植物根压研究进展

根压是植物根部产生的一种静水压力,广泛存在于多种植物中。在蒸腾作用很弱的情况下,根压不但可驱动水分从根部流向冠层叶片,缓解因白天强烈蒸腾而导致的水分亏缺,而且在木质部导管栓塞修复方面具有重要作用。虽然国内外学者对根压的产生已有一些解释,普遍接受的观点有渗透理论、代谢理论和水分向上共同运输假说等,但根压产生的机制至今仍是科学家争议的焦点之一。根压的测定方法虽有直接和间接测定、损伤和无损伤测定之分,但较为先进的根压测定技术仍需进一步改善和提升。受水通道蛋白、遗传因素、生境等因素的影响,根压的大小存在差异,即使是较低的根压也会影响农作物生长。在促进转运蛋白质、酶、氨基酸、激素及钙元素等在农作物木质部和韧皮部之间流通方面,适当大小的根压发挥重要作用,且有助于提高农作物产量。因此,加深对植物根压的认识和理解具有重要的生物学意义。该文从根压的定义和产生机制、具有根压的植物类群、根压的测定方法和大小、影响根压的主要因素及根压在植物科学研究领域的意义和影响等多个方面分别进行了归纳总结,并结合当前研究热点和研究成果,针对植物根压研究过程中遇到的问题和后续研究趋势及方向进行了展望。     

低剂量的毒物对生物有益？——比较毒物的单相剂量效应理论和双相剂量效应理论（3）

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论来描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,我们先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程。在第2和第3部分中,我们将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

低剂量的毒物对生物有益？——比较毒物的单相剂量效应理论和双相剂量效应理论（5）

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论来描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,我们先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程。在第2和第3部分中,我们将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

低剂量的毒物对生物有益？——比较毒物的单相剂量效应理论和双相剂量效应理论（6）

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论来描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,我们先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程。在第2和第3部分中,我们将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

低剂量的毒物对生物有益?——比较毒物的单相剂量效应理论和双相剂量效应理论（4）

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论来描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,我们先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程。在第2和第3部分中,我们将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

低剂量的毒物对生物有益？——比较毒物的单相剂量效应理论和双相剂量效应理论(1)

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程,在第2和第3部分中,将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

低剂量的毒物对生物有益？——比较毒物的单相剂量效应理论和双相剂量效应理论（2）

对于外源性和内源性的各种伤害性因素(统称为毒物)的攻击,生物有2种反应方式。在毒害作用超出生物应对能力的情况下,伤害程度与毒物的剂量成正比,可用毒物的单相剂量效应(linear no-threshold, LNT)理论来描述。然而在低剂量下,由于生物防御和修复机制的作用,不仅毒物的伤害作用可被大大降低,而且由于刺激了生物的主动反应,还可导致对生物有益的结果,其效应可用毒物的双相剂量效应(Hormesis)理论描述。毒物的双相剂量效应在生物界中是一种非常普遍的现象,在细菌、真菌、植物和动物(包括人)中都被发现。能引起双相剂量效应的伤害性刺激各式各样,包括电离辐射、活性氧、限食、高温、缺氧、缺血、机械伤害、重金属、化学毒物、抗生素、抗病毒药,以及植物中的一些活性成分等。在本文的第1部分中,先介绍毒物的单相剂量效应理论和双相剂量效应理论的内容及其历史发展过程,在第2和第3部分中,将分别以电离辐射和活性氧为例,具体讨论这2种理论对这些伤害性刺激的适用情形。大量科学研究的结果表明,低剂量的电离辐射对生物有益,而体育锻炼带来的体内活性氧的增加不仅对生物无害,还为实现体育锻炼有益效果所必须,证实了毒物双相剂量效应理论的正确性。     

杨树根压昼夜周期性及其影响因子

为研究土壤盆栽和溶液培养的‘84K’(Populus alba×P.glandulosa)离体根系的根压及根压昼夜节律,系统探讨影响根压的因素及它们与根压的关系,深入了解根压的产生机制及其节律调控,采用压力转换器法对‘84K’离体根系的根压进行了研究。该研究采用不同时间取样、摘叶、环割等试验处理,不同土壤温度、昼夜温差/恒温等测定条件研究了离体根压的节律,并进一步运用化学抑制剂研究根系呼吸代谢、根系导水率对根压的影响,并测定伤流液的渗透强度及其无机离子和可溶性糖含量日变化以探讨它们和根压昼夜节律的关系。研究发现,‘84K’离体根系的根压具有昼高夜低的周期节律,在上午至正午达到峰值,在20:00达到谷值。不同取样时间、不同培养介质对根压周期性有一定的影响,但总体昼高夜低没有改变。摘叶、环割、呼吸抑制剂或细胞膜水导抑制剂影响根压峰值的大小,但对根压的昼夜节律没有明显影响。摘叶、环割或呼吸抑制剂显著地降低了根压峰值,而水导抑制剂对根压峰值影响不大,说明根压峰值的大小主要受呼吸作用,包括呼吸底物的影响,而根系水导率对根压峰值的影响不大。根压峰值随土壤温度下降而下降、土壤温度还改变根压的昼夜节律。随温度的变化,最大根压和呼吸速率同步变化,意味着根压随温度的变化也是呼吸代谢在起作用。根系伤流液渗透强度白天高晚上低。伤流液无机离子含量以及可溶性糖含量的日变化和渗透强度变化一致,其中无机离子扮演更主要的角色。昼夜温差条件下的根压峰值显著大于昼夜常温条件下的根压峰值。总之,‘84K’根压呈现较稳定的昼夜周期性,昼高夜低。根压峰值大小主要受呼吸代谢的调控,呼吸抑制剂、呼吸代谢底物、影响呼吸作用的温度等都影响‘84K’根压峰值的大小。根系导水率对根压有一定的影响,但不显著。     

双相障碍抑郁发作及单相抑郁症患者血清T3、T4、TSH和BDNF水平的相关性分析

目的:分析双相障碍抑郁发作及单相抑郁症患者与血清三碘甲状腺原氨酸(T3)、甲状腺素(T4)、甲状腺激素(TSH)和脑源性神经营养因子(BDNF)水平的相关性。方法:选取2017年12月～2019年12月我院收治的120例抑郁症患者为研究对象,按照病情不同分为双相障碍抑郁发作组(n=50)、单相抑郁症组(n=70),同时选取同期于本院进行体检的30例健康者作为对照组,检测血清T3、T4、TSH和BDNF水平,并进行汉密尔顿抑郁(HAMD)量表评分,分析血清T3、T4、TSH和BDNF水平的相关性。结果:双相障碍抑郁发作组起病年龄低于单相抑郁症组(P0.05);治疗前双相障碍抑郁发作组和单相抑郁症组血清T3水平高于对照组,TSH、BDNF水平低于对照组(P0.05),双相障碍抑郁发作组血清T4水平高于对照组,单相抑郁症组和对照组血清T4水平比较差异无统计学意义(P0.05),双相障碍抑郁发作组血清T4水平高于单相抑郁症组,TSH、BDNF水平低于单相抑郁症组(P0.05);治疗后双相障碍抑郁发作组和单相抑郁症组血清T4水平低于对照组,双相障碍抑郁发作组血清T4水平低于单相抑郁症组(P0.05),且三组血清T3、TSH、BDNF水平比较差异无统计学意义(P0.05);治疗后双相障碍抑郁发作组认知障碍因子评分低于单相抑郁症组(P0.05);Spearman相关分析显示,血清T3、T4、TSH水平和HAMD评分与BDNF呈负相关,TSH水平与BDNF呈正相关(P0.05)。结论:抑郁症患者血清T3、T4、TSH和BDNF水平存在异常,可作为判断双相障碍抑郁发作及单相抑郁症的指标。     

双相躁狂抑郁症遗传方式的探讨 — X连锁显性遗传模式的检验

躁狂抑郁症遗传学研究已可以肯定遗传因 素是发病的重要因素之一Ell。自1959年Leonhard 认为遗传效应对双相躁狂抑郁症较单相 型更为突出后，Perris (1966), Angest (1966) 及Winokur (1967）等又提供资料支持这一看 法，但认为双相型与单相型的遗传背景有所不 同。一般以X连锁显性遗传和常染色体显性遗 传的推测为多，h,，一，，亦有许多学者认为此病属 多基因遗传〔a。国内有关的专题报告较少[2,3IO 一为了探讨双相躁狂抑郁症可能的遗传方式，我 们对40例双相躁狂抑郁症患者及其一级亲属 进行了家系研究。本文仅对其X连锁显性遗传 模式的问题进行分析和讨论。     

Determination of subcellular concentrations of soluble carbohydrates in rose petals during opening by nonaqueous fractionation method combined with infiltration–centrifugation method

Petal growth associated with flower opening depends on cell expansion. To understand the role of soluble carbohydrates in petal cell expansion during flower opening, changes in soluble carbohydrate concentrations in vacuole, cytoplasm and apoplast of petal cells during flower opening in rose (Rosa hybrida L.) were investigated. We determined the subcellular distribution of soluble carbohydrates by combining nonaqueous fractionation method and infiltration–centrifugation method. During petal growth, fructose and glucose rapidly accumulated in the vacuole, reaching a maximum when petals almost reflected. Transmission electron microscopy showed that the volume of vacuole and air space drastically increased with petal growth. Carbohydrate concentration was calculated for each compartment of the petal cells and in petals that almost reflected, glucose and fructose concentrations increased to higher than 100 mM in the vacuole. Osmotic pressure increased in apoplast and symplast during flower opening, and this increase was mainly attributed to increases in fructose and glucose concentrations. No large difference in osmotic pressure due to soluble carbohydrates was observed between the apoplast and symplast before flower opening, but total osmotic pressure was much higher in the symplast than in the apoplast, a difference that was partially attributed to inorganic ions. An increase in osmotic pressure due to the continued accumulation of glucose and fructose in the symplast may facilitate water influx into cells, contributing to cell expansion associated with flower opening under conditions where osmotic pressure is higher in the symplast than in the apoplast.     

Light-induced proton transport by chloroplasts suspended in fluid media at sub-zero temperatures: kinetics and stoichiometry.

1. Chloroplasts suspended in a medium containing ethanediol and water (1 : 1, v/v) at -16 degrees C show light-induced proton uptake and subsequent dark efflux. Proton uptake in continuous light showed biphasic kinetics. 2. A 1 ms flash caused a single turnover of the photochemical centres at -16 degrees C. Under the same conditions 3H+ were taken up from the external medium in the presence of methyl viologen as electron acceptor. 3. The flash-induced proton uptake was exponential and monophasic with t1/2 = 3 s. The flash-induced proton release into the thylakoid interior was biphasic, with half-times of less than 0.1 s and 3 s. The fast phase represented approximately 30% of the total release and may be correlated with the oxidation of water. 4. The half-time of reduction of cytochrome f in the dark following illumination in the presence of 2 mM NH4Cl (2.5 s) is similar to the half-time of the slow phase of proton release, suggesting a correlation between the kinetics of cytochrome f reduction and plastoquinol oxidation.     

Al uptake kinetics in roots of Melastoma malabathricum L. – an Al accumulator plant

The mechanism of Al uptake in melastoma (Melastoma malabathricum L.), which accumulates Al in excess of 10 000 mg kg^–1 in its leaves and roots, was investigated. Al uptake kinetics in excised melastoma roots showed a biphasic pattern, with an initial rapid phase followed by a slow phase. It was indicated that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. On the other hand, Al uptake rate in roots of whole melastoma plant was almost double that in excised roots. The difference of Al uptake rate between excised roots and whole plant seems to be due to transpiration-depended Al uptake. Results from a long-term experiment showed that different characteristics of Al accumulation between melastoma and barley was caused by the difference in capacity to retain Al in root symplast, rather than by the difference in uptake rate into symplast. Concentrations of oxalate in root symplastic and apoplastic fractions, and total oxalate in shoots and roots, did not change greatly with time of Al exposure compared to Al concentration, although oxalate is considered as a main Al ligand in tissue of melastoma. On the other hand, oxalate exudation to root apoplast was induced within 24 h of Al exposure; the role of such exudation was discussed.     

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

The positive xylem pressure (Px) in cucumber hypocotyls is a direct extension of root pressure and therefore depends on the root environment. Solutions of the electrolyte KCl (0-10 osm) reduced the hypocotyl Px transiently (biphasic response), while the Px reduction by mannitol solutions was sustained. The amplitudes of the induced Px reduction depended directly, and the degree of Px restoration after stress release depended indirectly, on the size of the initial positive Px indicating that mannitol released the root pressure by a mechanical rather than osmotic mechanism. Mannitol treatment and other means of root pressure reduction revealed two separate growth responses in the affected cucumber hypocotyls. Only steep Px drops (following root excision or root pressure release in mannitol) directly cause a rapid, transient drop in growth rate (GR). Both rapid and slow (after root incubation in KCN or NEM) decreases in root pressure, however, led to a sustained growth inhibition of cucumber hypocotyls after about 30 min. This delay characterizes the growth response as an indirect consequence of the Px change. Pea seedlings, which lacked root pressure and had a negative Px throughout, showed extremely small changes in epicotyl Px and GR after root incubation in mannitol. It is apparent that the higher sensitivity of cucumber growth to mannitol depended on the presence and release of root pressure.     

A mechanism of respiration-dependent water uptake enhanced by auxin

Summary There are many contradictory observations on the mechanohydraulic relation of growing higher plant cells and tissues. Graphical analysis of the simultaneous equations which govern irreversible wall yielding and water absorption has made more comprehensive the understanding of this relation when relative growth rate is plotted against turgor pressure. It suggests that some respiration-dependent and auxin sensitive process might regulate the difference of osmotic potential between cells and water source. Based on anatomical and electrophysiological knowledge of the pea stem xylem, we propose the wall canal system as the mechanism of respiration-dependent water uptake which is sensitive to auxin. This system consists of the xylem apoplastic walls, the xylem proton pumps, active solute uptake system and cell membranes. In the simplest case, third-order simultaneous differential equations are involved. Numerical analysis showed that net uptake of solutes enables water to be taken up against an opposing gradient of water potential. The behaviour of this wall canal system describes well the mechano-hydraulic relation of enlarging plant cells and tissues. Recent typical, but incompatible, interpretations of this relation are critically discussed based on our model.Abbreviations V the volume of enlarging symplast - the average extensibility of the wall - Pⁱ turgor pressure - Y the yield threshold of the wall - L the relative hydraulic conductance - the solute reflection coefficient of the plasmamembrane - Cⁱ the osmotic concentration of the symplast cells - C^x the osmotic concentration of the xylem vessels - P^x hydrostatic pressure in the xylem vessels - R the gas constant - T absolute temperature - ^o water potential of xylem fluid - ⁱ water potential of symplast cells     

Effects of anaerobic conditions on water and solute relations,and on active transport in roots of maize (Zea mays L.)

The effects of anoxia on water and solute transport across excised roots of young maize plants (Zea mays L. cv. Tanker) grown hydroponically have been studied. With the aid of the root pressure probe, root pressure (P_r), root hydraulic conductivity (Lp_r), and root permeability (P_sr), and reflection ( _sr) coefficients were measured using potassium nitrate (a typical nutrient salt) and sodium nitrate (an atypical nutrient salt) as solutes. During a period of 10–15 h, anaerobic treatment (0.0–0.2 g O₂·m^-3 in root medium) caused a decrease of root pressure by 0.01–0.28 MPa (by 10–80% of original root pressure) after a short transient increase. For a time period of 5 h, the decrease in the stationary root pressure was not reversible. Under anaerobic conditions, roots still behaved like osmometers and were not leaky. The root hydraulic conductivity measured in osmotic experiments (osmotic solute: NaNO₃) was smaller by one to two orders of magnitude than that measured in the presence of hydrostatic gradients. Both the osmotic and hydrostatic hydraulic conductivity decreased during anaerobic treatment by 28 and 44%, respectively, at a constant reflection coefficient of the solutes ( _sr=0.3–1.0). As with root pressure, changes in root permeability to water and solutes were not reversible within 5 h. Under aerobic conditions and at low external concentrations (31–59 mOsmol·kg^-1), osmotic response curves were monophasic for KNO₃, i.e. there was no passive uptake of solutes. Response curves became biphasic at higher concentrations (100–150 mOsmol·kg^-1)- For NaNO₃, response curves were biphasic at all concentrations. Presumably, this pattern was a consequence of the fact that potassium had already accumulated in the xylem. During anoxia, accumulation of potassium in the xylem was reduced, and biphasic responses were also obtained at lower potassium concentrations applied to the medium. The results are discussed in terms of a pump/leak model of the root in which anoxia affects both the active ion pumping and the permeability of the root to nutrient salts (leakage). The effects of anaerobiosis on the passive transport properties of the root (Lp_r, P_sr, _sr) are in line with the recently proposed composite transport model of the root.Abbreviations and Symbols A_r root surface area - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_r root pressure - P_sr permeability coefficient of root - _sr reflection coefficient of root The authors thank Mr. Walter Melchior for the curve-fitting program used to work out Lp_rh values from root pressure relaxations and Mr. Mohammad Hajirezai (Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth) for making the ATP measurements. The assistance of Mrs. Libuse Badewitz in making the drawings and the technical help of Mr. Burkhard Stumpf are also gratefully acknowledged.     

盐胁迫对大豆根系木质部压力和Na+吸收的影响

取栽培大豆的水培幼苗为材料,用木质部压力探针和原子吸收分光光度计测定了盐胁迫条件下其根木质部压力和伤流液中Na~+含量的变化,以分析大豆抗盐吸水的机制.结果表明:在25～150 mmol/L NaCl的浓度范围内,随着盐胁迫强度的增加,大豆根木质部负压力的绝对值逐渐增大,但相对负压力和根的径向反射系数则逐渐减小;木质部伤流液中Na~+含量逐渐增加,但Na~+的相对含量则逐渐降低.同时,虽然根系吸水所需的木质部负压力(压力势)及根木质部伤流液的渗透势随着盐胁迫强度的增加都有所下降,但两者共同作用使木质部水势下降的幅度远远小于根外溶液水势(渗透势)下降的幅度,即随着根外溶液盐浓度的升高,根木质部溶液的总水势逐渐高出根外溶液的水势.上述结果说明,在盐胁迫下大豆可以利用相对小的木质部负压力逆水势梯度吸水,且通过避免对Na~+的过量吸收来适应盐胁迫环境.     

A new concept of root exudation1

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch-activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m^?3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.     

Water Uptake by Different Regions of the Barley Root. Pathways of Radial Flow in Relation to Development of the Endodermis

The water uptake by different lengths of lateral roots and 1.0cm or 5.0 cm lengths of the seminal axes from differentregionsof the root were measured in potometers with the shoot in airat two humidity regimes. A model of the contribution by thesedifferent regions of the root to the total water absorptionby the plant agreed well with measurements of water uptake bythe whole root system. According to this model, about one halfof the water taken up by the main axis came from the older suberizedregions further than 10 cm from the tip, and together with itsassociated lateral roots this region provided 75% of the totalwater transpired. The development of State III endodermal cellswas correlated with decreases in both the water uptake by theolder regions of the root and the translocation of calcium.Thus in the younger regions of the root where water uptake ismaximal, the flow of water is principally apoplastic althoughthere is also likely to be flow via the symplast Despite a 43%difference in transpirational demand between the two humiditytreatments, the leaf water potentials remained constant, implyinga change in root resistance. This change in resistance mightbe explained if there were an apoplastic pathway within thesuberin lamellae of State III endodermal cells. The responseto the increased transpirational demand is met by the olderregions of the root, in particular by the zone of lateral emergencewhere an apoplastic pathway is known to exist as the Casparianband in the endodermis breaks down with the emergence of thelateral roots. Key words: Endodermis, Pathways, Water Uptake     

Water Flows in Higher Plants: Physiology,Evolution, and System Analysis

Water movement in higher plants is treated as a symplastic fluid flow incorporated into a unified hydrodynamic system comprising the apoplast and vessels (or tracheids). Since water flow is of vital necessity for algae (phylogenetic ancestors of higher plants), it can be stated that higher plants colonized land, still keeping connections with their former water habitat. It is argued that colonization of terrestrial areas by plants became possible due to the appearance and maintenance of a gradient of water chemical potential between the rhizosphere and atmosphere, which drives water flows. Autonomization of flows in the symplast is considered as a vector of evolution, whereas the gradient of water activity is a factor of evolution. The osmotic models of water uptake by roots are analyzed; the role of potassium circulation in water-transporting system is determined; and a mechanism of automatic coupling between CO₂ uptake through stomata and water evaporation from leaves is presented. An inherent property of the systems to explicitly or implicitly duplicate its structural or functional elements substantiates possible interactions between the mechanisms underlying opposite water flows in plants.