Reduced Root Hydraulic Conductivity Research Articles

Soil Water Deficit Reduced Root Hydraulic Conductivity of Common Reed (Phragmites australis).

Alterations in root hydraulics in response to varying moisture conditions remain a subject of debate. In our investigation, we subjected common reeds (Phragmites australis) to a 45-day treatment with four distinct soil moisture levels. The findings unveiled that, in response to drought stress, the total root length, surface area, volume, and average diameter exhibited varying degrees of reduction. Anatomically, drought caused a reduction in root diameter (RD), cortex thickness (CT), vessel diameter (VD), and root cross-sectional area (RCA). A decrease in soil moisture significantly reduced both whole- and single-root hydraulic conductivity (Lpwr, Lpsr). The total length, surface area, volume, and average diameter of the reed root system were significantly correlated with Lpwr, while RD, CT, and RCA were significantly correlated with Lpsr. A decrease in soil moisture content significantly influenced root morphological and anatomical characteristics, which, in turn, altered Lpr, and the transcriptome results suggest that this may be associated with the variation in the expression of abscisic acid (ABA) and aquaporins (AQPs) genes. Our initial findings address a gap in our understanding of reed hydraulics, offering fresh theoretical insights into how herbaceous plants respond to external stressors.

Physiological roles of Casparian strips and suberin in the transport of water and solutes.

Summary The formation of Casparian strips (CS) and the deposition of suberin at the endodermis of plant roots are thought to limit the apoplastic transport of water and ions. We investigated the specific role of each of these apoplastic barriers in the control of hydro‐mineral transport by roots and the consequences on shoot growth.A collection of Arabidopsis thaliana mutants defective in suberin deposition and/or CS development was characterized under standard conditions using a hydroponic system and the Phenopsis platform.Mutants altered in suberin deposition had enhanced root hydraulic conductivity, indicating a restrictive role for this compound in water transport. In contrast, defective CS directly increased solute leakage and indirectly reduced root hydraulic conductivity. Defective CS also led to a reduction in rosette growth, which was partly dependent on the hydro‐mineral status of the plant. Ectopic suberin was shown to partially compensate for defective CS phenotypes.Altogether, our work shows that the functionality of the root apoplastic diffusion barriers greatly influences the plant physiology, and that their integrity is tightly surveyed.

Beneficial role of acetylcholine in chlorophyll metabolism and photosynthetic gas exchange in Nicotiana benthamiana seedlings under salinity stress.

Acetylcholine (ACh) is believed to improve plant growth. However, regulation at biochemical and molecular levels is largely unknown. The present study investigated the impact of exogenously applied ACh (10µm) on growth and chlorophyll metabolism in hydroponically grown Nicotiana benthamiana under salt stress (150mm NaCl). Salinity reduced root hydraulic conductivity while ACh-treated seedlings exhibited a significant increase, resulting in increased relative water content. Salinity induced a reduction in chlorophyll biosynthetic intermediates, such as protoporphyrin-IX, Mg-photoporphyrin-IX and protochlorophyllide, which were significantly ameliorated in the presence of ACh. This influence of ACh on chlorophyll synthesis was confirmed by up-regulation of HEMA1, CHLH, CAO and POR genes. Gas exchange parameters, i.e. stomatal conductance, internal CO2 concentration and transpiration rate, increased with ACh, thereby alleviating the salinity effects on photosynthesis. In addition, the salinity-induced enhancement of lipid peroxidation declined after ACh treatment through modulation of the activity of the assayed antioxidant enzymes (superoxide dismutase and peroxidase). Importantly, ACh significantly reduced the uptake of Na and increased uptake of K, resulting in a decline in the Na/K ratio. Results of the present study indicate that ACh can be effective in ameliorating NaCl-induced osmotic stress, altering chlorophyll metabolism and thus photosynthesis by maintaining ion homeostasis, hydraulic conductivity and water balance.

Dehydrin MtCAS31 promotes autophagic degradation under drought stress

ABSTRACTDrought stress seriously affects crop yield, and the mechanism underlying plant resistance to drought stress via macroautophagy/autophagy is not clear. Here, we show that a dehydrin, Medicago truncatula MtCAS31 (cold acclimation-specific 31), a positive regulator of drought response, plays a key role in autophagic degradation. A GFP cleavage assay and treatment with an autophagy-specific inhibitor indicated that MtCAS31 participates in the autophagic degradation pathway and that overexpressing MtCAS31 promotes autophagy under drought stress. Furthermore, we discovered that MtCAS31 interacts with the autophagy-related protein ATG8a in the AIM-like motif YXXXI, supporting its function in autophagic degradation. In addition, we identified a cargo protein of MtCAS31, the aquaporin MtPIP2;7, by screening an M. truncatula cDNA library. We found that MtPIP2;7 functions as a negative regulator of drought response. Under drought stress, MtCAS31 facilitated the autophagic degradation of MtPIP2;7 and reduced root hydraulic conductivity, thus reducing water loss and improving drought tolerance. Taken together, our results reveal a novel function of dehydrins in promoting the autophagic degradation of proteins, which extends our knowledge of the function of dehydrins.Abbreviations: AIM: ATG8-interacting motif; ATG: autophagy-related; ATI1: ATG8-interacting protein1; BiFC: Biomolecular fluorescence complementation; CAS31: cold acclimation-specific 31; ConcA: concanamycin A; DSK2: dominant suppressor of KAR2; ER: endoplasmic reticulum; ERAD: ER-associated degradation; NBR1: next to BRCA1 gene 1; PM: plasma membrane; PIPs: plasma membrane intrinsic proteins; TALEN: transcription activator-like effector nuclease; TSPO: tryptophan-rich sensory protein/translocator; UPR: unfolded protein response; VC: vector control

Phytophthora infection in flooded citrus trees reduces root hydraulic conductance more than under non-flooded condition

Phythophthora infections in citrus trees may result in poor tree health. Infections are very common under wet conditions. However, the interaction of Phytophthora and flooding conditions has yet to be studied in citrus. In this study, Phytophthora was inoculated into the soil of potted grapefruit trees, and trees were either well-watered or flooded for six weeks; then, these trees were compared with trees without Phytophthora inoculation under the same irrigation regimes. Phytophthora reduced root hydraulic conductance in citrus even though the propagule count was very low when root hydraulic conductance was measured (at the end of the experiment). In the presence of flooding, Phytophthora significantly reduced root hydraulic conductance compared to non-flooded citrus trees inoculated with Phytophthora alone. Flooding reduced stomatal conductance and increased tree mortality more than Phytophthora, but Phytophthora decreased the number of leaves per tree and tended to decrease tree growth more than flooding.

Tolerance of peat-grown Scots pine seedlings to waterlogging and drought: Morphological, physiological, and metabolic responses to stress

Depending on the soil preparation method applied and peat characteristics, Scots pine (Pinus sylvestris L.) seedlings planted in prepared spots on forestry-drained peatlands may become more susceptible to extreme weather events such as drought or flooding. Only by studying its coping strategies can we eventually design methodology better suited for Scots pine regeneration on peat soils. In this study, we evaluated the tolerance of two-year-old seedlings to the two extremes of water-associated stress, drought and waterlogging, in unprocessed peat. Over one growing season in controlled conditions, drought distinctly reduced root and shoot growth in addition to photochemical efficiency (Fv/Fm) particularly in previous-year needles, whilst wet stress had little discernible impact. Drought also influenced polyamine metabolism by increasing free putrescine and spermine concentrations especially in current-year needles, whereas no impact was discerned in the wet treatment. Furthermore, reduced root hydraulic conductance (Kr) was observed in drought-stressed root systems. Apparently, waterlogging does not modify Scots pine seedling growth or vitality immediately, but rather in the longer term. However, no fatalities occurred in either of the stress treatments, this despite water availability in the Sphagnum peat reaching its lower (permanent wilting point) and upper (10% soil air content) limits. Maintenance of rather high photochemical efficiency despite severe drought stress would seem to indicate a potential for seedling recovery if water availability in the peat substrate improved.

Influence of salinity on pip gene expression in citrus roots and its relationship with root hydraulic conductance, transpiration and chloride exclusion from leaves

The present work studies the effect of salinity on PIP aquaporins gene expression in citrus roots and its relationship with root hydraulic conductance (Kr), transpiration rate (E) and chloride transport to leaves. To this end, ten-month-old seedlings of Cleopatra mandarin (CM), Carrizo citrange (CC) and Poncirus trifoliata (PT) were tested.No effect was detected of salt treatments on PIP1 and PIP2 aquaporin mRNA transcript abundances from citrus roots, although PIP1 expression in CM roots was lower than in CC and PT.The lowest Kr and E values were detected in CM, whereas PT had the highest. CC seedlings presented intermediate values for these parameters. Addition of HgCl2 to either control or salt solution (200mM NaCl) led to a decrease in Kr and E, thus implying aquaporin involvement. By contrast, salinity strongly reduced Kr and E in all plants, with this effect being unrelated to aquaporin activity.In salinized seedlings, E values appear to be related with Cl− concentration in leaves. Thus, CM seedlings treated with 80mM NaCl presented a lower Cl− uptake by leaves than PT, whereas this trend was intermediate in CC. Moreover, Hg2+ treatments significantly reduced leaf Cl− concentration in salt stressed plants, probably through the reduction of E.We can conclude that differences among genotypes in PIP1 expression affect Cl− exclusion from leaves, probably due to effects on water movement. Nevertheless, long-term salt treatments did not affect PIP expression in citrus plants, but reduced root hydraulic conductance and transpiration.

Mechanisms of Water Transport Mediated by PIP Aquaporins and Their Regulation Via Phosphorylation Events Under Salinity Stress in Barley Roots

Water homeostasis is crucial to the growth and survival of plants under water-related stress. Plasma membrane intrinsic proteins (PIPs) have been shown to be primary channels mediating water uptake in plant cells. Here we report the water transport activity and mechanisms for the regulation of barley (Hordeum vulgare) PIP aquaporins. HvPIP2 but not HvPIP1 channels were found to show robust water transport activity when expressed alone in Xenopus laevis oocytes. However, the co-expression of HvPIP1 with HvPIP2 in oocytes resulted in significant increases in activity compared with the expression of HvPIP2 alone, suggesting the participation of HvPIP1 in water transport together with HvPIP2 presumably through heteromerization. Severe salinity stress (200 mM NaCl) significantly reduced root hydraulic conductivity (Lp(r)) and the accumulation of six of 10 HvPIP mRNAs. However, under relatively mild stress (100 mM NaCl), only a moderate reduction in Lp(r) with no significant difference in HvPIP mRNA levels was observed. Sorbitol-mediated osmotic stress equivalent to 100 and 200 mM NaCl induced nearly identical Lp(r) reductions in barley roots. Furthermore, the water transport activity in intact barley roots was suggested to require phosphorylation that is sensitive to a kinase inhibitor, staurosporine. HvPIP2s also showed water efflux activity in Xenopus oocytes, suggesting a potential ability to mediate water loss from cells under hypertonic conditions. Water transport via HvPIP aquaporins and the significance of reductions of Lp(r) in barley plants during salinity stress are discussed.

Root signalling and modulation of stomatal closure in flooded citrus seedlings

In this work, we studied the sequence of responses induced by flooding in citrus plants, with the aim of identifying the signals that lead to stomatal closure. One-year-old seedlings of Carrizo citrange, grown in sand under greenhouse conditions, were waterlogged for 35 d and compared with normally watered well-drained plants. Significant decreases in stomatal conductance and transpiration were detected between flooded and control seedlings from a week after the beginning of the experiment. However ABA concentration in leaves only started to increase after three weeks of flooding, suggesting that stomata closed in the absence of a rise in foliar ABA. Therefore, stomatal closure in waterlogged seedlings does not appear to be induced by ABA, at least during the early stages of flood-stress. The low levels of ABA detected in roots and xylem sap from flooded seedlings indicated that it is very unlikely that the ABA increase in the leaves of these plants is due to ABA translocation from roots to shoots. We propose that ABA is produced in old leaves and transported to younger leaves. Flooding had no effect on water potential or the relative water content of leaves. Soil flooding reduced root hydraulic conductance in citrus seedlings. This effect was already evident after a week of waterlogging, and at the end of the experiment, flood-stressed seedlings reached values of root hydraulic conductance below 12% of that of control plants. This reduction was related to down-regulation of the expression of PIP aquaporins. In addition, whole plant transpiration was reduced by 56% after 35 d under flooding conditions. Flood-stress also decreased the pH of sap extracted from citrus roots. Evidence is presented suggesting that acidosis induced by anoxic stress in roots causes gating of aquaporins, thereby decreasing hydraulic conductance. Additionally, stomatal closure finely balances-out low pH-mediated losses of root hydraulic conductance therefore maintaining stable leaf hydration.

Ectomycorrhizas and water relations of trees: a review

There is plenty of evidence for improved nutrient acquisition by ectomycorrhizas in trees; however, their role in water uptake is much less clear. In addition to experiments showing improved performance during drought by mycorrhizal plants, there are several studies showing reduced root hydraulic conductivity and reduced water uptake in mycorrhizal roots. The clearest direct mechanism for increased water uptake is the increased extension growth and absorbing surface area, particularly in fungal species with external mycelium of the long-distance exploration type. Some studies have found increased aquaporin function and, consequently, increased root hydraulic conductivity in ectomycorrhizal plants while other studies showed no effect of ectomycorrhizal associations on root water flow properties. The aquaporin function of the fungal hyphae is also likely to be important for the uptake of water by the ectomycorrhizal plant, but more work needs to be done in this area. The best-known indirect mechanism for mycorrhizal effects on water relations is improved nutrient status of the host. Others include altered carbohydrate assimilation via stomatal function, possibly mediated by changes in growth regulator balance; increased sink strength in mycorrhizal roots; antioxidant metabolism; and changes in osmotic adjustment. None of these possibilities has been sufficiently explored. The mycorrhizal structure may also reduce water movement because of different fine root architecture (thickness), cell wall hydrophobicity or the larger number of membranes that water has to cross on the way from the soil to the xylem. In future studies, pot experiments comparing mycorrhizal and nonmycorrhizal plants will still be useful in studying well-defined physiological details. However, the quantitative importance of ectomycorrhizas for tree water uptake and water relations can only be assessed by field studies using innovative approaches. Hydraulic redistribution can support nutrient uptake during prolonged dry periods. In large trees with deep root systems, it may turn out that the most important function of mycorrhizas during drought is to facilitate nutrient acquisition.

Aerenchyma Formed Under Phosphorus Deficiency Contributes to the Reduced Root Hydraulic Conductivity in Maize Roots

AbstractRoot hydraulic conductivity has been shown to decrease under phosphorus (P) deficiency. This study investigated how the formation of aerenchyma is related to this change. Root anatomy, as well as root hydraulic conductivity was studied in maize (Zea mays L.) roots under different phosphorus nutrition conditions. Plant roots under P stress showed enhanced degradation of cortical cells and the aerenchyma formation was associated with their reduced root hydraulic conductivity, supporting our hypothesis that air spaces that form in the cortex of phosphorus‐stressed roots impede the radial transport of water in a root cylinder. Further evidence came from the variation in aerenchyma formation due to genotypic differences. Five maize inbred lines with different porosity in their root cortex showed a significant negative correlation with their root hydraulic conductivity. Shoot relative water content was also found lower in P‐deficient maize plants than that in P‐sufficient ones when such treatment was prolonged enough, suggesting a limitation of water transport due to lowered root hydraulic conductivity of P‐deficient plants.(Handling editor: Ming Yuan)

Is rootstock-induced dwarfing in olive an effect of reduced plant hydraulic efficiency?

We investigated the hydraulic architecture of young olive trees either self-rooted or grafted on rootstocks with contrasting size-controlling potential. Clones of Olea europea L. (Olive) cv 'Leccino' inducing vigorous scion growth (Leccino 'Minerva', LM) or scion dwarfing (Leccino 'Dwarf', LD) were studied in different scion/rootstock combinations (LD, LM, LD/LD, LM/LM, LD/LM and LM/LD). Shoots growing on LD root systems developed about 50% less leaf surface area than shoots growing on LM root systems. Root systems accounted for 60-70% of plant hydraulic resistance (R), whereas hydraulic resistance of the graft union was negligible. Hydraulic conductance (K = 1/R) of LD root systems was up to 2.5 times less than that of LM root systems. Total leaf surface area (A(L)) was closely and positively related to root hydraulic conductance so that whole-plant hydraulic conductance scaled by A(L) did not differ between experimental groups. Accordingly, maximum transpiration rate and minimum leaf water potential did not differ significantly among experimental groups. We conclude that reduced root hydraulic conductance may explain rootstock-induced dwarfing in olive.

Drought resistance of Ailanthus altissima: root hydraulics and water relations.

Drought resistance of Ailanthus altissima (Mill.) Swingle is a major factor underlying the impressively wide expansion of this species in Europe and North America. We studied the specific mechanism used by A. altissima to withstand drought by subjecting potted seedlings to four irrigation regimes. At the end of the 13-week treatment period, soil water potential was -0.05 MPa for well-watered control seedlings (W) and -0.4, -0.8 and -1.7 MPa for drought-stressed seedlings (S) in irrigation regimes S1, S2 and S3, respectively. Root and shoot biomass production did not differ significantly among the four groups. A progressively marked stomatal closure was observed in drought-stressed seedlings, leading to homeostasis of leaf water potential, which was maintained well above the turgor loss point. Root and shoot hydraulics were measured with a high-pressure flow meter. When scaled by leaf surface area, shoot hydraulic conductance did not differ among the treated seedlings, whereas root hydraulic conductance decreased by about 20% in S1 and S2 seedlings and by about 70% in S3 seedlings, with respect to the well-watered control value. Similar differences were observed when root hydraulic conductance was scaled by root surface area, suggesting that roots had become less permeable to water. Anatomical observations of root cross sections revealed that S3 seedlings had shrunken cortical cells and a multilayer endodermal-like tissue that probably impaired soil-to-root stele water transport. We conclude that A. altissima seedlings are able to withstand drought by employing a highly effective water-saving mechanism that involves reduced water loss by leaves and reduced root hydraulic conductance. This water-saving mechanism helps explain how A. altissima successfully competes with native vegetation.

Ecophysiological adaptations of black spruce ( Picea mariana ) and tamarack ( Larix laricina ) seedlings to flooding

Black spruce (Picea mariana (Mill.) B.S.P.) and tamarack (Larix laricina (Du Roi) K. Koch) are the predominant tree species in boreal peatlands. The effects of 34 days of flooding on morphological and physiolog- ical responses were investigated in the greenhouse for black spruce and tamarack seedlings in their second growing season (18 months old). Flooding resulted in reduced root hydraulic conductance, net assimilation rate and stomatal conductance and increased needle electro- lyte leakage in both species. Flooded tamarack seedlings maintained a higher net assimilation rate and stomatal conductance compared to flooded black spruce. Flooded tamarack seedlings were also able to maintain higher root hydraulic conductance compared to flooded black spruce seedlings at a comparable time period of flooding. Root respiration declined in both species under flooding. Sugar concentration increased in shoots while decreasing in roots in both species under flooding. Needles of flooded black spruce appeared necrotic and electrolyte leakage increased over time with flooding and remained signif- icantly higher than in flooded tamarack seedlings. No visible damage symptoms were observed in flooded tamarack seedlings. Flooded tamarack seedlings devel- oped adventitious roots beginning 16 days after the start of flooding treatment. Adventitious roots exhibited significantly higher root hydraulic conductivity than similarly sized flooded tamarack roots. Flooded black spruce lacked any such morphological adaptation. These results suggest that tamarack is better able to adjust both morphologically and physiologically to prolonged soil flooding than black spruce seedlings.

Responses of black spruce (Picea mariana) and tamarack (Larix laricina) to flooding and ethylene.

Black spruce (Picea mariana (Mill.) BSP) and tamarack (Larix laricina (Du Roi) K. Koch) are the predominant tree species in the boreal peatlands of Alberta, Canada, where low nutrient availability, low soil temperature and a high water table limit their growth. Effects of flooding for 28 days on morphological and physiological responses were investigated in greenhouse-grown black spruce and tamarack seedlings in a growth chamber. Flooding reduced root hydraulic conductance, net assimilation rate and stomatal conductance, and increased water-use efficiency (WUE) and needle electrolyte leakage in both species. Although flooded black spruce seedlings maintained higher net assimilation rates and stomatal conductance than flooded tamarack seedlings, flooded tamarack seedlings were able to maintain higher root hydraulic conductance than flooded black spruce seedlings. Needles of flooded black spruce developed tip necrosis and electrolyte leakage after 14 days of flooding, and these symptoms were subsequently more prominent than in needles of flooded tamarack seedlings. Flooded tamarack seedlings exhibited no visible injury symptoms and developed hypertrophied lenticels at their stem base. Application of exogenous ethylene resulted in a significant reduction in net assimilation, stomatal conductance and root respiration, whereas root hydraulic conductivity increased in both species. Thus, although flooded black spruce seedlings maintained a higher stomatal conductance and net assimilation rate than tamarack seedlings, black spruce did not cope with the deleterious effects of prolonged soil flooding and exogenous ethylene as well as tamarack.

PIP1 plasma membrane aquaporins in tobacco: from cellular effects to function in plants.

The molecular functions of several aquaporins are well characterized (e.g., by analysis of aquaporin-expressing Xenopus oocytes). However, their significance in the physiology of water transport in multicellular organisms remains uncertain. The tobacco plasma membrane aquaporin NtAQP1 was used to elucidate this issue. By comparing antisense plants that were inhibited in NtAQP1 expression with control plants, we found evidence for NtAQP1 function in cellular and whole-plant water relations. The consequences of a decrease in cellular water permeability were determined by measurement of transpiration rate and stem and leaf water potential as well as growth experiments under extreme soil water depletion. Plants impaired in NtAQP1 expression showed reduced root hydraulic conductivity and lower water stress resistance. In conclusion, our results emphasize the importance of symplastic aquaporin-mediated water transport in whole-plant water relations.

Hydraulic conductance in aspen (Populus tremuloides) seedlings exposed to low root temperatures.

Low root temperatures significantly reduced root hydraulic conductivity and increased resistance to water flow through the roots of aspen (Populus tremuloides Michx.) seedlings. Increased resistance to water flow could not be fully explained by the corresponding increase in water viscosity at low temperatures. The shapes of Arrhenius plots of root water flow and the activation energies were dependent on the direction, sequence and extent of temperature change. The Arrhenius plots suggested that the effect of low root temperature on root water flow was mediated by an effect on root metabolism. The low root temperatures tested did not induce root electrolyte leakage normally associated with cell membrane injury. Although a decrease in root temperatures to 7 or 4 degrees C induced a reduction in stomatal conductance, this reduction lagged the decline in root water flow by several hours. In contrast, when soil temperatures were raised from 4 or 7 degrees C to 25 degrees C, root water flow presumably increased, and stomatal conductance responded rapidly and was temporarily higher than before the cold treatment was imposed.

Offsetting effects of reduced root hydraulic conductivity and osmotic adjustment following drought.

Root hydraulic conductivity (L(p)) and leaf osmotic potential at full turgor (Psi(pi,o)) were measured in young, drought-stressed and nonstressed peach (Prunus persica (L.) Batsch), olive (Olea europaea L.), citrumelo (Poncirus trifoliata Raf. x Citrus paradisi Macf.) and pistachio (Pistachia integerrima L.). Drought stress caused a 2.5- to 4.2-fold reduction in L(p), depending on species, but Psi(pi,o) was reduced only in citrumelo and olive leaves by 0.34 and 1.4 MPa, respectively. No differences existed in L(p) among species for nonstressed plants. A simple model linking L(p) to osmotic adjustment through leaf water potential (Psi) quantified the offsetting effects of reduced L(p) and osmotic adjustment on the hypothetical turgor pressure difference between drought-stressed and nonstressed plants (DeltaPsi(p)). For olive, the 2.5-fold reduction in L(p) caused a linear decrease in DeltaPsi(p) such that the effect of osmotic adjustment was totally negated at Psi = -3.2 MPa. Thus, no stomatal closure would be required to maintain higher turgor in drought-stressed olive plants than in nonstressed plants over their typical diurnal range of Psi (-0.6 to -2.0 MPa). For citrumelo, osmotic adjustment was offset by reduced L(p) at Psi approximately -0.9 MPa. Unlike olive, stomatal closure would be necessary to maintain higher turgor in drought-stressed citrumelo plants than in nonstressed plants over their typical diurnal range of Psi (0 to -1.5 MPa). Regardless of species or the magnitude of osmotic adjustment, my analysis suggests that a drought-induced reduction in L(p) reduces or eliminates turgor maintenance through osmotic adjustment.

WATER RELATIONS OF MYCORRHIZAL AND PHOSPHORUS-FERTILIZED NON-MYCORRHIZAL CITRUS UNDER DROUGHT STRESS.

Rootstock seedlings of Carrizo citrange (Poncirus trifoliata (L.) Raf. x C. Sinensis (L.) Osbeck) and sour orange (Citrus aurontium L.) were grown in a sandy soil low in phosphorus (P) and either inoculated with Glomus intraradices Schenck & Smith (vesicular-arbuscular mycorrhizal; VAM) or fertilized with soluble P (non-mycorrhizal; NM). Five-month-old VAM and NM seedlings of each rootstock were comparable in size, P sufficiency, and relative growth rate whether they were kept well-watered or subjected to two drought-stress cycles of short duration. Under well-watered conditions, whole plant transpiration, leaf water status, and root hydraulic conductivity were similar for VAM and NM plants of each rootstock. During drought-stress and recovery periods, VAM plants also had very comparable whole-plant transpiration rates and leaf water potentials to NM plants, but mycorrhiza reduced root hydraulic conductivity of Carrizo citrange and sour orange 66 and 49%, respectively. These data do not support the hypothesis that mycorrhiza significantly enhance water relations of citrus under the drought-stress conditions studied.

Thermal and Water Relations of Roots of Desert Succulents

Two succulent perennials from the Sonoran Desert, Agave deserti Engelm. and Ferocactus acanthodes (Lem.) Britton and Rose, lose little water through their roots during drought, yet respond rapidly to light rainfall. Their roots tend to be shallow, although absent from the upper 20 mm or so of the soil. During 12–15 d after a rainfall, new root production increased total root length by 47 per cent to 740 m for A. deserti and by 27 per cent to 230 m for F. acanthodes; root dry weight then averaged only 15 per cent of shoot dry weight. The annual carbon allocated to dry weight of new roots required 11 per cent of shoot carbon dioxide uptake for A. deserti and 19 per cent for F. acanthodes. Elongation of new roots was greatest near a soil temperature of 30°C, and lethal temperature extremes (causing a 50 per cent decrease in root parenchyma cells taking up stain) were 56°C and -7°C. Soil temperatures annually exceeded the measured tolerance to high temperature at depths less than 20 mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentials above -2·6 MPa could produce new lateral roots, with 50 per cent of maximum elongation occurring near -1·4 MPa for both species. Non-droughted roots lost water when immersed in solutions with osmotic potentials below -0·8 MPa, and root hydraulic conductance decreased markedly below about -1·2 MPa. Pressure-volume curves indicated that, for a given change in water potential, non-droughted roots lost three to five times more water than droughted roots, non-droughted leaves, or non-droughted stems. Hence, such roots, which could be produced in response to a rainfall, will lose the most tissue water with the onset of drought, the resulting shrinkage being accompanied by reduced root hydraulic conductance, less contact with drying soil, and less water loss from the plant to the soil.