Water uptake by barley roots as affected by the osmotic and matric potential in the rhizosphere

Biomedical and Life Sciences Plant and Soil
Volume 94, Number 1, 143-146, DOI: 10.1007/BF02380596
U. Schleiff

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植物和土壤94,143-146（1986）                 Ms. 6080
9 1986 Martinus Ni/hoffPublishers, Dordrecht 在荷兰出版。

根际中大麦根系吸收水分受渗透和基质势人影响
U. SCHLEIFF
ETH-Zi~rich Department of Crop Science, CH-830 7 Eschikon-Lindau, 瑞士
1985年8月30日收到。1985年12月修订。
关键词 大麦 根际 耐盐性 土壤水势

摘要
盐碱土中根系对水的吸收率受到抑制的同时，根系周围土壤基质势和渗透水势也降低。遗憾的是，没有可靠的工具，可用于直接测量根际土壤中根的水吸收率降低的效果。本文展示了不同水势和基质势组合对根际土壤中大麦根系水吸收率的影响一些植物实验的结果。

降低土壤基质水势比起降低土壤渗透水势，使水吸收率降低到一个更大的程度。根据本实验的结果，砂土的总水势和当根系处在不同的基质势和渗透势时的水吸

简介
当根系对水分的吸收发生在盐碱土中时，近根系土壤水势下降更多，比起远离根系土壤。因此，在水耗尽期，根暴露在土壤基质势和水势，本质上不同于大块土壤。为了提高我们对生长在盐碱土中作物的水供应过程的理解，减少在根周围土壤渗透基质水势不得不定量地决定。
从各个田野和盆栽试验，得出的结论是，在一般情况下，相等的降低渗透和基质土壤水势对影响水供应和作物生长有一人相似的程度。因为作物抵抗土壤盐碱度和渗透胁迫的能力显著不同，这是众所周知，这个概念可能不适用所有作物。一些耐盐碱作物例如甜菜和大麦可以调整他们面子水势应对渗透胁迫，因此，渗透性土壤水分胁迫比起基质土壤水分胁迫对它们的水供应较少影响。基于这个假设这个实验的目的就是研究根系在不同的总土壤水势下从土壤中的水分吸收情况。

材料和方法
生长在一升的装有轻质土壤（370g黄土和1230g沙子的混合）盆的大麦植株。盆有 盖子覆盖，盖上有一个为了幼苗刺穿的小洞。盆的水容量是体积的20%。植物生长周期共分为两个阶段，最初栽培期36天，随后的实验阶段用4天（表1）。最初的栽培阶段是为了获得有茂密根系的良好的成年植株。被选择盐碱化处理的植物，被适应于加入10mmol/l氯化钠到水中的含盐生长环境，用于在最初的栽培期灌溉植物。被选择研究降低土壤基质势的影响的植株，不接受氯化钠。在在初始阶段结束时，由于土壤含水量下降到了3-4%，对应的土壤基质势0.1- 0.15兆帕，所有的植物枯萎。植物枯萎发生在相对高基质势中，因为植物暴露到干旱的生长环境中（30-35~29-40%空气相对温度）,根密度是极端高（~15cm/cm^3）.芽重量是1.4+-0.1g DM/shoot ,根重量1.0*0.1g DM/根（5盆平均数）

表1.
不同处理的氯化钠添加量在最初栽培期和处理期

实验期的目的是为了测量大麦根系暴露到不同渗透和基质土壤水势的根际土壤中时的水分吸收率。因此这个实验期开始靠用不同盐浓度（表1）150ml水/盆灌溉植株接近盆容量.灌溉3小时盆重被确定，植株暴露到模拟上面描述的干旱环境中。在接着的4天水消耗期间，土壤水损失（蒸发土壤水）由几次盆称重值决定。土壤基质和渗透水势被计算从土壤水容量。土壤的pF曲线靠暴露样品到相应的吸力而获得，较低的值被应用相应的压力。附加实验详情已经被报告。

结果和讨论
a)土壤水分状况的过程
相关的一些具有代表性的盆的土壤水势过程观测将被提交。在实验的开始（fig.1,0h）,所有盆的土壤基质水势是0.01Mpa ，土壤水容量大约15%。土壤溶液浓度不同导致在土壤渗透和总水势排列从水势0.01Mpa到1.0Mpa的不同。
在随后的水消耗期，土壤水势的显著不同被观察到。在非盐碱对照处理的土壤水容量下降到4%，在照明25小时后，相当于基质势的值为0.06Mpa.到实验结束时，土壤基质势的值达到0.3Mpa ,这将引起植株严重的萎蔫。
在用包含25mmol氯化钠/升的水灌溉盆时，土壤基质水势过程概念股最初25小时的照明相似。到实验结束时，基质势的值大约达到0.2Mpa.由于在最初栽培中，这个处理的土壤被轻微地盐碱化，更多的盐将被加入以灌溉水，植株已经被暴露到土壤的全水势，在实验开始的0.5Mpa。在接着的25h，the ~kT-value降低到大约1.2Mpa,主要是由于增加的土壤溶液盐浓度。在实验末，the ~T-Value大约是3.0Mpa,其中2.8Mpa是渗透势，仅仅有0.2Mpa是基质势。

b)根的水吸收
根的水吸收率的计算是基于1gDM形成一个长有150m的根的一个根土的假设。在实验期间根密度的增加被认为是不重要的对于水吸收，因此，不予考虑。
在水消耗期，土壤水势增加反过来影响根的水分吸收率。fig2展示了土壤fiT和暴露于非盐碱土壤和两个盐碱土壤的根的水分吸收率的关系。在第一个水消耗期（0-11小时）的平均的水吸收率改变在无盐碱的2.1 ，低盐碱的2.3和高盐碱的1.7.它是明显的，非常相似的吸收率发生在在非盐碱和低盐碱的处理的土壤中，本质上不同于他们的总水势。这些土壤T-Values的不同是唯一地被他们的滲透势值的不同所引起，然而基质势值是高的。显然，土壤渗透势对于根系水分吸收是较低重要性的。
在第二个照明期间（11-25小时），水分吸收率趋向更高比在第一时期，因为在温室中的空气温度是更高了。最高的平均水吸收率3.3是被发现在非盐碱盆，在土壤t值减少从15kpa到60kpa.在低盐碱盆中，水吸收率轻变低，然而t值的减少是显著的（从0.7到1.2Mpa）.最低的水吸收率1.5被观察到了在高盐碱环境，在那，t值减小从1.2到1.5Mpa.这些数据也显示土壤t值对于描述植物水供应不是今人满意的标准。在第三个照明期，水吸收率的对比也显示出了在1.1到1.3时相似的吸收率发生在非常不同的土壤水势中。
这些结果是相对于其他作者的发现,那些发现在作物中相似的水供应减少人，在减少的土壤的t值，t值独立的组件。几个原因能解释这些不同的结果。在这个实验中，一个耐受盐的作物被选择，它能够达到大的程度，来补偿在根际的渗透胁迫。其它的作者曾经选择豆子、柑橘和苜蓿，它们有较小的耐盐性。另一个原因可能是土壤的纹理与非常多的根结合在实验中。砂土包含很少的水，基质势0.1Mpa,甚至更低。相同程度渗透势的值能够被在盐碱和砂土中达到相对高土壤水分，但许多植物水分吸收因此轻微降低。
在这个实验中，水分吸收中基质势的值的作用可能是更引人注目的，由于高的根密度，根际基质势唯一地被测量，而非土壤层的基质势的平均值。这意味着植物供水不能靠来处外界根际和根生长进入非根际土壤的水通量提高。

总结本次实验的结果，土壤渗透和基质水势成分影响所有作物水分吸收到相同的程度，这是今人怀疑的。

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original text
http://www.springerlink.com/content/b36244880520810x/

Plant and Soil 94,143-146 (1986).
9 1986 Martinus Ni/hoffPublishers, Dordrecht. Printed in the Netherlands.
Ms. 6080
Water uptake by barley roots as affected by the osmotic and
matric potential in the rhizosphere
U. SCHLEIFF
ETH-Zi~rich Department of Crop Science, CH-830 7 Eschikon-Lindau, Switzerland
Received 30 August 1985. Revised December 1985.
Key words Barley Rhizosphere Salt tolerance Soil water potential
Summary The water uptake rates of roots in saline soils are depressed by the simultaneously
decreasing matric (r and osmotic (~o) water potentials in the soil surrounding the roots
(rhizospheric soil). Unfortunately there are no reliable tools available for direct measurements
of the effect of decreasing water potentials in the rhizospheric soil on the uptake rate of soil
water by roots. This paper presents some results of a vegetation technique for studying the
effect of different combinations of osmotic and matric water potentials in the rhizospheric
soil on the water uptake rates of barley roots.
Water uptake rates were reduced to a greater extent by decreasing soil matric water potentials
than by decreasing soil osmotic water potentials. According to the results of this experiment,
there was no relationship between the total soil water potential (r of a sandy soil
and the water uptake rates when the roots were exposed to different combinations of ~M and
fro-
Introduction
When water uptake by roots occurs in saline soils, the water potential of the soil near the
roots (rhizospheric soil) drops much more than in the soil further away from the roots. Thus,
during a period of water depletion roots can be exposed to soil osmotic and matric water
potentials that differ essentially from the bulk soil. In order to improve our understanding
of the processes of water supply to crops growing in saline soils, the effects of decreasing
osmotic and matric water potentials in the soil surrounding the roots have to be determined
quantitatively.
From various field and pot experiments, it was concluded that, in general, equivalent
decreases in osmotic and matric soil water potentials affect the water supply and the growth
of crops to a similar degree 2,3,4’5’8’9. Since it is well-known that crops differ significantly
in their ability to overcome soil salinity resp. osmotic stress, this concept might not be applicable
to all crops. Some salt tolerant crops such as sugar beets and barley may adapt their
leaf water potentials to osmotic stress and therefore their water supply should be less affected
by osmotic soil water stress than by matric soil water stress. Based on this assumption it was
the aim of this experiment to study the root water uptake from a soil with various combinations
of the total soil water potentials.
Materials and methods
Barley plants (one plant/pot, cv. Aura) were grown in l-liter pots filled with a light-textured
soil (mixture of 370g loess and 1230g sand). The pots were covered with a lid, in which a
small hole had been pierced for the seedling. The water capacity of the pots was 20 Vol.%.
The total growth period of the plants was divided into two periods, an initial cultivation
143
144 SHORT COMMUNICATION
phase of 36 days and a succeeding experimental phase of 4 days (Table 1). The aim of the
initial phase of cultivation was to obtain well grown plants having a dense root system. Plants
selected for salinization treatments were adapted to saline growth conditions by adding 10 mmol
NaC1/I to the water and used for irrigating the plants during the initial phase of cultivation.
Plants selected for studying the effect of decreasing soil matric potential did not received NaCI.
At the end of the initial phase, all plants were wilting due to a drop in soil water content to
3-4 Vol.% corresponding to a soil matric potential of–0.1 to –0.15 MPa. The wilting of the
plants occurred at these relatively high matric potentials since the plants were exposed to arid
growth conditions (30-35~ 29-40% relative air humidity) and the density of the roots was
extremely high (~ 15 cm/cm3). The shoot weight was 1.4 -+ 0.1 g DM/shoot and the root weight
1.0 x 0.1 g DM/root (average of 5 pots).
Table 1. Sodium chloride additions (mmol/pot) to the various treatments during the initial
phase of cultivation and during the treatment phase
Treatments (mmol NaC1/pot)
NaCl-conc. of water
(retool~l) Non saline 0 25 150 300 450
Initial phase
of cultivation 0 8 8 8 8 8
Treatment phase 0 0 3.75 22.5 45 67.5
Total 0 8 11.75 30.5 53 75.5
The aim of the experimental phase was to measure the water uptake rates of barley roots
exposed to a rhizospheric soil with different combinations of osmotic and matric soil water
potentials. The experimental phase was therefore begun by irrigating the plants near to pot
capacity with 150ml/pot water of different salt concentrations (Table 1). Three hours after
irrigation the pot weight was determined and the plants exposed to simulated aridity as described
above. During the following 4 days period of water depletion, the water losses of the
soil (= transpired soil water) were determined by weighing the pots several times. The matric
and osmotic water potentials of the soils were calculated from the soil water contents. The
pF-curve of the soil was obtained by exposing the samples to a corresponding suction, the
lower values (–80 to –1500kPa) by applying corresponding pressures. Additional experimental
details have already been reported 7.
Results and discussion
a) Course of the soil water status
Observations related to the course of the soil water potential in a few representative pots
will be presented. At the beginning of the experiment (Fig. 1, 0 h), the soil matric water potential
of all pots was –0.01 MPa at a soil water content around 15 Vol.%. Differences in the salt
concentrations of the soil solutions resulted in great differences in the osmotic and total water
potentials of the soil ranging from total water potentials of– 0.01 MPa to — 1.0 MPa.
During the succeeding periods of water depletion significant differences in the course of the
soil water potentials were observed. The soil water content in the nonsaline treatment dropped
after 25 hours of illumination to about 4 Vol.%, which corresponds to a ~PM-value of –0.06 MPa.
By the end of the experiment, the ~M-value of the soil had reached –0.3 MPa which caused
severe wilting of the plants.
In the pot irrigated with water containing 25 mmol NaC1/1, the course of the soil matrix
water potential was very similar during the first 25 h of illumination. By the end of the experiment,
a ~M-Value of about–0.2MPa had been reached. As the soil of this treatment was
slightly salinized during the initial phase of cultivation and further salts were added with the
irrigation water, the plants had already been exposed to a total water potential of the soil
(r of –0.5 MPa at the beginning of the experiment. During the following 25 h, the ~kT-value
SHORT COMMUNICATION 145
decreased to about –1.2 MPa, mainly due to the increasing salt concentration of the soil
solution. At the end of the experiment the ~T-Value was about–3.0 MPa,- 2.8 MPa of which
were osmotic and only –0.2 MPa were matric potential.
In the high saline treatment (150mmol NaCl/1), the drop in the ffM-value of the soil was
significantly lower. Following an illumination period of 25 h, the ~M-value was –0.02 MPa
which was significantly higher than in the nonsaline and lowsaline treatments. As the water
uptake rates of the plants were significantly reduced, the ffM-value had dropped to about
only — 0.04 MPa by the end of the experiment. Due to the salts dissolved in the soil solution,
the ~0T-Value of this soil had already reached — 1.0 MPa at the beginning of the experiment,
dropped to — 1.5 MPa 25 h later and reached — 2.5 MPa by the end of the experiment.
b) Water uptake by roots
The calculation of the water uptake rates by the roots is based on the assumption that a
root mass of 1 g DM forms a root length of 150 m 6, An increase in the rooting density during
the experimental phase was supposed to be of little importance for the water uptake and,
therefore, not taken into account.
During a period of water depletion, the water potential of the soil decreases which in turn
affects the water uptake rate of roots. Fig. 2 shows the relationship between fiT of the soil and
-o,o, 4 /
-qo2
-qo3
-~176 \ -q30L~ ‘ ~”‘– ~
0
irrigation water :
2×5 15xOmeq NoCI/I
\ \ I: 1
\ \1
[Val.*l.]
-15
– ,2,s
8
-7,s _~
-2s ~n
, tl , , , -0
-q2 -0.4-0.5 -10 -15 -2’0 -2; -3′.0 [MPG]
total water potential at soil
Fig. 1. Relationship between decreasing soil water (due to water depletion by roots) and total
water potential of the rhizospheric soil at various salinity levels.
cm r~t.10h J
saline
5 _~-
1-
o .0.’2 .; .& _o’.~ .,; .,.? .,; .,; _,:~ -2’otMPo] g
total soil water potential
Fig. 2. Water uptake rates of barley roots in relation to the decreasing water potential of the
rhizospheric soil due to root water uptake at different salinity levels.
illumi~lion inle~ol:
x 0 tO llh
25 rnt:~zI I~k3CI/I irrig waler 9 11 10 25h
9 ?25 tO 39h
S o 39 lo 53 h
146 SHORT COMMUNICATION
the water uptake rates of the roots exposed to a nonsaline soil and two salinized soils. The
average water uptake rates during the first water depletion period (0-11 h) varied between
2.1 ~1 in the nonsaline, 2.3 t~l in the lowsaline and 1.7 ~l/cm root. 10 h in the highsaline treatment.
It is obvious that the very similar uptake rates in the nonsaline and low saline treatments
occurred in soils differing essentially in their total water potentials. The differences in the
~/T-Values of these soils were exclusively caused by differences in their Co-values, whereas the
CM-Values were high (> — 0.015 MPa). Obviously the soil osmotic potential was of low importance
for the water uptake by the root system.
During the second illumination interval (11-25 h), the water uptake rates tended to be
higher than during the first period since the air temperature in the greenhouse was higher.
The highest average water uptake rate of 3.3#1 was found in the nonsaline pot at a ~T-Value of
the soil decreasing from — 15 kPa to –60 kPa. In the lowsaline pot the water uptake rate was
slightly lower (2.7~1/cm root. 10h), whereas the decrease in the ,VT-Value was significant
(from –0.7 to –1.2 MPa). The lowest water uptake rate of 1.5 ~1 was observed in the highly
saline soil in which the CT-Value decreased from — 1.2 to — 1.5 MPa. These data show too that
the CT-Values of the soils were unsatisfactory criteria for characterizing the water supply to
the plants. The comparison of the water uptake rates during the third period of illumination
shows also that very similar uptake rates of 1.1 to 1.3 ul occurred in soils of very different soil
water potentials (~T from –0.1 MPa to — 1.6 MPa).
These results are in contrast to the findings of other authors 3’s’9, who found similar reductions
in crop water supply at decreasing CT-Values of the soil, independent of the components
of the qJT-Value. Several reasons could be responsible for these different results. In this
experiment, a salt tolerant crop was chosen, which was able, to a great extent, to compensate
for the osmotic stress in the rhizosphere. The other authors had selected beans, citrus and
alfalfa, which are less salt tolerant. Another reason could have been the texture of the soil
in combination with the extremely high rooting density in this experiment. A sandy soil contains
little water at a r of- 0.1 MPa or even lower. A ~0 o-value of the same degree can
be reached in a saline, sandy soil at relatively high soil water contents, but the water uptake of
many plants is thereby reduced only slightly.
The effect of lower qJM-Values on the water uptake may have been more dramatic in this
experiment as, due to the high rooting density, the qJM-Value of the rhizosphere exclusively was
measured and not the qJM-Value of a soil layer in average. This means that the water supply
of the plants could not be improved by a water flux from outside the rhizosphere and by roots
growing into nonrhizospheric soil.
In summarizing the results of this experiment, it is doubtful that the osmotic and matric
component of the soil water potential affect the water uptake of all crops to the same degree.
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