Relative Contribution of Apices and Mature Tissues to ABA Synthesis in Droughted Maize Root Systems

We investigated whether different parts of maize root systemscould contribute to ABA synthesis, and whether a previous cycleof soil dehydration-rehydration would modify the ability ofroots to synthesize ABA. Maize (Zea mays L.) root tissues, i.e.mature primary root sections, young and unbranched primary rootsections, secondary roots and primary root tips, from both wellwatered plants and previously drought-rewatered plants, weresubjected to different degrees of dehydration and their ABAconcentration changes were assayed. All categories of rootsfrom always well watered plants, including mature tissues containingno apex, could synthesize ABA when dehydrated. Mature primaryroot sections and their previously associated secondary rootsaccumulated less ABA in response to dehydration than the youngprimary roots and primary root tips did, and their ABA accumulationwas not substantial until dehydration was below 65% of relativewater content (RWC). Previous soil dehydration-rehydration cyclessubstantially reduced ABA accumulation in these roots in responseto dehydration again. Young primary root sections and primaryroot tips accumulated ABA much more sensitively in responseto dehydration than mature root sections, although considerablevariations existed among different batches of young primaryroot sections. Results are discussed in the context of the relativecontribution of different categories of roots to ABA synthesiswhen the root system is in drying soil. We concluded that primaryroot apices should not contribute by more than 2% to the totalABA synthesis by the root system. (Received December 15, 1995; Accepted April 19, 1996)     

The use of aeroponics to investigate antioxidant activity in the roots of <Emphasis Type="Italic">Xerophyta viscosa</Emphasis>

In order to ultimately understand the whole plant mechanism of attaining desiccation tolerance, we undertook to investigate the root tissues of the resurrection plant Xerophyta viscosa, as previous work has only been conducted on the leaf tissues of resurrection plants. An aeroponic plant growth system was designed and optimised to observe the root’s response to desiccation without the restrictions of a soil medium, allowing easy access to roots. Successful culture of both X.viscosa and the control, Zea mays, was achieved and dehydration stress was implemented through reduction of nutrient solution spraying of the roots. After drying to the air dry state (achieved after 7 days for roots and 10 days for shoots), rehydration was achieved by resumption of root spraying. X.viscosa plants survived desiccation and recovered but Z. mays did not. The activity of the antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase and quantities of ascorbate and glutathione were determined during root desiccation. There was an initial decline in activity in all enzymes upon drying to 80% RWC, but activity thereafter remained constant, at rates indicative of potential metabolic activity, to the air-dry state. This data suggests that these enzymes are not denatured by desiccation of the root tissue. Ascorbate and glutathione content remained constant at concentrations of 70 and 100 μM, respectively during drying. Thus root tissues appear to retain antioxidant potential during drying, for use in recovery upon rehydration, as has been reported for leaf tissues of this and other resurrection plants.     

Reverse flow in roots of Sesbania rostrata measured using the constant power heat balance method

This investigation was performed to examine qualitatively and quantitatively the reverse flow in partially dried roots of Sesbania rostrata using the constant power heat balance method. First, a semi-empirical technique for estimating sheath conductance of sap-flow sensors without assuming that sap flow is zero at night was proposed. Sap flow measured with the heat balance method was compared with water uptake as measured by a potometric method. Sap flow was overestimated by 56·1% for a 3·3-mm-diameter root, and by 40·0% for 6·1 mm and 33·3% for 8·8 mm roots. However, high correlation coefficients between the rates of water uptake and sap flow demonstrated that calibration would provide reliable values for root sap flow. To detect reverse flow, a split root experiment was conducted using a S. rostrata plant with its root system divided between dry and wet compartments. Daily sap flow of the drying compartment declined whereas that in ‘wet’ root increased, suggesting that the decrease in water uptake by ‘dry’ roots was offset by the ‘wet’ roots. Reverse flow was observed at night in the root on the dry side of the container when the soil water potential was less than –0·30 MPa. The total amount of water released into the soil during the night period was estimated to be 22·5 g.     

Accumulation rate of ABA in detached maize roots correlates with root water potential regardless of age and branching order

How much ABA can be supplied by the roots is a key issue for modelling the ABA-mediated influence of drought on shoot physiology. We quantified accumulation rates of ABA ( S _ABA) in maize roots that were detached from well-watered plants and dehydrated to various extents by air-drying. S _ABA was estimated from changes in ABA content in root segments incubated at constant relative water content (RWC). Categories of root segments, differing in age and branching order, were compared (root branches, and nodal roots subdivided into root tips, subapical unbranched sections, and mature sections). All categories of roots accumulated ABA, including turgid and mature tissues containing no apex. S _ABA measured in turgid roots changed with root age and among root categories. This variability was largely accounted for by differences in water content among different categories of turgid roots. The response of S _ABA to changes in root water potential ( Ψ _root) induced by dehydration was common to root tips, nodal roots and branches of several ages, while this was not the case if root dehydration was expressed in terms of RWC. Differences among root categories in the response of S _ABA to RWC were due to different RWC values among categories at a given Ψ _root, and not to differences in the response of S _ABA to Ψ _root.     

Root porosity and oxygen movement in waterlogging-tolerant Trifolium tomentosum and -intolerant Trifolium glomeratum

Trifolium tomentosum and T. glomeratum are small (< 0·5 mg) seeded pasture legumes which are considered to be waterlogging tolerant and intolerant, respectively. The root porosity of the two species was compared for plants raised for 10 d in aerated nutrient solution and then transferred to either aerated (0·25 mol O₂ m^–3) or ‘hypoxic’ (0·031–0·069 mol O₂ m^–3) solutions for a further 7 and 21 d. After 21 d, T. tomentosum developed a root porosity of 11·2% in ‘hypoxic’ solution, which was significantly higher than the 6·1% developed by T. glomeratum. When grown in aerated solution, T. tomentosum also had a larger constitutive porosity (6·7%) than T. glomeratum (3·9%). Cylindrical root-sleeving O₂ electrodes were used to measure the rates of radial O₂ loss (ROL) from roots of the two species when in an O₂-free medium. In general, roots previously grown in ‘hypoxic’ solution had higher rates of ROL than roots grown in aerated solution. Moreover, the rates of ROL along the main root of T. tomentosum were ≈ 5-fold faster than from equivalent locations along roots of T. glomeratum. Manipulations of the shoot O₂ concentration resulted in rapid changes in ROL near the root tip of T. tomentosum plants raised in aerated or ‘hypoxic’ solutions, whereas for T. glomeratum ROL only increased for roots of plants raised in ‘hypoxic’ solution. Thus, the cortical air spaces in roots of both species raised in ‘hypoxic’ solution formed a continuous, low resistance pathway for O₂ diffusion from the shoots to the roots. ROL from the lateral roots was also evaluated and it was 3-fold faster from T. tomentosum than from T. glomeratum. Moreover, ROL from lateral roots of T. tomentosum was 10–20-fold higher than from a position on the primary root axis the same distance from the root/shoot junction. Relatively, high rates of ROL were also recorded for young (40 mm in length) lateral roots of T. glomeratum which were previously grown in ‘hypoxic’ solution, but the ROL was low for the older lateral roots of this species. The substantial amounts of ROL from the lateral roots may limit O₂ supply to the lower parts of the primary root axis, so that the laterals probably become the main functional root system in waterlogged soils.     

Influence of temperature and soil drying on respiration of individual roots in citrus: integrating greenhouse observations into a predictive model for the field

In citrus, the majority of fine roots are distributed near the soil surface – a region where conditions are frequently dry and temperatures fluctuate considerably. To develop a better understanding of the relationship between changes in soil conditions and a plant’s below‐ground respiratory costs, the effects of temperature and soil drying on citrus root respiration were quantified in controlled greenhouse experiments. Chambers designed for measuring the respiration of individual roots were used. Under moist soil conditions, root respiration in citrus increased exponentially with changes in soil temperature (Q₁₀ = 1·8–2·0), provided that the changes in temperature were short‐term. However, when temperatures were held constant, root respiration did not increase exponentially with increasing temperatures. Instead, the roots acclimated to controlled temperatures above 23 °C, thereby reducing their metabolism in warmer soils. Under drying soil conditions, root respiration decreased gradually beginning at 6% soil water content and reached a minimum at <2% soil water content in sandy soil. A model was constructed from greenhouse data to predict diurnal patterns of fine root respiration based on temperature and soil water content. The model was then validated in the field using data obtained by CO₂ trapping on root systems of mature citrus trees. The trees were grown at a site where the soil temperature and water content were manipulated. Respiration predicted by the model was in general agreement with observed rates, which indicates the model may be used to estimate entire root system respiration for citrus.     

Combined effects of spray‐drying conditions and postdrying storage time and temperature on Salmonella choleraesuis and Salmonella typhimurium survival when inoculated in liquid porcine plasma

The objective of this study was to determine the effectiveness of the spray‐drying process on the inactivation of Salmonella choleraesuis and Salmonella typhimurium spiked in liquid porcine plasma and to test the additive effect of immediate postdrying storage. Commercial spray‐dried porcine plasma was sterilized by irradiation and then reconstituted (1:9) with sterile water. Aliquots of reconstituted plasma were inoculated with either S. choleraesuis or S. typhimurium, subjected to spray‐drying at an inlet temperature of 200°C and an outlet temperature of either 71 or 80°C, and each spray‐drying temperature combinations were subjected to either 0, 30 or 60 s of residence time (RT) as a simulation of residence time typical of commercial dryers. Spray‐dried samples were stored at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days. Bacterial counts of each Salmonella spp., were completed for all samples. For both Salmonella spp., spray‐drying at both outlet temperatures reduced bacterial counts about 3 logs at RT 0 s, while there was about a 5·5 log reduction at RT 60 s. Storage of all dried samples at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days eliminate all detectable bacterial counts of both Salmonella spp.

Significance and Impact of the Study

Safety of raw materials from animal origin like spray‐dried porcine plasma (SDPP) may be a concern for the swine industry. Spray‐drying process and postdrying storage are good inactivation steps to reduce the bacterial load of Salmonella choleraesuis and Salmonella typhimurium. For both Salmonella spp., spray‐drying at 71°C or 80°C outlet temperatures reduced bacterial counts about 3 log at residence time (RT) 0 s, while there was about a 5.5 log reduction at RT 60 s. Storage of all dried samples at either 4.0 ± 3.0°C or 23.0 ± 0.3°C for 15 days was effective for eliminating detectable bacterial counts of both Salmonella spp.     

Some effects of the growth-retardant glyphosine [N, N-bis (phosphonomethyl) glycine] on the sugar-beet crop

Four experiments tested a range of doses of the growth-retardant glyphosine on sugar-beet crops between 1970 and 1972. The period of time between application and harvest was varied. One experiment (1972) in nutrient mist culture examined its effect on seedling root development. The growth-retardant decreased root, sugar and top yield of sugar beet at all doses in excess of 0·56 kg a.i. ha^-1 at all treatment times. It increased the α-amino nitrogen content of beet roots significantly (P < 0·05) with doses in excess of 1·12 kg a.i. ha^-1. When applied to the leaves of seedlings with their roots in nutrient mist culture, glyphosine at 500 ppm and 2000 ppm of a.i. in aqueous solution slowed or stopped root elongation.     

New national and regional bryophyte records, 8

Abstract

Monoclea forsteri Hook. is a thalloid liverwort species that is found in damp habitats and can, therefore, be expected to be sensitive to dehydration. It does, however, have some unique chemical constituents and anatomical features that could play a role in dealing with the adverse effects of water deficits. Corresponding to the habitat, M. forsteri lost its turgor at high relative water content (RWC≈0.90) and did not survive drying below 20% RWC. Moreover, the gametophytes showed an increase in malondialdehyde content and a depletion of the ascorbate pool during dehydration, indicating oxidative damage. Cellular constituents did not affect turgor pressure during drying and electrolyte leakage from the cells was greatly increased at RWC<0.20. Photosynthetic processes seemed not to be affected by the loss of turgor, but a decline appeared to correlate with an increase in electrolyte leakage. A speedy and fully sustained recovery from dehydration was realized from water contents above 30% and seemed only to be possible if membrane integrity could be preserved. Anatomical characteristics within M. forsteri gametophytes deserve further investigation to better understand their physiological functions.     

Shoot apex removal does not alter autoregulation of Modulation in soybean

The suppression of new nodule development in soybean (Glycine max (L.) Merr.) has been previously demonstrated to involve the shoot through reciprocal grafts between the wild-type cultivar Bragg and its supernodulating mutant nts382. Using the same grafting technique, but modified through the excision of the shoot apex region and emerging lateral shoots, we show here that autoregulation of nodule number still existed despite apex removal. This radical treatment lowered total nodule number per plant as well as root, shoot and nodule dry weight. Bragg shoots grafted onto nts382 roots gave wild-type nodulation (26 nodules, 15mg total nodule mass) as compared to nts382 shoots grafted onto Bragg roots (340 nodules, 277 mg total nodule mass). Specific nodule mass differed between supernodulating (about 0·5-1·0mg per nodule) and wild-type nodulating (2·3 mg per nodule) plants. In contrast to other growth characteristics, apex removal did not affect specific nodule size, except in plants with wild-type shoots and nts382 (supernodulation) roots. Apex removal only slightly affected the percentage of nodule weight per total root weight in nts382, but had a severe effect in wild type. Growth reductions varied between the normal and supernodulating plants. The fact that autoregulation of nodulation still functions in plants devoid of functional shoot apices suggests that the autoregulation signal may not be derived from the apex regions and that the leaf may be a likely source.     

Xylem and cell turgor pressure probe measurements in intact roots of glycophytes: transpiration induces a change in the radial and cellular reflection coefficients

Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ_r) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ_r assumed values of the order of about 0·2–0·4. The σ_r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m^?2 s^?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ_r= 1. Further increase of the light intensity to about 400 μmol m^?2 s^?1 resulted in a slight but significant decrease of the σ_r values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m^?2 s^?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ_r values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m^?2 s^?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σ_r values of the respective glycophytes, the σ_c values of the root cortical cells of wheat and barley were close to unity, whereas σ_c for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m^?2 s^?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σ_r values, the σ_c values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.     

Genotypic variation in 'early warning signals' from roots in drying soil: intrinsic differences in ABA synthesising capacity rather than root density determines total ABA 'message' in cowpea (Vigna unguiculata L.)

In a comparison of six cowpea cultivars, we determined the variation in abscisic acid (ABA) production as an ‘early warning signal’ produced in response to drought stress. By imposing drought only to the upper 20 cm rooting zone, we compared the rates of ABA synthesis relative to (i) total root mass and (ii) inherent variation per unit root mass. We were able to relate the intensity of the stress response to these two factors, and determine which is quantitatively more important as the primary signal indicating responsiveness to drought stress. Plants were grown in 1.2 m long columns and a soil drying treatment imposed in such a way that that upper roots were in dry soil and deep roots in soil at field capacity. Relative water contents (RWC) of stressed plants were similar and not significantly different from those of well watered controls. However, roots accumulated ABA in the dehydrated zone, where root water content ranged from 10–12 g g^?1 DW. The soil moisture contents and root ‐water contents in the dry zone were similar for each of the different varieties. However, the ABA contents were significantly different in drought‐stressed (upper) roots and ranged from 7.82 nmol g^?1 DW in cv. APC 689 to 16.02 nmol g^?1 DW in cv. APC 370, such that for varieties with similar overall root weights (e.g. APC 580 and APC 540) the different ABA contents were related to the capacity for ABA synthesis. The relationship between stomatal conductance and total root ABA was assessed, with a negative relation (r= 0.90, n= 24, P= 0.05) suggesting that the intrinsic capacity of cowpea varieties for ABA synthesis could play an important role in regulating stomatal conductance in a drying soil and provide useful selection criteria for tolerance to drought stress.     

Dehydration of isolated roots of seven <Emphasis Type="Italic">Lupinus</Emphasis> species induces synthesis of different amounts of free,but not conjugated,abscisic acid

Abscisic acid (ABA) is recognised as an important hormone involved in root-to-shoot communication of drought stress in plants. This study aimed to determine whether isolated roots can produce both free and conjugated ABA (ABA–glucose ester) and whether Lupinus species vary in the synthesis of ABA in the roots when dehydrated. The concentration of free and conjugated ABA at 100 and 50% root water content was measured in the distal 10 mm of the roots of 3- to 5-day-old seedlings of seven Lupinus species with and without 10⁻⁵ M tetcyclacis, an inhibitor of the oxidative breakdown of ABA. When the root tips were exposed to tetcyclacis, the concentration of free ABA increased by 20% on average, suggesting that oxidative breakdown of free ABA was limited in the isolated Lupinus roots. The concentration of free ABA of the fully hydrated plants varied significantly among genotypes and more than doubled on average across genotypes with dehydration of the root tips to 50% water content. The concentration of conjugated ABA also varied significantly with species, but was only one-tenth the concentration of free ABA in the roots and did not change significantly with root dehydration or the inhibition of oxidative metabolism. The production of free ABA in response to the water deficit varied with species from +470% in L. digitatus to +33% in L. angustifolius. The small concentration and lack of increase of conjugated ABA with water deficit suggests that it is unlikely to have an important role as a root signal in response to soil drying in Lupinus species.     

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     

Comparative evaluation of the hygienic efficacy of an ultra-rapid hand dryer vs conventional warm air hand dryers

Aims: To compare an ultra‐rapid hand dryer against warm air dryers, with regard to: (A) bacterial transfer after drying and (B) the impact on bacterial numbers of rubbing hands during dryer use. Methods and Results: The Airblade? dryer (Dyson Ltd) uses two air ‘knives’ to strip water from still hands, whereas conventional dryers use warm air to evaporate moisture whilst hands are rubbed together. These approaches were compared using 14 volunteers; the Airblade? and two types of warm air dryer. In study (A), hands were contaminated by handling meat and then washed in a standardized manner. After dryer use, fingers were pressed onto foil and transfer of residual bacteria enumerated. Transfers of 0–10⁷ CFU per five fingers were observed. For a drying time of 10 s, the Airblade? led to significantly less bacterial transfer than the other dryers (P < 0·05; range 0·0003–0·0015). When the latter were used for 30–35 s, the trend was for the Airblade to still perform better, but differences were not significant (P > 0·05, range 0·1317–0·4099). In study (B), drying was performed ± hand rubbing. Contact plates enumerated bacteria transferred from palms, fingers and fingertips before and after drying. When keeping hands still, there was no statistical difference between dryers, and reduction in the numbers released was almost as high as with paper towels. Rubbing when using the warm air dryers inhibited an overall reduction in bacterial numbers on the skin (P < 0·05). Conclusions: Effective hand drying is important for reducing transfer of commensals or remaining contaminants to surfaces. Rubbing hands during warm air drying can counteract the reduction in bacterial numbers accrued during handwashing. Significance and Impact of the Study: The Airblade? was superior to the warm air dryers for reducing bacterial transfer. Its short, 10 s drying time should encourage greater compliance with hand drying and thus help reduce the spread of infectious agents via hands.     

Some physiological comparisons between the resurrection grass, Eragrostis nindensis, and the related desiccation-sensitive species, E. curvula

Both the poikilochlorophyllous resurrection grass, Eragrostisnindensis, and the desiccation sensitive species, E.curvula, dehydrate to a relative water content (RWC) of less than5% in two weeks. On rewatering, most E. nindensisleaves (except the older, outer ones) rehydrate and resume normal metabolicactivity within a few days, whereas E. curvula does notrecover. There is a controlled loss of photosynthetic pigments, paralleled witha gradual shutdown in gas exchange during dehydration of E.nindensis. On rehydration respiration resumes almost immediately butphotosynthesis only restarts at 70% RWC by which time chlorophyll hasbeen resynthesised and anthocyanin content reduced. In contrast, photosyntheticactivity in E. curvula is maintained down to 40%RWC, after which further drying results in a sudden breakdown of thephotosynthetic system and its pigments. At this point, electrolyte leakage andincreases F_V/F_M decreases such that belowca. 40% RWC, metabolism is irreparably damaged.Interestingly, the older outer leaf in most tillers of E.nindensis does not rehydrate. These leaves show signs of membranedamage and curl in an irregular manner similar to those of E.curvula during dehydration.     

Intra- and inter-plant variation in xylem cavitation in Betula occidentalis

A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.     

Increased Synthesis of ABA in Partially Dehydrated Root Tips and ABA Transport from Roots to Leaves

Zhang, J. and Davies, W. J. 1987. Increased synthesis of ABAin partially dehydrated root tips and ABA transport from rootsto leaves.—J. exp. Bot. 38: 2015–2023. Isolated root tips of pea (Pisum sativum L. cv. Feltham First)and Commelina communis L. were air-dried until they lost between10% and 40% of their fresh weight, followed by a period of incubationat these reduced water contents. These treatments resulted inincreased ABA production, suggesting that root tips of bothspecies have the capacity to synthesize ABA in increased amountswhen water deficits develop in the root. The ABA concentrationin pea roots increased linearly as turgors fell below about0·15 M Pa and relative water contents (R WC) fell below90%. Commelina roots produced more ABA when RWC fell below asimilar value but the threshold turgor for increased ABA productionin Commelina roots was around 0·30 MPa. Roots of intact plants loaded with ABA as a result of incubationin solutions of varying concentrations provided ABA to leaveswhich resulted in increased ABA concentrations in the leaveswhen these were assayed several hours later. This occurred whenthese roots were not contributing substantially to transpirationalflux. Leaves on shoots that were enclosed and darkened and thereforenot transpiring, did not accumulate ABA from ‘loaded’roots. A role for root-sourced ABA in root-to-shoot communication ofthe effects of soil drying is discussed. Key words: ABA, roots, water relations     

Evidence that the induction of crassulacean acid metabolism by water stress in Mesembryanthemum crystallinum (L.) involves root signalling

The aim of this study was to investigate whether the root system of Mesembryanthemum crystallinum (L.) plays a role in triggering the induction of crassulacean acid metabolism (CAM) during water stress. Depriving well-irrigated plants of water, by allowing the soil surrounding the roots to dry, caused increased daily losses in leaf relative water content (RVVC) and mesophyll cell turgor pressure. The RWC of the roots also declined. Subsequently plants exhibited physiological characteristics of CAM photosynthesis (i.e. diurnal fluctuations in leaf titratable acidity and nocturnal net CO₂ fixation). When the root system of plants was divided equally between two soil compartments and one half deprived of water, plants exhibited physiological characteristics of CAM without prior changes in leaf RWC content or mesophyll cell turgor pressure. Only the RWC of the water-stressed portion of the roots was reduced. These data suggest that in water-stressed plants daily changes in leaf water relations greater than those observed in well-irrigated plants, are not essential to trigger CAM expression. It is probable that a reduction in soil water availability can be perceived by the roots of M. crystallinum and that this information is conveyed to the leaves triggering the transition from C₃ to CAM photosynthesis.