Thermal and Water Relations of Roots of Desert Succulents

Two succulent perennials from the Sonoran Desert, Agave desertiEngelm. and Ferocactus acanthodes (Lem.) Britton and Rose, loselittle water through their roots during drought, yet respondrapidly to light rainfall. Their roots tend to be shallow, althoughabsent from the upper 20 mm or so of the soil. During 12–15d after a rainfall, new root production increased total rootlength by 47 per cent to 740 m for A. deserti and by 27 percent to 230 m for F. acanthodes; root dry weight then averagedonly 15 per cent of shoot dry weight. The annual carbon allocatedto dry weight of new roots required 11 per cent of shoot carbondioxide uptake for A. deserti and 19 per cent for F. acanthodes.Elongation of new roots was greatest near a soil temperatureof 30°C, and lethal temperature extremes (causing a 50 percent decrease in root parenchyma cells taking up stain) were56°C and -7°C. Soil temperatures annually exceeded themeasured tolerance to high temperature at depths less than 20mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentialsabove -2·6 MPa could produce new lateral roots, with50 per cent of maximum elongation occurring near -1·4MPa for both species. Non-droughted roots lost water when immersedin solutions with osmotic potentials below -0·8 MPa,and root hydraulic conductance decreased markedly below about-1·2 MPa. Pressure-volume curves indicated that, fora given change in water potential, non-droughted roots lostthree to five times more water than droughted roots, non-droughtedleaves, or non-droughted stems. Hence, such roots, which couldbe produced in response to a rainfall, will lose the most tissuewater with the onset of drought, the resulting shrinkage beingaccompanied by reduced root hydraulic conductance, less contactwith drying soil, and less water loss from the plant to thesoil. Agave deserti, Ferocactus acanthodes, roots, soil, temperature, water stress, drought, Crassulacean acid metabolism, succulents     

A Finite-element Model of Radial and Axial Conductivities for Individual Roots: Development and Validation for Two Desert Succulents

A morphologically explicit numerical model for analysing wateruptake by individual roots was developed based on a conductornetwork, with specific conductors representing axial or radialconductivities for discrete root segments. Hydraulic conductivity(L_p; m s^–1 MPa^–1) was measured for roots of Agavedeserti Engelm. and Opuntia ficus-indica (L.) Miller by applyinga partial vacuum to the proximal ends of excised roots in solution.L_p was also measured for 40- to 80-mm segments along a root,followed by measurements of axial conductivity and calculationof radial conductivity. Predicted values of L_p for entire rootsbased on two to ten segments per root averaged 1.04±0.07(mean±s.e. mean for n = 3) of the measured L_p for A.deserti and 1.06±0.10 for O. ficus-indica. The modelalso closely predicted the drop in water potential along theroot xylem (^xylem); when a tension of 50 kPa was applied tothe proximal ends of 0.2 m-long roots of A. deserti and O. ficus-indica,the measured ^xylem to midroot averaged 30 kPa compared witha predicted decrease of 36 kPa. Such steep gradients in ^xylemsuggest that the driving force for water movement from the soilto young distal roots may be relatively small. The model, whichagreed with an analytical solution for a simple hypotheticalsituation, can quantify situations without analytical solutions,such as when root and soil properties vary arbitrarily alonga root. Agave deserti, electrical circuit analog, hydraulic conductivity, Opuntia ficus-indica, water potential     

Soil O2 and CO2 Effects on Apparent Cell Viability for Roots of Desert Succulents

Roots of desert succulents occupy the upper layers of porous,well-aerated soils. However, roots of Agave deserti, Ferocactusacanthodes, and Opuntia ficus-indica all tolerated many daysof soil anoxia; 0% O₂ in the soil gas phase for 30 d reducedthe fraction of cells taking up the vital stain neutral red,an average of only 18% for the cortex and 6% for parenchymacells within the stele of perennial established roots. Ephemeralrain roots, induced by watering as branches on the establishedroots, were more susceptible to 0% O₂ in the soil gas phase;19 d abolished stain uptake for cortical cells and 32 d forstelar parenchyma cells. Soil CO₂ levels above the 0.1% observedin the root zone in the field rapidly reduced uptake of neutralred; the fraction of cortical cells taking up the stain decreased30% in 10 h at 0.5% CO₂ and was abolished in 9 h at 2% and 7h at 10% CO₂ averaged for the three species. Rain roots weresomewhat more susceptible than established roots to elevatedsoil CO₂ levels, and stelar parenchyma cells were much lesssusceptible than were cortical cells. When uptake of the vitalstain was abolished by elevated soil CO₂, no anatomical evidenceof cellular damage was observed. For A. deserti exposed to 2%CO₂, the pH of macerated root tissue decreased about 0.35 pHunit over 10 h; CO₂ apparently entered the cells, lowered theintracellular and/or cell wall pH, and prevented the accumulationof neutral red. Elevated soil CO₂ also inhibits root respirationfor the three desert succulents considered. Hence, the restrictionof such species to porous soils may reflect the relatively rapidinhibiting effects of elevated soil CO₂ levels rather than arequirement for high soil O₂ levels, consistent with the observationthat desert soils tend to have low gas-phase CO₂ levels near0.1% compared with 1% or more in the root zone of non-desertspecies. Key words: Agave deserti, Ferocactus acanthodes, neutral red, Opuntia ficus-indica, pH     

Allometric Root/Shoot Relationships and Predicted Water Uptake for Desert Succulents

Root morphology, shoot morphology, and water uptake for Agavedeserti and Ferocactus acanthodes of various sizes were studiedusing allometric relationships (y = ax^b) and a previously developedwater uptake model. Shoot surface area increased with shootvolume with an exponent b of 0.75 for both species. Root lengthand the ground area explored by the roots increased with shootsurface area with b's of 0.72 for A. deserti and 0.92 for F.acanthodes. Various sized individuals had about the same ratioof root length to explored ground area, with higher values occurringfor A. deserti. Predicted water uptake averaged over the exploredground area was approximately constant over a 10⁴-fold rangein shoot surface area, suggesting that shoot size confers nointraspecific competitive advantage for water uptake. For theroot lengths per explored ground area observed in the field,water uptake was predicted to be 85 per cent of maximal; wateruptake could be increased by the production of more rain roots.When differences in shoot volume were accounted for by allometry,small plants had relatively less shoot surface area and relativelymore root length per shoot volume than did large plants, whichmay be important for the water relations of seedling establishment. Agave deserti, Ferocactus acanthodes, allometry, desert succulents, root distribution, root length, seedling growth, seedling establishment, shoot surface area, shoot volume, water uptake     

A Two-dimensional Model for Water Uptake by Desert Succulents: Implications of Root Distribution

Water uptake by Agave deserti and Ferocatus acanthodes was predictedusing a two-dimensional simulation model in which the soil arounda plant was divided into a series of layers and concentric cylindricalshells. Root lengths in 0.05 m thick soil layers were determinedfor both species in the field, where mean root depths were only0.11 m for A. deserti and 0.10 m for F. acanthodes. For a yearwith average precipitation (159 mm), 42 per cent of the annualprecipitation could be taken up by A. deserti and 25 per centby F. acanthodes. Predicted water uptake by both species wasgreater from the upper soil layers (above 0.15 m) for averageand dry years, but was greater from the deeper layers for awet year. The actual root distribution for both species ledto more water uptake than when all of the roots were in a singlelayer. The large number of days per year when the soil temperaturesexceeded 57 °C (the temperature for 50 per cent inhibitionof uptake of a vital stain by root cells) may exclude rootsfrom the 0.00–0.05 m soil layer, even though water uptakewhen all roots were located there was predicted to be maximal.Therefore, the observed root distribution of A. deserti andF. acanthodes may be limited near the soil surface by high temperaturesand at maximum depths by water availability for all but wetyears. Agave deserti, Ferocactus acanthodes, desert succulents, root system, root distribution, soil temperature, water uptake     

Presence of Two Different Membrane-bound, KCl-stimulated Adenosine Triphosphatase Activities in Maize Roots

Recent publications have indicated that a KCl-stimulated ATPase from cereal roots is specifically associated with plasmalemma-enriched membrane fractions. However, in previous work we found that relatively high specific activities of this enzyme were also associated with a membrane fraction which did not contain plasmalemma. In an attempt to clarify this discrepancy, we have investigated the effect of density gradient composition upon the association of the enzyme with different membrane fractions isolated from the roots of Zea mays L. (WF9 × M14).     

Xerophytic Structures in the Roots of Desert Shrubs

The primary body of the root of 30 perennial dicotyledonousspecies growing in various habitats in the Israeli Negev wasexamined. Significant quantitative differences which were foundbetween them could be related to their natural environmentalconditions. Extreme hydro- and xerohalophytes develop wide casparianstrips in their endodermis. A gradient in the ratio width of casparian strips/width of the radial wall of the endodermis was established. The ratio is high in halophytes, and low inless extreme habitats such as river beds, sandy dunes, and loesssteppes. Plants populating the extremely dry hammadas, and alsohydrohalophytes, have a small number of cell layers in the cortexof their roots. In less extreme habitats the cortex is moredeveloped. The area of the primary tracheae in transverse section of theroot was found not to be correlated to the habitat. However,it shows a good correlation with the depth of the root system,being larger in deep-rooting plants. The percentage of species having ring porous arrangement ofthe tracheae in the secondary xylem of the root is relativelyhigh. The periderm is highly developed in perennial roots.     

The Influence of Leaf Windows on the Utilization and Absorption of Radiant Energy in Seven Desert Succulents

Four fluorescence parameters [F_v/F_m = the intrinsic efficiency of energy conversion via photosystem 2 (PS2); F_v/F_m= the efficiency of energy conversion via PS2 in the light; P = fraction of absorbed radiant energy utilized for photosynthesis; and D = fraction of absorbed radiant energy dissipated as heat] were measured on leaves of seven species of succulents having epidermal windows. While the function of leaf windows has reportedly been to increase absorption of radiant energy and, hence, the rate of photosynthesis in these species, recent evidence indicates that this translucent portion of epidermal tissue, lacking chlorophyll, may also result in photoinhibition in these species, especially for those with growth habits aboveground. Species with aboveground and belowground growth habits were compared with their leaf windows covered with reflective tape and with windows unobstructed. Results showed no increase in photoinhibition for these species resulting from the radiant energy penetrating the window tissue. Although the efficiency of the photosynthetic mechanism was not significantly influenced by the additional radiant energy provided by the window for individual species, there were significant differences in the efficiencies of radiant energy capture (F_v/F_m) and utilization (P) between the two growth habits. Species with an aboveground growth habit were less efficient in radiant energy utilization compared with the species having a belowground growth habit.     

Changes in Structure and Hydraulic Conductivity for Root Junctions of Desert Succulents as Soil Water Status Varies

Variations in hydraulic conductivity (L_P) and the underlying anatomical and morphological changes were investigated for main root-lateral root junctions of Agave deserti and Ferocactus acanthodes under wet, dry, and rewetted soil conditions. During 21 d of drying, L_P and radial conductivity (L_R) increased threefold to fivefold at junctions of both species. The increase in L_R was accompanied by the formation of an apoplastic pathway for radial water movement from the surface of the junction to the stele for A. deserti and by the rupture of periderm by emerging primordia of secondary lateral roots for F. acanthodes. During 7 d of rewetting, L_R decreased for junctions of A. deserti, as apoplastic water movement was not apparent, but L_R was unchanged for F. acanthodes. Axial conductance (K_h) decreased during drying for both species, largely because of embolism related to the degradation of unlignified cell wall areas in tracheary elements at the root junction. The resulting apertures in the cell walls of such elements would admit air bubbles at pressure differences of only 0.12-0.19 MPa. Rewetting restored K_h for both species, but not completely, due to blockage of xylem elements by tyloses. About 40% of the primary lateral roots of the monocotyledon A. deserti abscised during 21 d of drying. For the dicotyledon F. acanthodes, which can form new conduits in its secondary xylem, only 10% of the primary lateral roots abscised during 21 d of drying, consistent with the much greater frequency of lateral roots that persist during drought in the field compared with the case for the sympatric A. deserti.     

Membrane-bound Adenosine Triphosphatase Activities of Oat Roots

Homogenates of oat (Avena sativa cv. Goodfield) roots contained at least five membrane-associated adenosine triphosphatase (ATPase) activities. The membrane-bound ATPases were separated on sucrose gradients and distinguished by membrane density, pH optima, sensitivity to monovalent salts, and substrate specificity.     

黑刺菝葜根提取物的抑菌活性研究

以常见6种细菌和5种真菌为供试菌。采用滤纸片水平扩散法。对黑刺菝葜根的4种溶剂提取物和剩余水相大孔吸附树脂5种乙醇梯度洗脱物共9种提取物进行了室内抗菌生物活性测试,并对其主要的化学成分-常规的抗菌活性物质黄酮和甾体皂苷类化合物进行了定量测定。结果表明：黑刺菝葜根剩余水相大孔吸附树脂100％乙醇的洗脱物表现出极强的广谱性抗菌活性,80％乙醇的洗脱物对绿木霉、大肠杆菌显示出极强抑菌力;黑刺菝葜根的石油醚提取物、乙酸乙酯提取物和正丁醇提取物均对枯草芽孢杆菌、大肠杆菌显示较强的抗性;根的石油醚提取物和乙酸乙酯提取物对金黄色葡萄球菌、蜡状芽孢杆菌和巨大芽孢杆菌等有较强的抑制作用;根中所含的常规抑菌活性成分黄酮的含量为2．83％;甾体皂苷元的含量为4．55％。     

Two new Umbelliferae of the Chihuahuan Desert

Two apparently calcicolous species of apioid Umbelliferae,Aletes calcicola andDonnellsmithia coahuilensis, are described from the Chihuahuan Desert of Coahuila, Mexico.     

铝胁迫对小麦根系液泡膜ATP酶、焦磷酸酶活性和膜脂组成的效应

研究了铝胁迫下耐铝性不同的两个小麦品种根细胞液泡膜ＡＴＰ 酶、焦磷酸酶活性和膜脂的变化。与对照相比,经２０ 和１００μｍｏｌ／Ｌ的ＡｌＣｌ３ 处理后,耐铝品种Ａｌｔａｓ ６６ 的液泡膜Ｈ＋ＡＴＰ 酶和Ｃａ２＋ＡＴＰ 酶活性迅速下降; 铝敏感品种Ｓｃｏｕｔ ６６ 液泡膜Ｈ＋ＡＴＰ酶和Ｃａ２＋ＡＴＰ 酶活性则在２０ μｍｏｌ／Ｌ 时增加,１００ μｍｏｌ／Ｌ时下降。焦磷酸酶活性在Ａｌｔａｓ６６ 中下降,在Ｓｃｏｕｔ ６６ 中增加。与对照相比,在ＡｌＣｌ３ ２０ μｍｏｌ／Ｌ处理时,液泡膜磷脂含量增加,Ａｌｔａｓ ６６ 中的增加比Ｓｃｏｕｔ ６６ 更为明显;１００ μｍｏｌ／Ｌ 时,Ｓｃｏｕｔ ６６ 液泡膜磷脂含量迅速下降,而Ａｌｔａｓ ６６ 下降不显著。两品种小麦在铝处理后根液泡膜糖脂结合半乳糖含量均高于对照,而Ａｌｔａｓ ６６ 中的含量又高于Ｓｃｏｕｔ６６ 。铝处理后,两品种小麦根液泡膜的棕榈酸和油酸含量增加,亚麻酸含量下降, 不饱和指数也随之下降,其中Ｓｃｏｕｔ６６ 下降更为明显。表明Ａｌｔａｓ ６６ 根细胞液泡膜比Ｓｃｏｕｔ６６ 对铝胁迫有更强的适应能力。     

Hydraulic Conductances of the Soil, the Root-Soil Air Gap, and the Root: Changes for Desert Succulents in Drying Soil

Water movement to and from a root depends on the soil hydraulicconductivity coefficient (L^soil), the distance across any root-soilair gap, and the hydraulic conductivity coefficient of the root(L^P). After analytical equations for the effective conductanceof each part of the pathway are developed, the influences ofsoil drying on the soil water potential and L^soil are describedduring a 30 d period for a loamy sand in the field. The influencesof soil drying on L^P for three desert succulents, Agave deserti,Ferocactus acanthodes, and Opuntia ficus-indica, are also describedfor a 30 d period. To quantify the effects of soil drying onthe development of a root-soil air gap, diameters of 6-week-oldroots of the three species were determined at constant watervapour potentials of –1.0 MPa and –10 MPa as wellas with the water vapour potential decreasing at the same rateas soil drying during a 30 d period. The shrinkage observedfor roots initially 2·0 mm in diameter averaged 19% duringthe 30d period. The predominant limiting factor for water movementwas L^P of the root for the first 7 d of soil drying, the root-soilair gap for the next 13 d, and L^soil thereafter. Compared withthe ease of water uptake from a wet soil, the decrease in conductancesduring soil drying, especially the decrease in L^soil causedthe overall conductance to decrease by 3 x 10³-fold during the30 d period for the three species considered, so relativelylittle water was lost to the dry soil. Such rectifier-like behaviourof water movement in the soil-root system resulted primarilyfrom changes in L^soil and, presumably, is a general phenomenonamong plants, preventing water loss during drought but facilitatingwater uptake after rainfall. Key words: Agave deserti, Ferocactus acanthodes, Opuntia ficus-indica, rectification, soil water potential, water movement     

Possible Involvement of Abscisic Acid in Increases in Activities of Two Vacuolar H+-Pumps in Barley Roots under Aluminum Stress

Levels of abscisic acid (ABA) in barley roots increased upontreatment with AlCl₃. Treatment with AlCl₃ or ABA increasedboth ATP-dependent and PP_i-dependent H⁺-pumping activities intonoplast-enriched membrane vesicles. Increase in the H⁺-pumpingactivities caused by aluminum stress could result from increasedlevels of ABA. ¹Present address: Department of Botany, Faculty of Science,Hirosaki University, Hirosaki, Aomori, 036 Japan     

Matric Bound Water of Water Tissue from Succulents

Matric bound water was measured as water retained by frozen and thawed tissue after desorption on a pressure membrane filter under 20 bars nitrogen gas pressure. Central water-storage tissue and peripheral chlorenchyma from leaves or stems of 15 taxonomically diverse non-halophytic succulent species were investigated. Matric bound water as a per cent of the dry weight averaged higher in water storage than in chlorenchyma tissue but lower than values reported for many mesophytic leaves. Matric bound water as a proportion of the total water held, however, was lower in water tissues. Osmotic potentials were generally high (solute contents low). It is concluded that matric or osmotic forces cannot account, in any unique way, for the high water content of water tissues. This appears to depend, instead, on the enormous ability of the thin-walled cells to take up available water and expand.     

Secondary Metabolites and Antimycobacterial Activities from the Roots of Ficus nervosa

Bioassay‐guided fractionation of the active AcOEt‐soluble layer led to the isolation of two new pyranocoumarins, 3‐hydroxyxanthyletin ( 1 ) and 3‐methoxyxanthyletin ( 2 ), along with 22 known compounds including four simple coumarins, i.e., xanthyletin ( 3 ), umbelliferone ( 4 ), scopoletin ( 5 ), and (+)‐(S)‐marmesin ( 6 ); nine flavonoids, i.e., carpachromene ( 7 ), parvisoflavone B ( 8 ), alpinumisoflavone ( 9 ) genistein ( 10 ), 2′‐hydroxygenistein ( 11 ), prunetin ( 12 ), cajanin ( 13 ), apigenin ( 14 ), and (2S)‐naringenin ( 15 ); three benzenoids, i.e., 4‐hydroxybenzaldehyde ( 16 ), vanillin ( 17 ), and (S)‐lasiodiplodin ( 18 ); five steroids, i.e., ergosterol peroxide ( 19 ), a mixture of 6β‐hydroxystigmast‐4‐en‐3‐one ( 20 ) and 6β‐hydroxystigmasta‐4,22‐dien‐3‐one ( 21 ), and a mixture of β‐sitosterol ( 22 ) and stigmasterol ( 23 ); and one triterpenoid, i.e., oleanolic acid ( 24 ) from the roots of Ficus nervosa. Their structures were elucidated on the basis of extensive 1D‐ and 2D‐NMR as well as MS analyses. Among these isolates, 3‐hydroxyxanthyletin ( 1 ), genistein ( 10 ), prunetin ( 12 ), and (2S)‐naringenin ( 15 ) showed antimycobacterial activities against Mycobacterium tuberculosis H₃₇R_V in vitro with MIC values of 16, 35, 30, and ≤2.8 μg/ml, respectively.