Examination of the electrode impedance spectrum showed thatelectrode insertion produced a damaged collar, 0·4–0·5mm wide, around the electrode. This was confirmed by visualobservation of the damage in P. obtusifolia leaf. Key words: Peperomia obtusifolia L., Brassica oleracea L. (cabbage), electrical impedance, equivalent circuit, electrode polarization     

Growth Rate and Water Relations of Citrus Leaf Flushes

Growth rates of seasonal leaf flushes of ‘Valencia’orange [Citrus sinensis (L.) Osbeck] were measured and waterrelations characteristics of young (new) and over-wintered (old)citrus leaves were compared. New flush leaves had lower specificleaf weights and lower midday leaf water potentials than comparablyexposed old leaves. Spring and summer flush new leaves had higherosmotic potentials than old leaves. These differences becamenon-significant as the new leaves matured. During summer conditions,water-stressed new leaves reached zero turgor and stomatal conductancealso began to decrease in them at higher leaf water potentialsthan in old leaves. Old leaves were capable of maintaining openstomata at lower leaf water potentials. Opened flowers and newflush leaves lost more water, on a dry weight basis, than flowerbuds, fruit or mature leaves. The results illustrate differencesin leaf water potential and stomatal conductance which can beattributed to the maintenance of leaf turgor by decreases inleaf osmotic potentials as leaves mature. These changes in citrusleaf water relations are especially important since water stressresulting from high water loss rates of new tissues could reduceflowering and fruit set. Citrus sinensis (L.) Osbeck, orange, Citrus paradisi Macf., grapefruit, growth rate, leaf water relations, osmotic potential, water potential, stomatal conductance     

Effects of varying air and soil moisture on the water relations and growth of sugar beet

Sugar beet were grown for short periods with different amounts of moisture in the soil and air. Growing plants in wet soil (23 % moisture on dry weight) compared with dry soil (15% moisture) increased growth of the shoots and roots and plant dry weights by 15% in young plants and 10% in mature plants. Growing plants in wet air containing 10.9 g m^-3 of water (equivalent to a saturation deficit of 2.5 mb) compared with dry air containing 6.4 g m^-3 of water (saturation deficit = 8.5 mb) increased the dry weights of both young and mature plants by 8%, mostly by increasing the sizes of their storage roots. Wet air and wet soil increased the net assimilation rates of both young and mature plants. Wet soil, but not wet air, increased leaf areas of young plants by accelerating leaf expansion, and both increased the leaf area of mature plants by slowing senescence of the older leaves. Wet soil increased the water potential of the leaves of both young and mature plants and, by doing so, increased their stomatal conductances and rates of photosynthesis. Wet air also increased stomatal conductances and rates of photosynthesis of leaves of plants of both ages, but without changing their water potentials. Stomatal conductances and photosynthetic rates were greater for young leaves than mature on the same plant and at the same water potential. It is suggested that at certain stages in the crops growth photosynthetic efficiency could be increased by applying additional water as a mist to increase the moisture content of the air around the crop.     

A comparison of leaf crystal macropatterns in the two sister genera Piper and Peperomia (Piperaceae)

? Premise of the study: This is the first large-scale study comparing leaf crystal macropatterns of the species-rich sister genera Piper and Peperomia. It focuses on identifying types of calcium oxalate crystals and their macropatterns in leaves of both genera. The Piper results are placed in a phylogenetic context to show evolutionary patterns. This information will expand knowledge about crystals and provide specific examples to help study their form and function. One example is the first-time observation of Piper crystal sand tumbling in chlorenchyma vacuoles. ? Methods: Herbarium and fresh leaves were cleared of cytoplasmic content and examined with polarizing microscopy to identify types of crystals and their macropatterns. Selected hydrated herbarium and fresh leaf punches were processed for scanning electron microscopy and x-ray elemental analysis. Vibratome sections of living Piper and Peperomia leaves were observed for anatomical features and crystal movement. ? Key results: Both genera have different leaf anatomies. Piper displays four crystal types in chlorenchyma-crystal sand, raphides, styloids, and druses, whereas Peperomia displays three types-druses, raphides, and prisms. Because of different leaf anatomies and crystal types between the genera, macropatterns are completely different. Crystal macropattern evolution in both is characterized by increasing complexity, and both may use their crystals for light gathering and reflection for efficient photosynthesis under low-intensity light environments. ? Conclusions: Both genera have different leaf anatomies, types of crystals and crystal macropatterns. Based on Piper crystals associated with photosynthetic tissues and low-intensity light, further study of their function and association with surrounding chloroplasts is warranted, especially active crystal movement.     

Pathway of phloem loading in the C3 tropical orchid hybrid Oncidium Goldiana

The apoplast of mature leaves of the tropical orchid OncidiumGoldiana was perfused with 0.5 mM p-chloromercuribenzenesulphonicacid (PCMBS) via the transpiration stream in order to test themode of phloem loading. The efficacy of introducing PCMBS byperfusion was shown by saffranin O dye movement in the veinsand leaf apoplast in control experiments. Photoassimilate exportas the result of phloem loading was measured by collection of¹⁴CO₂-derived photoassimilates from the basal cut-ends of intactleaves. Phloem loading and translocation of photoassimilates was inhibitedby 89% in leaves perfused with PCMBS for 1 h. The effect ofPCMBS on leaf photosynthesis was minimal. The amount of radiocarbonfixed by PCMBS-treated leaves averaged 89% of control leavesperfused with distilled water. A negative correlation betweenthe total amount of photoassimilate exuded and the calculatedconcentration of PCMBS in the leaf apoplast was also observed.The results indicate that phloem loading in Oncidium Goldianaoccurs via the apoplastic pathway. Key words: Phloem loading, apoplast, PCMBS, tropical orchid