Development of Chlorenchyma and Window Tissues in Leaves of Peperomia columella

Window leaves consist primarily of tissues specialized for waterstorage (window tissue) and photosynthesis (chlorenchyma). Theobjective of our research was to determine when these specializationsoccur during leaf development in Peperomia columella, a succulentwindow plant, native to deserts of South America. We measuredabsolute and relative volumes of leaf tissues. Young leavesconsist of approximately 75% chlorenchyma and 12% window tissue,suggesting that they are structurally specialized primarilyfor photosynthesis rather than water storage. In mature leavesthe percentages of chlorenchyma and window tissue are approximately20% and 58%, respectively, indicating that specialization forwater storage occurs during later stages of leaf development.The percent window tissue decreases in mature leaves, but increasesin young leaves with water stress.Copyright 1993, 1999 AcademicPress Chlorenchyma, Peperomia columella, succulent, window plant     

Transpiration Stream of Desert Species: Resistances and Capacitances for a C3, a C4, and a CAM Plant

Transpiration rates and water potentials of three sympatricdesert perennials, a C3 subshrub (Encelia farinosa), a C₄ bunchgrass(Hilaria rigida), and a CAM succulent (Agave deserti), wereanalysed using an electrical circuit analogue that includedresistances and capacitances for the leaves, stems, and roots.The water storage capability of the organs differed considerably,capacitance ranging over 1000-fold from the thin leaves of H,rigida to the massive leaves of A. deserti, although the capacitanceper unit volume varied only 1.9-fold. The diurnal changes inwater storage could support maximum transpiration rates of H.rigida for 4 min, E. farinosa for 7 min, and A. deserti for16 h. The time constant for equilibration of water from storageto the xylem ranged from 29 s for roots of H. rigida to 52 minfor leaves of A. deserti. Resistances for such movement wererelatively low for the succulent leaves of A. deserti and wereup to about 50-fold higher for the three organs of E. farinosa.Xylem resistances calculated using the Hagen-Poiseuille lawand measured xylem dimensions were 2.1- to 2.1-fold lower thanresistances estimated from observed water potential drops, adiscrepancy which is in agreement with other published data.Contrary to data on other plants, the xylem resistances in theroots and leaves of E. farinosa and H. rigida averaged only15% of the stem xylem resistance. Key words: Capacitance, Xylem resistance, Transpiration stream, Desert     

Root Water Uptake, Leaf Water Storage and Gas Exchange of a Desert Succulent: Implications for Root System Redundancy

A technique used for hydroponics was adapted to measure instantaneousroot water uptake from the soil for a leaf succulent CAM species,Agave deserti. Comparisons were made to previously modelledwater fluxes for A. deserti and to Encelia farinosa, a non-succulentC₃species. Net CO₂uptake and transpiration forA. deserti underwell-watered conditions occurred primarily at night whereasroot water uptake was relatively constant over 24 h. Leaf thicknessdecreased when transpiration commenced and then increased whenrecharge from the stem and soil occurred, consistent with previousmodels. A drought of 90 d eliminated net CO₂uptake and transpirationand reduced the water content of leaves by 62%. Rewetting theentire root system for 7 d led to a full recovery of leaf waterstorage but only 56% of maximal net CO₂uptake. Root water uptakewas maximal immediately after rewetting, which replenished rootwater content, and decreased to a steady rate by 14 d. Whenonly the distal 50% of the root system was rewetted, the timefor net CO₂uptake and leaf water storage to recover increased,but by 30 d gas exchange and leaf water storage were similarto 100% rewetting. Rewetting 10 or 20% of the root system resultedin much less water uptake; these plants did not recover leafwater storage or gas exchange by 30 d after rewetting. A redundancyin the root system of A. deserti apparently exists for dailywater uptake requirements under wet conditions but the entireroot system is required for rapid recovery from drought.Copyright1999 Annals of Botany Company Agave deserti Engelm., desert, drought, gas exchange, rewetting, roots, succulent, water uptake.     

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     

Effects of Drought on CAM and Water Relations in Plants of Peperomia carnevalii

The mechanisms underlying the drought tolerance of Peperomiacarnevalii Steyermark (Piperaceae), a succulent herb growingin the understorey of seasonally dry forests, were examined.Crassulacean acid metabolism (CAM) was studied in the fieldand laboratory, and measurements of water status were made inplants subjected to drought in the greenhouse. Nocturnal acidaccumulation and day and night-time CO₂assimilation rates weregreatest in watered plants and decreased in drought. The proportionof CO₂recycled through CAM in droughted plants, with nocturnalCO₂uptake close to zero, was higher than in watered plants.Maximum quantum yield of chlorophyll fluorescence remained unchangedduring drought, but the PSII quantum yield at the photosyntheticphoton flux density at which the plants were grown was significantlydecreased. Leaf anatomy consists of a chlorophyll-less hydrenchymalocated beneath the upper epidermis, and a two-layered mesophyll.Leaves nearer to the apex are thinner than those nearer to thebase of the shoot. Drought caused a reduction in leaf thicknessdue to shrinkage of the hydrenchyma, but not of the mesophyll.This was associated with the occurrence of a gradient of osmoticpotential between these tissues. Comparison of water loss fromthin leaves of watered and droughted plants, either partly defoliatedat the lower nodes or intact, suggested that water moved fromthe thick to the thin leaves. This process was related to theoccurrence of a gradient of water potential between the thickand the thin leaves. Drought tolerance in P. carnevalii is achievedby the operation of CAM and the occurrence of water movementwithin and between leaves. Copyright 2000 Annals of Botany Company Crassulacean acid metabolism, fluorescence, hydrenchyma, mesophyll, Peperomia carnevalii, water relations     

Spectral properties of heavily glaucous and non-glaucous leaves of a succulent rosette-plant

Summary Comparisons of reflection, transmittance, and absorptance spectra of heavily glaucous leaves, glaucous leaves from which the glaucescence was experimentally removed, and naturally occurring non-glaucous leaves of a single species (Dudleya brittonii Johansen) reveal that glaucescence, a powdery wax coating on the leaf surface, is responsible for very high reflectance of ultraviolet radiation (UV) and high reflectances of visible and near-infrared radiation. UV reflectance (up to 83% in UV-B) measured for glaucous leaves of D. brittonii, a succulent rosette-plant occurring in fissures in steep volcanic outcrops, is higher than that reported for any other plant species. Non-glaucous leaves of the species reflect about 10% in UV. I hypothesize that the high UV reflectance of glaucous leaves is ecologically significant in reducing damage to dehydrated leaves from visible and UV-B radiation, thus promoting longevity of the leaves important both in conservation of the mineral nutrient capital of these succulent plants and in their acquisition and storage of water. This and other demonstrated and hypothesized functions of glaucescence are discussed in terms of the evolutionary significance of glaucescence in succulent plants.     

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.     

Developmental patterns in anatomy are shared among separate evolutionary origins of stem succulent and storage root-bearing growth habits in Adenia (Passifloraceae)

The architecture of flowering plants is astonishingly diverse. To understand evolutionary patterns and processes that account for this diversity, I investigated developmental anatomy of storage roots and stems of 58 species in the genus Adenia (Passifloraceae) using an explicit phylogenetic context. Because expanded storage roots and stem succulence evolved multiple times in Adenia, patterns of transition between succulent and nonsucculent forms were analyzed using a comparative test that accommodates phylogenetic uncertainty. I tested the innervation hypothesis, wherein I expected the evolution of vascular strands to be correlated with evolutionary increases in water storage tissue if evolution of vascular strands facilitates transport through water and starch storage structures. Not only is evolution of vascular strands in stems statistically coupled with evolutionary increases in parenchyma storage tissue, most lineages that evolved expanded storage roots also evolved vascular strands in these roots in parallel to succulent stems. I proposed that vascular strands and closely associated storage parenchyma found in both roots and shoots of Adenia comprise a homologous unit. A switch-like evolutionary mechanism that alters the spatial expression of this unit between roots and shoots can account, in large part, for transitions between markedly different habits such as storage-rooted herbs and succulent-stemmed shrubs.     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

The influence of the parasitic angiosperm Striga gesnerioides on the growth and photosynthesis of its host, Vigna unguiculata

Cowpea plants (Vigna unguiculata) infected with the root hemiparasiticangiosperm Striga gesnerioides accumulated less biomass thanuninfected plants over a growth period of 60 d. The allometricrelationship between shoot and root dry weight was similar inparasitized plants relative to control plants, as was the proportionof dry matter partitioned into leaf, stem and root tissue. However,infected plants failed to make any significant investment ofdry matter in pods. The rate of photosynthesis of the youngestfully expanded leaf of parasitized plants was significantlylower than for control plants. The lower rates of photosynthesiswere not attributable to stomatal limitation, a loss of chlorophyllor to an accumulation of carbohydrate. The depression of photosynthesisin the young leaves was transient. As control leaves aged, photosynthesisdeclined. This also occurred in Striga infected plants, butto a lesser extent resulting in higher rates of photosynthesisin mature leaves when compared to those of uninfected plants.The foliar nitrogen content of parasitized plants was higherthan control plants consistent with the slower rate of photosyntheticdecline of older leaves. The data are discussed with respectto the influence of parasitic weeds on host growth and photosynthesis. Key words: Cowpea, hemiparasite, allometry, nitrogen     

Water relations of stem succulent trees in north-central Baja California

Summary Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 to 4 kg H₂O/m³ of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H₂O/m³. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m^–2 s^–1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of elastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanism allowing for the maintenance of leaf turgor in these stem succulent trees. The low transpiration rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.     

骆驼蓬营养器官的旱生结构

 骆驼蓬是生长在荒漠或盐碱地区的旱生植物,具有显著的旱生结构特征。主根粗壮,其中柱周围产生2或3轮呈同心环状排列的异常维管束。这种异常结构对旱生植物具有重要的生态学意义。茎肉质,其皮层、韧皮部,尤其是髓内具有发达的贮水薄壁组织细胞。叶片肉质,光滑无毛。其表皮细胞的外切向壁有较厚的角质层,表面具皱纹状突起。气孔器与表皮细胞平齐,面积较大,而密度较小。栅栏组织发达,为环栅型,包围着发达的贮水组织。叶脉维管束不发达。其叶表面积与体积之比远比中生植物小。根据以上营养器官形态结构特征,可以认为骆驼蓬是典型的多浆汁旱生植物。     

多肉植物离体再生研究进展

通过品种、外植体类型、培养基、外源植物激素的选择等几方面对国内多肉植物的离体再生研究现状进行了综述。发现多肉植物的离体再生研究多以种子、叶片、茎段、花器官等为外植体;不定芽诱导阶段常用的培养基类型为MS培养基,不定根诱导常用1/2 MS培养基;6-苄氨基腺嘌呤(6-BA)、萘乙酸(NAA)和激动素(KT)作为常见的外源植物激素,被广泛运用于多肉植物愈伤组织诱导、不定芽诱导、不定根诱导等离体再生各阶段;分析了当前存在的问题和发展前景,以期为日后多肉植物的相关研究提供可行性建议。     

The Role of Roots in Control of Bean Shoot Growth

Restriction of root growth by growing bean plants (Phaseolusvulgaris L.) in very small pots led to the development of dwarfplants. The leaves of those plants were smaller and their internodesshorter than those of control plants which were grown in largerpots and had developed a more extensive root system. A largequantity of starch—much more than in control plants —accumulated in the leaves and shoots of the dwarf plants. Increasingthe amount of minerals which was supplied to the roots, enhancedleaf growth of the control plants but failed to affect the dwarfones, in spite of the fact that in both cases the treatmentincreased the content of N, P and K in all the plant organs.The leaf water content was similar in both treatments, but theleaf water potential was higher in the dwarf plants. Exogenousapplication of gibberellic acid (GA₃) to the dwarf plants overcamethe reduction of stem growth completely, and that of the leavespartially. Application of the cytokinin, benzyladenine (BA)did not affect stem growth, but increased that of the primaryleaves. A combined supply of GA₂ + BA restored completely thegrowth of the stem and the primary leaves, and partially thatof the trifoliate leaves. It is concluded that a limited rootsystem restricts shoot growth through an hormonal system inwhich at least gibberellins and cytokinins are involved, andthat the dwarfing is not a consequence of mineral or assimilatedeficiency, or due to water stress. Phaseolus vulgaris L., leaf growth, stem growth, root restriction, gibberellic acid, benzyladenine, cytokinin     

Phase Change in Hedera helix L: II. THE POSSIBLE BOLE OF ROOTS AS A SOURCE OF SHOOT GIBBERELLIN-LIKE SUBSTANCES

When synthesis was estimated by the agar diffusion techniqueboth basal and lateral adventitious roots of Hedera helix L.in the juvenile growth phase were shown to synthesize gibberellin-likesubstances. Seedlings and cuttings from juvenile ivy could begrown in water culture for several weeks. When roots were excisedfrom these plants shoot growth was reduced in comparison withthat of intact plants. The stems, apices, and leaves of derootedseedlings and cuttings contained lower levels of extractablegibberellin-like substances than comparable organs of intactplants. The major zone of gibberellin-like activity in intactplants co-chromatographed with gibberellins A₁ and A₃. Whenthese gibberellins were applied to plants grown in culture theywere found to promote growth of intact but not derooted plants.These findings are discussed in relation to the role of rootfactors and particularly gibberellins in phase change.     

Water recycling in leaves of Lithops (Aizoaceae)

Lithops plants consist of a pair of succulent leaves inserted on a short stem; in each growing season, young leaves develop in a cavity formed between the older pair. Young leaves can take up water from the older pair allowing the plant to maintain growth and leaf expansion even without external supply of water. Recycling water between vegetative organs is one of the possible adaptation strategies of plants under drought stress, but it had never been demonstrated experimentally in Lithops. The methodology used to verify the existence of water redistribution from old leaves to young leaves was fluorescence microscopy, using two dyes to follow the water pathway inside the plant: Sulforhodamine G (SRG) and 5(6)-carboxyfluoroscein diacetate (CFDA). In Lithops fluorescent tracers loaded into old leaves were found in young leaves, in 74% of the cases for SRG, in 59% of the cases for CFDA. Our data demonstrate that young leaves take up water from the old ones following both a symplastic and an apoplastic pathway. Water recycling is therefore one of the adaptive responses of these plants allowing them to perform at least a complete growth cycle even during prolonged drought stress periods, using the water stored in the older leaves.     

The Physiology of a Wilty Pea: Abscisic Acid Production under Water Stress

‘Wilty’ (JI 1069), a mutant of Pisum sativum, hasbeen examined for its ability to produce abscisic acid (ABA)under water stress. ABA was measured using combined gas chromatography-massspectrometry and multiple-ion-monitoring employing a deuteratedinternal standard. In intact droughted plants, ‘Wilty’produced less ABA than a non-wilty line (JI 1194) and maximumproduction was delayed. Detached leaves of the wilty mutantlost significantly more water than control leaves but did notshow an increase in ABA content. Non-stressed mutant materialfrom both intact plants and isolated leaves contained less ABAthan control tissue. Key words: Pea, Wilty mutant, Abscisic acid     

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     

Apical Dominance in Phaseolus vulgaris L.

Although determinations of the ABA content of lateral buds ofPhaseolus vulgaris revealed no difference between decapitatedand intact control plants in the first 12 h following decapitation,a relative decrease in the ABA content of lateral buds of decapitatedplants was detectable 24 h following decapitation. Shoot decapitationwas also observed to result in a decrease in the ABA contentof stem tissue. The application of IAA to the stem of decapitatedplants prevented these changes and increased the ABA contentof stem tissue relative to that of intact plants. The levelsof IAA and ABA were also determined in the stem tissue fromthe nodes of intact bean plants. The possible interdependenceof these two plant hormones was further investigated by a studyof [2_–14ClABA metabolism. The results are discussed inrelation to the possible role of these hormones in apical dominance. Key words: Apical dominance, Abscisic acid, Indole-3-acetic acid     

Environmental Influences on CAM Activity of Cissus quadrangularis

Cissus quadrangularis, Vitacea, has a succulent stem and showsCrassulacean Acid Metabolism (CAM). Environmental control ofCAM is shown with respect to water supply, day temperature,and light intensity. C. quadrangularis, under moderate drought,has a high capacity for dark CO₂ fixation and high efficiencyof water use (nocturnal transpiration ratio = 8). Severely droughtedplants of C. quadrangularis still show dark CO₂ fixation althoughat a reduced rate, which allows a maintenance level metabolism.On the other hand, high temperature and light intensity promoteCAM activity in well-watered plants resulting in luxuriant andfast growth. Data indicate that under natural conditions CAMshould effectively contribute to the successful adaptation ofC. quadrangularis to its environment.