Hydraulic Conductivity Characteristics of Desert Plant Organs: Coping with Drought Tolerance Strategy

Lower leaf water potential (ψ1) was observed in young rice plants grown in wet and highly compacted soil. Our objectives were to establish why the reductions of ψ1 occurred and to find the effect of plant hydraulic conductance on ψ1. One seedling of a lowland cultivar Nipponbare and an upland cultivar Senshou were grown in plastic pipes with a diameter of 5.4 cm and a height of 45 cm filled with a soil mixture. Soil bulk density (SBD) was set at four levels and wet soil conditions were maintained. In a growth chamber ψ1, transpiration rate per unit leaf area (TL), leaf area and root length were measured at 35 days after sowing. In both cultivars, plant hydraulic conductance (CP = –TL/ψ1) decreased as SBD increased and there was a positive correlation between CP and root length per unit leaf area. The greater decreases in root length than in leaf area, in the soil with a high bulk density, were suggested to reduce CP, thus resulting in lower ψ1. In both cultivars, root hydraulic conductance per unit root length (GR), estimated using a pressure-flux method, increased with increase in SBD. The increases in CR were accompanied by the increases in TL per unit root length at a high SBD. We suggest that the suppression of root length in rice by highly compacted soil causes lower GP which in turn reduces ψ1 even if the soil is wet. CR, water absorption rate per unit root length and the diameter of the primary roots, clearly increased under highly compacted soil conditions, but this might not be able to compensate for the greater reduction in root length than in leaf area, and so may not permit CP and ψ1 to be maintained.

The influence of phosphorus deficiency on growth and nitrogen fixation of white clover plants.

The effects of abscisic acid (ABA) on aquaporin content, root hydraulic conductivity (Lpr), whole plant hydraulic conductance, and leaf growth are controversial. We addressed these effects via a combination of experiments at different scales of plant organization and tested their consistency via a model. We analyzed under moderate water deficit a series of transformed maize (Zea mays) lines, one sense and three antisense, affected in NCED (for 9-cis-epoxycarotenoid dioxygenase) gene expression and that differed in the concentration of ABA in the xylem sap. In roots, the mRNA expression of most aquaporin PIP (for plasma membrane intrinsic protein) genes was increased in sense plants and decreased in antisense plants. The same pattern was observed for the protein contents of four PIPs. This resulted in more than 6-fold differences between lines in Lpr under both hydrostatic and osmotic gradients of water potential. This effect was probably due to differences in aquaporin activity, because it was nearly abolished by a hydrogen peroxide treatment, which blocks the water channel activity of aquaporins. The hydraulic conductance of intact whole plants was affected in the same way when measured either in steady-state conditions or via the rate of recovery of leaf water potential after rewatering. The recoveries of leaf water potential and elongation upon rehydration differed between lines and were accounted for by the experimentally measured Lpr in a model of water transfer. Overall, these results suggest that ABA has long-lasting effects on plant hydraulic properties via aquaporin activity, which contributes to the maintenance of a favorable plant water status.

Drought is one of the major abiotic stresses limiting the advancement of the pear industry in the arid and semiarid regions of China. Currently, the production of pear seedlings mainly depends on the grafting propagation of rootstocks from Pyrus betulaefolia Bunge and Pyrus calleryana Decne. However, limited information is available on the drought stress potential of these rootstocks. Here, we conducted pot experiments with controlled watering conditions on both rootstocks' seedlings and compared their growth, plant hydraulic conductivity, and stem sap flow velocity. Both rootstocks' seedlings were challenged with three drought regimes i.e. light (LD), moderate (MD), and heavy (HD) drought stress. The results indicated that P. betulaefolia performed better in growth than P. calleryana under the same drought regimes. Under LD, MD, and HD the relative growth of the new shoots, leaf-specific canopy hydraulic conductivity, leaf, stem, and root hydraulic conductivity, and stem sap flow velocity of P. betulaefolia exhibited reduction as compared to control (CK). Similar reduction trends were noted for P. calleryana, with slight variations in the case of stem-specific canopy hydraulic conductivity.

We studied the effect of scion donor-tree age on the physiology and growth of 6- to 7-year-old grafted Scots pine (Pinus sylvestris L.) trees (4 and 5 years after grafting). Physiological measurements included photosynthethetic rate, stomatal conductance, transpiration, whole plant hydraulic conductance, needle nitrogen concentration and carbon isotope composition. Growth measurements included total and component biomasses, relative growth rates and growth efficiency. Scion donor trees ranged in age from 36 to 269 years at the time of grafting. Hydraulic conductance was measured gravimetrically, applying the Ohm's law analogy, and directly, with a high-pressure flow meter. We found no effect of scion donor-tree age on any of the variables measured. There was, however, great variation within scion donor-tree age groups, which was related to the size of the grafted trees. Differences in size may have been caused by variable initial grafting success, but there was no indication that grafting success and age were related. At the stem level, hydraulic conductance scaled with total leaf area so that total conductance per unit leaf area did not vary with crown size. However, leaf specific hydraulic conductance (gravimetric), transpiration, photosynthesis and stomatal conductance declined with increasing total tree leaf area and needle width. We hypothesize that needle width is inversely related to mesophyll conductance. We conclude that canopy and needle size and not scion donor-tree age determined gas exchange in our grafted trees.

Anthropogenic activities are increasing cadmium (Cd) concentrations in agricultural soils. Cadmium can be absorbed by plant roots and modify the physiology of the plant. Carbon exchange rate (CER) and leaf conductance to water vapor (gs) were measured on the fifth leaf of two soybean (Glycine max [L.] Merr.) cultivars (Illini insensitive and Richland sensitive) for 6 consecutive days; Cd(NO3)2 was added to the hydroponic solution to achieve a final concentration of 50 µmol. At the end of the experiment, stomata length and width, mesophyll limitation to photosynthesis, root hydraulic conductance, relative water content (RWC), and Cd concentration in leaves, stems, and roots were measured on treated and control plants. Cadmium progressively reduced CER and gs to about 50% after 6 d of treatment. This was more evident in Richland than in Illini and was not linked with leaf RWC and mesophyll limitation to photosynthesis. After 6 d, the apparent root hydraulic water conductivity was 67% lower in the Cd‐treated plants than in controls. The primary mechanism affected by Cd‐induced stress in soybean is root water uptake, and this reduction is consistent with the decrease in stomatal opening and conductance, and therefore, in photosynthesis.

Despite considerable investigations of diurnal water use characteristics in different plant functional groups, the research on daily water use strategies of woody bamboo grasses remains lacking. We studied the daily water use and gas exchange of Sinarundinaria nitida (Mitford) Nakai, an abundant subtropical bamboo species in Southwest China. We found that the stem relative water content (RWC) and stem hydraulic conductivity (K(s)) of this bamboo species did not decrease significantly during the day, whereas the leaf RWC and leaf hydraulic conductance (K(leaf)) showed a distinct decrease at midday, compared with the predawn values. Diurnal loss of K(leaf) was coupled with a midday decline in stomatal conductance (g(s)) and CO(2) assimilation. The positive root pressures in the different habitats were of sufficient magnitude to refill the embolisms in leaves. We concluded that (i) the studied bamboo species does not use stem water storage for daily transpiration; (ii) diurnal down-regulation in K(leaf) and gs has the function to slow down potential water loss in stems and protect the stem hydraulic pathway from cavitation; (iii) since K(leaf) did not recover during late afternoon, refilling of embolism in bamboo leaves probably fully depends on nocturnal root pressure. The embolism refilling mechanism by root pressure could be helpful for the growth and persistence of this woody monocot species.

Radial transport of water in excised barley (Hordeum distichon, cv. Villa) roots was measured using a new method based on the pressure-probe technique. After attaching excised roots to the probe, root pressures of 0.9 to 2.9 bar were developed. They could be altered either by changing the root pressure artificially (with the aid of the probe) or by changing the osmotic pressure of the medium in order to induce water flows across the root. The hydraulic conductivity of the barley roots (per cm(2) of outer root surface) was obtained in different types of experiments (initial water flow, pressure relaxations, constant water flow) and was (0.3-4.3)·10(-7) cm s(-1) bar(-1). The hydraulic conductivity of the root was by an order of magnitude smaller than the hydraulic conductivity of the cell membranes of cortical and epidermal cells (0.8-2.2)·10(-6) cm s(-1) bar(-1). The half-times of water exchange of these cells was 1-21 s and two orders of magnitude smaller than that of entire excised roots (100-770 s). Their volumetric elastic modulus was 15-305 bar and increased with increasing turgor. Within the root cortex, turgor was independent of the position of the cell within a certain layer and turgor ranged between 3 and 5 bar. The large difference between the hydraulic conductivity of the root and that of the cell membranes indicates that there is substantial cell-to-cell (transcellular plus symplasmic) transport of water in the root. When it is assumed that 10-12 membrane layers (plasmalemma plus tonoplast) in the epidermis, cortex and endodermis form the hydraulic resistance to water flow, a value for the hydraulic conductivity of the root can be calculated which is similar to the measured value. This picture for water transport in the root contradicts current models which favour apoplasmic water transport in the cortex.

Abstract. Stomatal regulation and whole plant hydraulic signaling affect water fluxes and stress in plants. Land surface models and crop models use a coupled photosynthesis–stomatal conductance modeling approach. Those models estimate the effect of soil water stress on stomatal conductance directly from soil water content or soil hydraulic potential without explicit representation of hydraulic signals between the soil and stomata. In order to explicitly represent stomatal regulation by soil water status as a function of the hydraulic signal and its relation to the whole plant hydraulic conductance, we coupled the crop model LINTULCC2 and the root growth model SLIMROOT with Couvreur's root water uptake model (RWU) and the HILLFLOW soil water balance model. Since plant hydraulic conductance depends on the plant development, this model coupling represents a two-way coupling between growth and plant hydraulics. To evaluate the advantage of considering plant hydraulic conductance and hydraulic signaling, we compared the performance of this newly coupled model with another commonly used approach that relates root water uptake and plant stress directly to the root zone water hydraulic potential (HILLFLOW with Feddes' RWU model). Simulations were compared with gas flux measurements and crop growth data from a wheat crop grown under three water supply regimes (sheltered, rainfed, and irrigated) and two soil types (stony and silty) in western Germany in 2016. The two models showed a relatively similar performance in the simulation of dry matter, leaf area index (LAI), root growth, RWU, gross assimilation rate, and soil water content. The Feddes model predicts more stress and less growth in the silty soil than in the stony soil, which is opposite to the observed growth. The Couvreur model better represents the difference in growth between the two soils and the different treatments. The newly coupled model (HILLFLOW–Couvreur's RWU–SLIMROOT–LINTULCC2) was also able to simulate the dynamics and magnitude of whole plant hydraulic conductance over the growing season. This demonstrates the importance of two-way feedbacks between growth and root water uptake for predicting the crop response to different soil water conditions in different soils. Our results suggest that a better representation of the effects of soil characteristics on root growth is needed for reliable estimations of root hydraulic conductance and gas fluxes, particularly in heterogeneous fields. The newly coupled soil–plant model marks a promising approach but requires further testing for other scenarios regarding crops, soil, and climate.

Plant water transport and aquaporins in oxygen-deprived environments

Changes in hydraulic conductance cause the difference in growth response to short-term salt stress between salt-tolerant and -sensitive black gram (Vigna mungo) varieties

Different root low temperature response of two maize genotypes differing in chilling sensitivity

As a soil dries, the roots growing in it may shrink and retain only partial contact with the soil. The steady-state model described here calculates the effect of root shrinkage on the water inflow across soils and roots with various hydraulic conductivities. Relative to standard conditions, typical of a secondary root in a loam at field capacity, the water inflow was very sensitive to the root radius, the root shrinkage, the root hydraulic conductivity and the water potential in the bulk soil and at the root endodermis. The inflow was insensitive to the density of rooting, and to the soil hydraulic conductivity except in dry sandy soil where this was lower than the root hydraulic conductivity. Loss of full contact can decrease the inflow by a factor of up to about three. In very dry conditions, vapour transfer across the air gap between the root and soil surfaces can contribute usefully to the total water inflow.

Leaflet expansion rates for fifteen soybean cultivars

The basis used for expression of carbon dioxide exchange rate (CER) of single leaves may alter the relative ranking of genotypes being selected for a breeding program. Therefore, field and growth chamber experiments were conducted on 16 genotypes of tall fescue (Festuca arundinacea Schreb.) to determine the relative merit of expressing CER data on a leaf area or leaf weight basis. Genotypes were grown in plots in the field and in pots in the growth chamber. The CER was measured using an infrared gas analyzer. Herbage yield was determined. Significant genotypic variation within experiments was found for CER when expressed on either a leaf area or leaf weight basis. The correlation coefficient between CER on a leaf area and leaf weight basis ranged from 0.82 to 0.94 for the field experiments and was 0.76 for the growth chamber experiment. Comparision of coefficients of variation (CV's) associated with the two methods of expressing CER indicated slightly higher variation in data based on leaf weight. The CV's for the field experiments were between 11.5 and 16.5%, suggesting that either method of expression of CER for tall fescue could be satisfactorily used to detect genotypic differences. Leaf area is suggested as the preferred base because of its association with radiation interception. Further, genetic control of CER per unit leaf area appears to be independent from specific leaf weight.