Pathways and processes of water and nutrient movement in roots

Recent work in our laboratory provides evidence for a revised view of the functioning of roots of maize, and probably of all the grasses. The development of coherent soil sheaths on the distal 30-cm of these roots, and the loss of the sheaths further back, led us to investigate the differences in surface structure, anatomy, carbon exudation and microflora of the sheathed and bare zones. The significant differences are summarized. But the fact which underlies all these differences is the maturation of the late metaxylem (LMX). In the sheathed zones the LMX elements are still alive and non-conducting; only the early metaxylem (EMX) and protoxylem are open. In the bare zones they are open vessels. This leads directly to the dryness of bare zones and the wetness of sheathed zones, and indirectly to the other differences noted. Branch root junctions are shown to be structures of great significance. Besides connecting the branches to the axile systems, they serve also to connect the EMX and LMX vessels, and contain a tracheid barrier which prevents air embolisms entering the main vessels. These discoveries force us to revise the traditional view of water uptake by the root hair zone, and to suggest that much water must also enter bare roots, possibly via the laterals. There is some published evidence for this. The living LMX elements of the sheathed zone accumulate large concentrations of potassium which must joint the transpiration water at the transition to the bare zone. Calculations suggest that this may be only a tenth of the requirement of a mature plant, and that the balance may enter the bare zones with the transpiration water.     

Different populations of bacteria associated with sheathed and bare regions of roots of field-grown maize

It has been previously shown that soil sheaths cling tightly to some portions of all axile roots and cover all but the growing tips of the young roots of field-grown maize. These sheaths overlie immature regions of the roots which have intact epidermal cells with root hairs, and living, thus non-conducting, late metaxylem elements. Loss of the soil sheath in the proximal region coincides with the opening of these large metaxylem vessels. Now, total, and viable counts have been recorded of bacteria associated with the root surface and adhering soil of sheathed and bare regions. These showed some common features, in that populations of similar size were associated with the two root regions in plants beginning to flower. Each population included about the same numbers of bacteria that were viable on each of three selective media (nitrogen-free, Pseudomonas F or MacConkey). However, more spore-formers capable of growth on nitrogen-free media and more fluorescent bacteria were isolated from the sheathed regions. Actinomycetes were absent from sheathed but plentiful on bare regions.The high numbers of diverse types of bacteria associated with both root surfaces can be related to the previously demonstrated similarity in amounts of organic carbon released from each region. The proliferation of actionomycetes on the bare roots and their exclusion from sheathed roots may in part be due to the lower water status of the bare region, which is related to its greater axial conducting capacity. Thus the distribution of the two types of root surface within an individual root system has important implications for the choice of root and rhizosphere sampling techniques and for root bacterization work.     

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.     

Living Vessel Elements in the Late Metaxylem of Sheathed Maize Roots

The two types of nodal roots of field-grown maize, sheathedand bare, were found to have such different water conductivitiesthat an investigation of the anatomy of their large metaxylemvessels was made. While the vessels of the bare roots were openfor scores of centimetres, those of the sheathed roots werefound to be not vessels but developing vessel elements, withcross walls at 1 mm intervals, and protoplasts. The cross wallsbetween the elements had several unique histochemical properties.Previous investigators have often failed to find the cross wallsbecause they are very easily dislodged during the usual methodsof tissue preparation. They are best identified by microdissectionof fresh xylem. The living elements persist in the late metaxylemup to 20 – 30 cm from the tip. As the roots become longerthan this both the cross walls and the soil sheaths disappearand there is a transition to a bare root with open vessels inthe proximal region. The soil sheath persists a little longerthan the cross walls. The two types are thus stages in a developmentalsequence through which all nodal roots pass. A fundamental differencebetween the two types is in their water status, since the estimatedconductive capacity of a bare root is about 100 times greaterthan that of a sheathed root. These observations point to theneed for a reassessment of the published work on transport ofions into the xylem of grass roots through a reinvestigationof the ‘maturity’ of their xylem vessels. Grass roots, dimorphic roots, ion secretion to xylem, soil sheaths, xylem vessels, xylem differentiation, water conduction, Zea mays L     

Accumulation of potassium by differentiating metaxylem elements of maize roots

The previous demonstration that the large late metaxylem vessels of field-grown maize ( Zea mays L. cv. Rosella) roots do not lose their crosswalls until they are 20–30 cm from the tip, and that the presence of a soil sheath outside the root was indicative of immature vessels within, greatly strengthened the hypothesis that ion accumulation into these roots was by uptake into living xylem element vacuoles. Proposals that salt movement into the xylem was by leakage or secretion into dead vessels became much less plausible. Potassium concentration in the vacuoles of late metaxylem elements was measured by X-ray microanalysis in unetched fracture faces of bulk, frozen-hydrated pieces of sheathed roots, and found to be in the range 150–400 m M . Potassium concentration in open vessels of bare roots, measured both with the microprobe and by spectrophotometry of aspirated sap, was in the range of 5 to 25 m M . It is concluded that uptake of potassium (and possibly other ions) is into living xylem elements, and that its release to the transpiration stream occurs by the breakdown of their crosswalls and the addition of their vacuoles to the solution in the vessels above.     

Modeling soil–root water transport and competition for single and mixed crops

A knowledge of above and below ground plant interactions for water is essential to understand the performance of intercropped systems. In this work, root water potential dynamics and water uptake partitioning were compared between single crops and intercrops, using a simulation model. Four root maps having 498, 364, 431 and 431 soil-root contacts were used. In the first and second cases, single crops with deep and surface roots were considered, whereas in the third and fourth cases, roots of two mixed crops were simultaneously considered with different row spacing (40 cm and 60 cm). Two soils corresponding to a clay and a silty clay loam were used in the calculations. A total maximum evapotranspiration of 6 mm d^-1 for both single or mixed crops was considered, for the mixed crops however, two transpiration distributions between the crops were analyzed (3:3 mm d^-1, or 4:2 mm d^-1 for each crop, respectively). The model was based on a previous theoretical framework applied to single or intercropped plants having spatially distributed roots in a two-dimensional domain. Although water stress occurred more rapidly in the loam than in the clay, due to the rapid decrease of the soil water reserve in the loam, the role of the root arrangement appeared to be crucial for water availability. Interactions between the distribution of transpiration among mixed crops and the architecture of the root systems which were in competition led to water movements from zones with one plant to another, or vice versa, which corresponded to specific competition or facilitation effects. Decreasing the distances between roots may increase competition for water, although it may determine greater water potential gradients in the soil that increase lateral or vertical water fluxes in the soil profile. The effects of the root competition on water uptake were quite complicated, depending on both environmental conditions, soil hydrodynamic properties, and time scales. Although some biological adaptive mechanisms were disregarded in the analysis, the physically 2-D based model may be considered as a tool to study the exploitation of environmental heterogeneity at microsite scales.     

The Effect of Rapid Temperature Increase on the Growth Rate of Wheat Leaves

The growth rate of the first leaf of eight-day-old wheat plants was measured using a DLT-2 highly sensitive linear displacement transducer. Leaf extensibility was evaluated from the growth rate under the increase in the pulling force by 2 g. An increase in the air temperature resulted in the doubling of the transpiration rate and immediate slowing of the leaf growth followed by the leaf shrinkage. However, growth was later resumed almost completely. Heat treatment did not induce any changes in the leaf extensibility, indicating that cell-wall mechanical properties were not changed. Growth retardation was supposed to result from a decrease in the water content in the leaf tissues because the balance between water influx from roots and its loss through transpiration was shifted toward the water loss. An initial drop in the relative water content (RWC) indicates such a misbalance. Subsequent growth resumption coincided with a decreased water deficiency. Since the rate of transpiration was not reduced, RWC and growth rate restoring evidently occurred due to the activated water uptake by roots, which can be explained by the increased hydraulic permeability detected in our experiments.     

黄土高原陕北丘陵沟壑区不同立地条件下刺槐水分生理生态特性研究

对黄土高原丘陵沟壑区不同立地条件下刺槐水分生理生态特性进行了初步研究.结果表明,不同立地刺槐林地0～500cm土壤平均含水量(2003年)分别为阳坡6.96%、半阳坡7.62%、半阴坡8.06%、阴坡8.87%,阴坡、半阴坡与阳坡差异达极显著,与半阳坡达显著水平,半阳坡与阳坡差异显著,而阴坡与半阴坡差异不显著.不同立地条件下刺槐叶片相对含水量和饱和亏与各立地土壤含水量关系密切,阳坡刺槐叶片相对含水量和叶水势始终维持在较低水平,而半阴坡和阴坡尤其是阴坡维持在较高水平.刺槐日蒸腾平均值大小顺序为阴坡(4.07μg·cm^-2·s^-1)＞半阴坡(3.89μg·cm^-2·s^-1)＞半阳坡(3.05 μg·cm^-2·s^-1)＞阳坡(2.70μg·cm^-2·s^-1),各立地刺槐蒸腾出现较大差异的时间在11:00和13:00.不同立地条件除阳坡外其刺槐蒸腾速率均与光照强度显著相关,各立地均与大气相对湿度显著相关,与土壤水分密切相关.阴坡生物量最高(8.50gk·株^-1),高于其他3种立地,阳坡最低(5.79kg·株^-1).     

Response of stem sap flow and leaf photosynthesis in <Emphasis Type="Italic">Tamarix chinensis</Emphasis> to soil moisture in the Yellow River Delta,China

Soil moisture is the main limiting factor for vegetation growth at shell ridges in the Yellow River Delta of China. The objective of this study was to explore the soil moisture response of photosynthetic parameters and transpiration in Tamarix chinensis Lour., a dominant species of shell ridges. Leaf photosynthetic light-response parameters and sap flow were measured across a gradient of relative soil water content (RWC), from drought (23%) to waterlogging (92%) conditions. Leaf photosynthetic efficiency and stem sap flow of T. chinensis showed a clear threshold response to soil moisture changes. Leaf net photosynthetic rate, water-use efficiency (WUE), light-saturation point, apparent quantum yield, maximum net photosynthetic rate, and dark respiration rate peaked at moderately high RWC, decreasing towards high and low values of RWC. However, peak or bottom RWC values substantially differed for various parameters. Excessively high or low RWC caused a significant reduction in the leaf photosynthetic capacity and WUE, while the high photosynthetic capacity and high WUE was obtained at RWC of 73%. With increasing waterlogging or drought stress, T. chinensis delayed the starting time for stem sap flow in the early morning and ended sap flow activity earlier during the day time in order to shorten a daily transpiration period and reduce the daily water consumption. The leaf photosynthetic capacity and WUE of T. chinensis were higher under drought stress than under waterlogging stress. Nevertheless, drought stress caused a larger reduction of daily water consumption compared to waterlogging, which was consistent with a higher drought tolerance and a poor tolerance to waterlogging in this species. This species was characterized by the low photosynthetic capacity and low WUE in the range of RWC between 44 and 92%. The RWC of 49–63% was the appropriate range of soil moisture for plant growth and efficient physiological water use of T. chinensis seedlings.     

Seasonal variation of water uptake of a Quercus suber tree in Central Portugal

Hydraulic redistribution (HR) is the phenomenon where plant roots transfer water between soil horizons of different water potential. When dry soil is a stronger sink for water loss from the plant than transpiration, water absorbed by roots in wetter soil horizons is transferred toward, and exuded into dry soil via flow reversals through the roots. Reverse flow is a good marker of HR and can serve as a useful tool to study it over the long-term. Seasonal variation of water uptake of a Quercus suber tree was studied from late winter through autumn 2003 at Rio Frio near Lisbon, Portugal. Sap flow was measured in five small shallow roots (diameter of 3–4 cm), 1 to 2 m from the tree trunk and in four azimuths and at different xylem depths at the trunk base, using the heat field deformation method (HFD). The pattern of sap flow differed among lateral roots as soil dried with constant positive flow in three roots and reverse flow in two other roots during the night when transpiration ceased. Rain modified the pattern of flow in these two roots by eliminating reverse flow and substantially increasing water uptake for transpiration during the day. The increase in water uptake in three other roots following rain was not so substantial. In addition, the flux in individual roots was correlated to different degrees with the flux at different radial depths and azimuthal directions in trunk xylem. The flow in outer trunk xylem seemed to be mostly consistent with water movement from surface soil horizons, whereas deep roots seemed to supply water to the whole cross-section of sapwood. When water flow substantially decreased in shallow lateral roots and the outer stem xylem during drought, water flow in the inner sapwood was maintained, presumably due to its direct connection to deep roots. Results also suggest the importance of the sap flow sensor placement, in relation to sinker roots, as to whether lateral roots might be found to exhibit reverse flow during drought. This study is consistent with the dimorphic rooting habit of Quercus suber trees in which deep roots access groundwater to supply superficial roots and the whole tree, when shallow soil layers were dry.     

The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.     

A coupled model of stomatal conductance, photosynthesis and transpiration

A model that couples stomatal conductance, photosynthesis, leaf energy balance and transport of water through the soil–plant–atmosphere continuum is presented. Stomatal conductance in the model depends on light, temperature and intercellular CO₂ concentration via photosynthesis and on leaf water potential, which in turn is a function of soil water potential, the rate of water flow through the soil and plant, and on xylem hydraulic resistance. Water transport from soil to roots is simulated through solution of Richards’ equation. The model captures the observed hysteresis in diurnal variations in stomatal conductance, assimilation rate and transpiration for plant canopies. Hysteresis arises because atmospheric demand for water from the leaves typically peaks in mid‐afternoon and because of uneven distribution of soil matric potentials with distance from the roots. Potentials at the root surfaces are lower than in the bulk soil, and once soil water supply starts to limit transpiration, root potentials are substantially less negative in the morning than in the afternoon. This leads to higher stomatal conductances, CO₂ assimilation and transpiration in the morning compared to later in the day. Stomatal conductance is sensitive to soil and plant hydraulic properties and to root length density only after approximately 10 d of soil drying, when supply of water by the soil to the roots becomes limiting. High atmospheric demand causes transpiration rates, LE, to decline at a slightly higher soil water content, θ_s, than at low atmospheric demand, but all curves of LE versus θ_s fall on the same line when soil water supply limits transpiration. Stomatal conductance cannot be modelled in isolation, but must be fully coupled with models of photosynthesis/respiration and the transport of water from soil, through roots, stems and leaves to the atmosphere.     

Effects of water stress on the water relations of Phaseolus vulgaris and the drought resistant Phaseolus acutifolius

The tepary bean ( Phaseolus acutifolius Gray var. latifolius ), a drought resistant species, was compared under water stress conditions with the more drought susceptible P. vulgaris L. cvs Pinto and White Half Runner (WHR). In order to better understand the basis for the superior drought resistance of tepary, this study was designed to determine the relationships among leaf water potential, osmotic potential, turgor potential, and relative water content (RWC).
Plants were prestressed by withholding irrigation water. These stress pretreatments changed the relation between leaf water potential and relative water content of both species so that prestressed plants had lower water potentials than controls at the same leaf RWC. Tepary had lower water potentials at given RWC levels than Pinto or WHR; this can account for part of the superior resistance of tepary. In all genotypes, prestressed plants maintained osmotic potentials approximately 0.2 MPa lower than controls. Tepary reached osmotic potentials that were significantly lower (0.15 to 0.25 MPa) than Pinto or WHR. Both control and prestressed tepary plants had 0.05 to 0.25 MPa more turgor than Pinto or WHR at RWC values between 65 and 80%. Both prestressed and control tepary plants had greater elasticity (a lower elastic modulus) than Pinto or WHR. This greater turgor of tepary at low RWC values could be caused by several factors including greater tissue elasticity, active accumulation of solutes, or greater solute concentration.
Tepary had significantly lower osmotic potentials than the P. vulgaris cultivars, but there was little difference in osmotic potential between Pinto and WHR. Knowledge of differences in osmotic and turgor potentials among and within species could be useful in breeding for drought resistance in Phaseolus.     

Involvement of root ABA and hydraulic conductivity in the control of water relations in wheat plants exposed to increased evaporative demand

We studied the possible involvement of ABA in the control of water relations under conditions of increased evaporative demand. Warming the air by 3°C increased stomatal conductance and raised transpiration rates of hydroponically grown Triticum durum plants while bringing about a temporary loss of relative water content (RWC) and immediate cessation of leaf extension. However, both RWC and extension growth recovered within 30 min although transpiration remained high. The restoration of leaf hydration and growth were enabled by increased root hydraulic conductivity after increasing the air temperature. The use of mercuric chloride (an inhibitor of water channels) to interfere with the rise on root hydraulic conductivity hindered the restoration of extension growth. Air warming increased ABA content in roots and decreased it in shoots. We propose this redistribution of ABA in favour of the roots which increased the root hydraulic conductivity sufficiently to permit rapid recovery of shoot hydration and leaf elongation rates without the involvement of stomatal closure. This proposal is based on known ability of ABA to increase hydraulic conductivity confirmed in these experiments by measuring the effect of exogenous ABA on osmotically driven flow of xylem sap from the roots. Accumulation of root ABA was mainly the outcome of increased export from the shoots. When phloem transport in air-warmed plants was inhibited by cooling the shoot base this prevented ABA enrichment of the roots and favoured an accumulation of ABA in the shoot. As a consequence, stomata closed.     

Soil solute concentration and water uptake by single lupin and radish plant roots

Application of computer assisted tomography to gamma and X-ray attenuation measurements and Na⁺-LIX microelectrodes were used to determine the spatial distributions of soil water content and Na⁺ concentrations respectively near single roots of eighteen day old lupin and radish plants. These quantities were monitored at root depths of 3, 6 and 9 cm and at zero, 2, 4, 6, and 8 hour intervals from the diurnal commencement of transpiration. The plants were subjected to two levels of transpirational demand and five Na⁺ soil solution concentration levels. Water extraction rates for the lupin and radish roots increased continuously with time but were substantially reduced with increasing Na⁺ concentration in the treatment. Water uptake was uniform along the length of the essentially constant diameter lupin roots but decreased along the tapering radish roots as the diameter and hence the surface area per unit length of the roots decreased. The accumulation of Na⁺ at the root surfaces of both plants increased gradually with time in a near linear fashion and was slightly higher under the higher transpiration demand. These increases were not exponential as would be expected with non-absorption by the roots and this is considered to be due to back diffusion at the relatively high water contents used. At these water contents matric potentials had a much smaller influence on transpiration than osmotic potentials. The relationships between leaf water potentials (Ψ₁) and osmotic potentials at the root surfaces were linear with the decreases in Ψ₁ almost exactly reflecting the decreases in Ψ_π indicating rapid plant adjustment. Leaf water potentials decreased progressively with time and the relationships between leaf water potential and the transpiration rate were also linear supporting the suggestion of constant plant resistances at any given concentration.     

Drought adaptation of field grown wheat in relation to soil physical conditions

In order to investigate the effects of soil texture on possible non-hydraulic signals under field conditions, spring wheat plants (Triticum aestivum L. cv. Cadensa) grown in sand and loam soils and with a well developed root system were exposed to slow soil drying in the late vegetative stage of growth. Soil water potential and content were measured daily at different depths and plant responses were measured in flag leaves. When the average soil water potential in the top soil layers (0–25 cm depth in sand and 0–45 cm depth in loam) dropped to –60 or –70 kPa and the lower soil layers were still at field capacity, morning xylem [ABA] (0.03–0.04 vs. 0.06–0.08 mmol m-3) and midday leaf ABA concentration increased (250–300 vs. 400–450 ng/g DW) and leaf conductance decreased relatively to well-watered (control) plants (0.75–0.88 vs. 0.64–0.70 mol m-2 s-1). These responses took place before any decrease in leaf water potential occurred as compared with control plants, indicating that they were triggered by root-borne signals due to reduced root water status in the top soil layers. At this stage the soil water content was as low as 6% by volume, the fraction of roots in ‘wet’ soil was 0.12 and relative available soil water was 45% in sand and still high 20%, 0.48 and 70%, respectively, in loam of the whole soil profile indicating that roots were responding to soil water availability and not soil water content at a certain evaporative demand. In addition, similar responses occurred at high and low evaporative demands (3.4–5.2 vs. 0.6–4.0 mm/day of potential evapotranspiration). This revised version was published online in July 2006 with corrections to the Cover Date.     

Occurrence of Inverted Water Potential Gradients between Soil and Bean Roots

Measurements with a pressure chamber were made of the xylem water potential of leaves, shoots and roots from bean plants (Pkaseolus vulgaris L. cv. Processor) grown with a 12 hour dark period and natural or artificial light conditions during the day. The water potentials were measured at the end of a dark period and during the light period. Measurements taken at the end of the dark period indicated normal potential gradients within the soil/plant system (leaf < shoot < root < soil), when the matric potential of soil water was relatively high (above ?0.02 bar), and the gradients then also remained normal during the day (natural light). When the soil water potential was ?1 bar or lower in the morning, however, the root xylem water potential was higher than the soil water potential; at very low soil water potentials (< ?4 bar) it remained higher during most of the day. In this case also leaf and shoot xylem water potentials were higher than the soil water potential in the early morning, although decreasing rapidly in daylight. Under artificial light, both leaf and root water potentials were higher than the soil water potential throughout the whole diurnal cycle when the latter potential was below ?4 bar. From measurements of stomatal diffusion resistance, transpiration, relative water content of leaves and of changes in the matric potential of soil water, it was concluded that when the matric potential of soil water was low, water could be taken up by the plant against a water potential gradient. Because leaf xylem water potential was always lower than root xylem water potential, the mechanism involved in the inversion of water potential gradient must be localized in the roots, and probably related to ion uptake. Symbols and abbreviations used in the text: Ψ: Plant water potential (thermocouple psychrometer); Ψx: Xylem water potential (pressure chamber); Ψs: Osmotic potential of xylem sap; Ψm: Matric potential of soil water; RWC: Relative water content.     

The expansion of maize root-cap mucilage during hydration. 3. Changes in water potential and water content

Root-cap mucilage from aerial nodal roots of maize has been found to have water potential values of −11 MPa or lower when air dried. The value approaches 0 MPa within 2 min of hydration in distilled water. In this time the expanding gel absorbs only about 0.3% of the water content of fully expanded mucilage. It is concluded that the root-cap mucilage per se has almost no capacity to retain water in the rhizosphere. Any function that it may play in the slowing of root desiccation would be indirect. For example, mucilage might decrease pore size between and within soil aggregates by pulling the particles together in a cycle of nocturnal efflux of water from the root surface, and diumal dyring during transpiration.     

三峡库区濒危植物疏花水柏枝的生理生化特性研究

对三峡库区特有濒危植物疏花水柏枝的光合作用、蒸腾作用、水势等生理特性以及丙二醛(MDA)、过氧化物酶(POD)和多酚氧化酶(PPO)活性等进行分析测定。结果显示,疏花水柏枝在水淹胁迫后,能快速地恢复其光合与蒸腾生理作用。植株在秋季和夏初的光合作用和蒸腾作用的日动态呈单峰曲线,最高值出现在中午。夏初的光合强度与蒸腾强度一般高于秋季,表明夏初是该物种的生长旺季。该物种10月份水势的日动态在-0.97～1.82MPa之间变动,水势与光合作用与蒸腾作用呈显著负相关。该物种虽是一种对水淹和干旱胁迫适应能力较强的物种,但土壤水分状况仍对植物的生长有较大影响。植株的生长发育阶段对疏花水柏枝的抗逆性有影响,在花前的抗逆性总体高于花后。还对疏花水柏枝的迁地保护提出了相应的建议。     

Changes in water relations and in some physiological functions of bean under very light osmotic shock induced by polyethylene glycol

Bean plantlets ( Phaseolus vulgaris L. cv. Topcrop) were stressed at the age of 16–18 days by gradual (2–8%) or abrupt addition of 6% (w/v) polyethylene glycol Mw 6000 (PEG 6000) to Hoagland solution. Leaf conductance, photosynthesis, internal CO₂ partial pressure (C_i), relative water content (RWC), water content/dry weight (H₂O/DW), apoplastic PEG concentrations and weight of leaves, stems and roots were determined. Leaf conductance, photosynthesis and C_i were determined on non-detached primary leaves, and leaf potentials (water, osmotic and turgor potentials) were investigated in freshly detached (non-rehydrated) primary leaves, both in treated and control plants; RWC and osmotic potential were also assessed at the null turgor point. Low PEG 6000 concentrations induced early and evident decrease in leaf conductance and photosynthesis, whereas C_i decreased only moderately and tended to recover during advanced stress. There were moderate though significant decreases in RWC and H₂O/DW, no change or increases in water potential, no significant changes in osmotic potential and a moderate but significant increase in turgor potential. Even when referred to null turgor point, RWC significantly decreased and osmotic potential was unchanged. It was concluded that apoplastic PEG 6000 accumulation at evaporating sites would account for the early decrease in conductance which would also justify the unchanged or the prevalent increase in water potential and turgor potential. The subsequent PEG diffusion and concentration in the leaf apoplastic water would have induced the RWC and H₂O/DW decrease and the final turgor flexion documented.