Schleiff: Crop Salt Tolerance and Root Environment


Research Aspects for Crop Salt Tolerance under Irrigation

with Special Reference to Root Environment


Dr. Uwe Schleiff: D-38289 Wolfenbuettel; P.O.Box 1934; schleiff@salinity.de
International Expert for Irrigation&Salinity - Fertilizers&Crops – Soils&Environment;
http://salinity.schleiff.net

Dedicated to my dear colleague Prof. Dr. Dr. Ewald Schnug on the occasion of his 50th birthday

Key words: rhizosphere, saline irrigation, saline soils, salt tolerance, root morphology, root water uptake, soil water availability


Abstract

Due to lack of good quality water for human consumption and industry in many countries irrigated agriculture is exposed to increasing pressure to expand the use of brackish and even high-saline waters for crop production. The application of lower quality waters requires special management practices, which are not yet fully understood with respect to optimize plant growth on saline soils. Generally the evaluation of crop salt tolerance is based on yield losses related to the average salinity of the rooted soil layer. However this concept often fails in cases when brackish water is applied, e.g. under high frequency irrigation. The presented approach reflects the effect of horizontal water and salt movement in saline soils within periods of soil water depletion by roots of different properties on root water uptake. When roots absorb water from a saline environment they create gradients of salt concentration and soil osmotic water potential between bulk and rhizospheric soil. Consequently progress in the understanding of root water uptake under saline conditions has to consider salt dynamic processes happening in the contact zone between soil and plant, the rhizosphere. During periods of water depletion salts continuously accumulate in the rhizospheric soil achieving concentrations that are several times higher than in the bulk soil. Roots of relatively salt sensitive young maize may accumulate salt concentrations in the rhizospheric soil solution corresponding to an ΨO-value of -0.8 to -0.9 MPa and salt tolerant barley roots to an ΨO of about -2.5 to -3.0 MPa. In general decreasing soil matric water potentials affected root water uptake significantly stronger than decreasing soil osmotic water potentials. First experiments indicate that interactions between soil and roots of different morphology may be one key factor for crop salt tolerance and breeding of salt tolerant crops. Roots of rape were 5-fold more efficient in their water uptake from saline soils than roots of onions, when the uptake rates were related to root masses.

1. Introduction

Worldwide, but especially in most developing countries, irrigated agriculture plays an increasing role for food supply of the growing population. According to FAO it is assumed that a share of about 30% of the irrigated area suffers from salinity problems, which corresponds to an area of more than 100 million ha worldwide. For some countries the percentage of salt-affected land is even higher: for Egypt, Pakistan and Iraq 35 to 50% of the irrigated land is considered as saline. It is estimated that yearly 10 million ha of irrigated land are abandoned because of salinity problems (Hamdy, 1999). A land resource of 1 billion ha (saline deserts, coastal soils) can be developed for food production, when more salt-tolerant plants such as halophytic species are taken into consideration for cultivation (Boeer&Ghais, 1999).

In the near future the share of low quality saline waters and saline soils in irrigated agriculture will strongly increase, as in many developing countries there is a strong pressure on good quality water sources. This pressure comes from competing users demanding for good quality water such as growing industries and increasing number of households due to population growth. The competition for fresh water between nations may even cause military threats as e.g. between India and Bangladesh for water of the Ganges River (Farakka Barrage; Schleiff, 1999; Shiva, 2002).

Several countries, where fresh water supply of growing population became already critical, follow the strategy to replace good quality waters by marginal waters (brackish as well as treated waste water) for agriculture (Kuck, 1999). So we consider the scientific preoccupation with plant growth under saline soil conditions and salt tolerance of irrigated crops not just as an academic exercise, but as research of high practical relevance for agriculture and environment of many developing countries.

Recent decades have already achieved much progress in the application of saline waters for crop irrigation. One significant indicator for the growing experience is the adjustment of critical ranges for the evaluation of irrigation water quality (Table 1). In 1954 EC-values of waters in the range of 0.75-2.25 dS/m were considered as highly saline, whereas almost 50 years later in 1992 water of 10-25 dS/m is classified as highly saline.

Table 1: Evaluation of irrigation water salinity changing with time

EC of water in dS/m (=mS/cm)

CLASSIFICATION

USDA, 1954

FAO, 1976

FAO, 1992

non-saline

< 0,25

< 0,75

< 0,7

slightly saline



0,7 - 2,0

moderately saline

0,25 - 0,75

0,75 - 3,0

2 - 10

highly saline

0,75 - 2,25

> 3,0

10 - 25

very highly saline

2,25 - 5,0


25 – 45

seawater: > 45dS/m



2. Aspects of Salinity Management in Irrigated Agriculture

The safe use of saline waters for sustainable crop production is a challenge that has to consider besides technical aspects the economic, climatic, social and hydro-geological situation (Hamdy, 1995). Consequently there is no single way to combat salinity harms at field level as well as at project and regional level.

The objective of Fig.1 is to summarize the most important technical and environmental aspects that may contribute to affect saline agriculture. It may serve as a useful tool to identify possible causes of salinity harms under definite field conditions. Based on the results of such a complex analysis a bundle or single measures to improve the present situation can be elaborated.

A second purpose of Fig.1 is to demonstrate the complexity of the problem and that there is no chance to cover all aspects within such a publication. However Fig.1 is also very helpful to


Fig. 1: Management strategies for saline agriculture (Hamdy, 1995, modified after Schleiff)




place our ideas and research activities within this complex and vast field of strategies to improve management of crop growth under saline conditions.

The concern of our research activities with respect to crop salt tolerance is basically placed in the root surrounding soil volume, the contact zone between plant and soil, at the soil/root-interface. But before offering our research concept for further progress in the management of saline soils and to improve crop salt tolerance by specific breeding goals (root morphology) the principles of present guidelines for a sustainable use of saline irrigation waters will be presented.


3. Rootzone Salinity for Crop Salt-Tolerance Evaluation

The present evaluation of crop salt tolerance for irrigated agriculture considers the average root zone salinity expressed as ECse (electrical conductivity of the soil saturation extract in dS/m) as critical parameter for plant growth. The classification presented in Table 2 is an extract of results gained from systematic field data analysis all over the world (Hoffman, 1981).

Table 2: The threshold of average rootzone salinity (electrical conductivity of soil saturationextract = ECse in dS/m) of field crops (Rhoades et al.1992; modified after Schleiff)

Crop sensitivity

Threshold(ECSE in dS/m)

Crop example

sensitive

0-1.5

bean, pea, onion, carrot, orange, peach, clover

moderately sensitive

1.5-3.0

maize, broad bean, alfalfa, tomato, grape

moderately tolerant

3.0-6.0

sorghum, soybean, wheat, red beet

tolerant

6.0-10.0

barley, cotton, sugar beet, date palm

very tolerant

>10

Species of Atriplex, Agropyron, Kochia, Salsola

no plant growth

>30-40

(estimated)


In agronomic terms a crop’s salt tolerance is appraised from the relative yield of the crop on a saline soil as compared with its yield on a normal, non-saline soil under comparable growing conditions (Hoorn, van & Alphen, van, 1994). The relative crop yield (Y in %) at a given average soil salinity (ECse in dS/m) for a specific crop can be calculated from the following equation (Maas & Hoffman, 1977):

(1) Y = 100 – b (ECse – a)

where: ‘b’ = the yield loss in % per unit (1 dS/m) increase of soil salinity
‘a’ = soil salinity threshold value: ECse, where yield decrease starts

Both values ‘a’ and ‘b’ are crop specific and were published by Maas & Hoffman (1977) and Ayers & Westcot (1985). This approach can be helpful to compare the relative advantage of crops or cropping patterns with respect to yield and economic value.

As crops differ significantly in their salt tolerance, they tolerate different levels of soil salinity. If these crop specific levels are exceeded e.g. due to salt import by saline irrigation water yield losses or even crop failure may occur. Consequently the permanent control of soil salinity avoiding excessive soil salinity is a key activity for sustainable use of saline soils and waters.

Leaching of salts from the rooted soil layer by excessive soil water percolating into deep and non-rooted sub-soil is the main activity to manage saline crop production. It is now the objective of a professional irrigation management to find out the amount of water required for an economic crop yield (crop water requirement, CWR in mm) and to control soil salinity at a crop and yield specific level.


Consequently there is a basic need for the quantitative determination of the water requirement for salt leaching from the root-zone (LR=leaching requirement), which is expressed as a fraction (LF = leaching fraction: LF = water leached [mm]/water applied [mm]) of the crop water requirement (CWR in mm) determined for non-saline conditions and follows the following equation:

(2) LR = LF = ECi / ECdw = Wdr / Wi

where: ECi = EC of irrigation water in dS/m
ECdw = EC of drainage (=percolation) water in dS/m
Wi = applied irrigation water in mm
Wdr = percolated drainage water in mm

The presented approach takes into consideration the ECi of the applied irrigation water, which has to be known, and the ECdw of the drainage water, which can be accepted according to the salt-tolerance of the crop and can be taken from Table 3.

Table 3: Tolerated salt concentrations of drainage waters (ECDW of soil solution belowrootzone) for crops of different salt-tolerance

Salt-tolerance of crop

tolerated ECDW

sensitive

4 - 8

moderately tolerant

8 - 16

tolerant

16 - 24

halophytes

24 - 50

Example: the LF for a maize crop (moderately tolerant: ECdw = 10dS/m) under irrigation with slightly saline irrigation water (ECi = 2.0dS/m) is 0.2 (2/10) resp. LR = 20% of the crop water requirement for non-saline conditions. When the CWR is 600mm/season, an LR of 120mm has to be applied to control soil salinity, which is a total of 720mm/season.

Soil salinity under controlled irrigation with saline waters never is homogeneous all over the rooted soil layer as assumed in Table 2. When irrigation follows the rules to cover the CWR and the LR, there is developing a gradient of soil salinity, which increases from the surface versus the bottom layer of the rooted soil. This gradient is the result of leaching processes, which differ in the various soil layers of the profile and can be calculated according to an approach presented more detailed by Ayers & Westcot (1985).

This approach does not regard the root-zone as a 1-layered profile, but as a 4-layered profile, where the root water uptake follows a pattern reflecting a rooting density. The top one-quarter is expected to contribute 40%, the lowest quarter only 10% of the total CWR:

40% from the top one-quarter

The gradient of soil water salinity (ECfc = ~ECdw) for each quarter of the root-zone can be calculated, when the effect of soil water extraction from the different soil layers on the LF from each layer is considered quantitatively.



The consequences of a stepwise calculation of salt-leaching on soil water salinity for a given situation are demonstrated in Fig.3. In average soil water salinity of all the rooted soil in both cases is about 6 dS/m as calculated from ECdw of the relevant layers:

(3) 1-layer-profile: (ECi+ECdw)/2 = (2+10)/2 = 6.0dS/m
4-layer-prof.: (ECdw0+ECdw1+ECdw2+ECdw3+ECdw4)/5 = (2+3+4.8+8+12)/5 = 5.96dS/m

But in case that salinity evaluation follows the 4-layered profile, it is clear that salinity of the upper 3 layers is significantly lower as calculated for the 1-layered profile. Consequently salinity stress to crops tends to be overestimated when soil water salinity evaluation is based on the 1-layered profile only.


Fig. 2: Soil salinity developing after long-term use of saline irrigation water (ECi = 2 dS/m)
in a rooted 4-layered soil profile as compared to a 1-layered profile

When salinity of soil water (ECdw) in a soil layer is known, soil salinity (ECse = EC of soil saturation extract) can be calculated by dividing ECdw with 2 for medium textured soils and 4 with sandy soils:

(4) Loamy soil: ECse = ECdw/2 = 6/2 = 3 dS/m
Sandy soil: ECse = ECdw/4 = 6/4 = 1.5 dS/m

Under practical field conditions salinity of the percolating soil water often deviates from the calculated concentration. This deviation reflects that not all soil water percolating through a profile participates in salt leaching. The reduced leaching efficiency comes from the fact that part of the soil water entering a soil layer does not mix with all the soil solution. The leaching efficiency coefficient (fi) was introduced to take leaching inefficiency into consideration. fi tends to affect water requirement for salt leaching more on heavy soils (soil water by-passing in large pores and cracks, when dry: e.g. fi 0.5 to 0.8) than on medium or sandy textured soils under frequent irrigation (e.g. fi 0.9 to 1.0). More details are presented by Hoorn & Alphen (1994).

Doubtless the 4-layers-concept approaches reality of vertical salt distribution under controlled irrigation better than the simpler 1-layer-concept for most cases and thus contributes to improve the calculation of the water required for effective leaching.

Water requirement to keep soil salinity at a crop specific level is also affected by irrigation technic. Experience shows that LF often is lower under high-frequency drip irrigation (daily or more) than under low-frequency irrigation according to following equation:

(5) LF=ECi / (2 * maxECse)

where maxECse defines the theoretical maximal ECse at zero yield potential at which crop growth ceases. This value is high (~25-30 dS/m) for salt tolerant plants (barley, sugar beet, cotton, date palm) and low (5-10 dS/m) for salt sensitive plants (groundnut, phaseolus bean, pepper, orange, strawberry; Ayers & Westcot 1985).

Basically the concept of high frequency irrigation at smaller application rates assumes a lower salinity stress to plants as changes in soil osmotic and matric water potential are smaller between water applications. However there are cases where this concept failed. Pasternak et al. (1986) reported from field experiments with tomatoes drip irrigated with water of 10dS/m. Tomato yields were 20% lower when daily irrigated as compared to an irrigation cycle of 2-days with the same total water quantity.

Similar results were found in pot experiments with maize irrigated with water of 5.3dS/m (Schleiff 1983a). Yields were 25% higher when water was applied every 3 days as compared to the every day treatment. From higher Chloride contents of leaves under the daily irrigation cycle it was concluded that plants were exposed to higher soil salinity stress. The results indicate that the concept to reduce salinity stress to plants by frequent irrigations is not of general validity. This indicates that not all mechanisms on water acquisition from saline soils are fully understood.

4. Research on Horizontal Salt Distribution around Roots

The presented overview has shown that there exists a theoretically clear and generally accepted concept for saline crop irrigation and its effects on bulk soil salinity, crop yields and plant growth. The concept even considers the effects on root water uptake rates following the rooting density, which usually decreases with soil depth and affects salt leaching.

However the concept only takes into consideration the vertical movement of soil water and dissolved salts, which predominates at soil water contents exceeding field capacity and is the key process for salt leaching. It does not consider that there occurs also a horizontal movement of soil water and salts, which predominates during periods of soil water extraction by plant roots following a water application. Leaf transpiration is the driving force that initiates the horizontal flow of water from a distance into the rhizospheric soil surrounding the root surface (rhizosphere: part of rooted soil volume directly altered by roots as compared to the bulk soil). Water and most nutrients are taken up by roots, whereas most salts translocated into the rhizospheric soil are excluded from uptake. Consequently during periods of water depletion salts accumulate in the soil solution close to the roots at simultaneously decreasing soil water contents. The effect of this basic process within irrigation cycles on shoot water supply has been already discussed earlier (Schleiff 1982a).

Riley & Barber (1970) and Sinha & Singh (1976) first observed an accumulation of easily soluble salts around roots of soybeans and maize. From field experiments with onions in Saudi Arabia using drainage waters of different salinities (2.4 to 8dS/m) Schleiff (1979, 1980, 1981) concluded that salt accumulation in the rhizospheric soil is not unlimited. In pot experiments with maize, barley and sugar beets it was shown that the degree of salt accumulation in the rhizospheric soil solution might be plant specific. For young maize plants representing a less salt-tolerant a maximum osmotic potential around -0.8 to -0.9 MPa cannot be exceeded (Schleiff 1981b). Using micro suction cups Vetterlein&Jahn (2002) confirmed this critical value for the rhizospheric soil solution of young maize roots. Using a specific vegetation technique Schleiff (1987a) determined for more salt-tolerant crops such as barley and sugar beet maximum values around -3.0 MPa that were found after an adaptation of shoots to saline conditions (Schleiff 1982b, 1982c, 1982d, 1983b). For young sugar beet a close relationship between the osmotic water potential of weakly wilting leaves and the rhizospheric soil solution was reported (Schleiff 1982b). This observation principally opens the possibility to estimate the maximum salt concentration around roots (where root water uptake and transfer of salts from the bulk soil into the rhizosphere is close to zero) from the shoot osmotic potential at wilting.

The presented results clearly indicate that the osmotic potential in soil solutions contacting the root surface is one key factor for root water uptake and plant water supply and that there are large differences among plants. But there are further factors involved in the complex process of root water acquisition under saline soil conditions.

From field observations it is also known that salinity stress may alter root morphology. Roots of 5- to 6-months old sweet potatoes growing saline soils were very fine, but no tubers were produced, whereas the shoots looked quite good (Schleiff 1989). Plants adapted to saline soils by improving the efficiency of their root system for water uptake. Production of tubers was neglected, as all assimilates transferred down to the roots were required for acquisition of soil water.

From experience especially with perennial crops it is well-known that roots are involved in plant salt-tolerance. Rootstocks of grape, citrus and avocado affect salt-tolerance of the shoot significantly (Rhoades & al., 1992, p.39). It is not always clear, which root property is effective. The exclusion of damaging ions (Cl, Na) from the shoot is often mentioned as one decisive factor.

.


Fig. 3: Salt dynamic processes in the rhizosphere of crops under irrigation with saline
water and the effect on crop salt-tolerance (after Schleiff, 2003)

Fig.3 is presented in order to sketch a research proposal that is expected to supplement our present knowledge on crop salt tolerance and optimizing irrigation procedures with saline waters. The graph is combining the effect of the already well-known approach (vertical profile of soil salinity) to evaluate crop salt tolerance with interactions between root and soil properties at the soil-root interface (horizontal salinity profile), which are not yet understood. Basically the research concept follows selected aspects of root and soil properties that were identified earlier to enlarge our understanding on nutrient acquisition by root and soil factors as summarized by Jungk (2002).

Basically Fig.3 considers the three following factors playing an important role for crop salt tolerance and water acquisition by roots under saline soil conditions:

The vertical salinity profile under controlled irrigation as shown in Fig.3 reflects the depth and density of the root system (see under chapter 3). However it is not just the relative rooting depth as shown schematically in Fig. 2 that affects crop salt tolerance, but it is the absolute rooting depth that determines the rooted soil volume. The right half of Fig.3 indicates that a shallow root system is expected to support the sensitivity of plants to soil salinity, whereas a deep root system (left half of Fig.3) tends to improve the crop salt tolerance, as the access to a larger volume of easily available soil water is facilitated. Principally there is no further increase in the salinity of the soil water, when the water leaves the rooted soil layer (drainage water) and effects of by-pass water and saline groundwater are excluded.

Especially under (semi)arid climates (high transpiration rates) water depletion by roots is expected to create horizontal salinity profiles around roots within irrigation cycles. However the development of salinity profiles between bulk and rhizospheric soil is expected to vary among plants and soil moisture conditions. Fig.3 distinguishes for crops of different salt-tolerance between two points of time: curve B, which is the salinity profile at the beginning of a period of water depletion (soon after a water application) and curve A, which shows the salinity profile at the end of a period of water depletion (before following water application).

The curve B shows for both crops that there is just a small gradient between soil salinity of the bulk and rhizospheric soil soon after a water application. This gradient principally will be affected by the application rate. Higher application rates will reduce this gradient stronger than lower application rates. Only very high application rates will achieve the disappearance of the salinity gradient. During the following period of soil water depletion a continuous flow of saline soil solution from the bulk soil to the root surface will initiate a salt accumulation in the rhizospheric soil. As reported earlier in this paper (chapter 3), the continuously increasing salinity of the rhizospheric soil decreases root water uptake and consequently affects plant growth. The ‘maximum salt concentration of the rhizospheric soil solution’ (expressed as osmotic potential in -MPa) as outlined in Fig.3 indicates the plant specific salinity level, which ceases root water uptake. This salinity level principally is lower for salt-sensitive crops and higher for salt-tolerant crops. It may vary between -0.6 MPa for very salt-sensitive crops (e.g. phaseolus vulgaris), around -2.0 to -3.0 MPa for salt-tolerant crops and up to -4.0 MPa for halophytes.

Fig.3 indicates the principal effect of root hairs for crop salt-tolerance. There are large differences in the volume of the rhizospheric cylinders among plants (Junk 2002). Length of root hairs, their density and life span are some very relevant factors for nutrient uptake, which may also interfere with plant water supply under saline soil conditions. This is concluded from the fact that the transpiration-induced salt-buildup close to the roots may occur in rhizospheric soil volumes respectively water volumes that differ significantly. When a definite amount of saline soil solution is transferred from the bulk soil into a small rhizocylinder to cover the transpiration need, there will occur a sharp increase of salinity (=strong decrease of soil osmotic water potential). In case the same amount of saline soil solution is transferred into a larger rhizocylinder, a significantly lower increase of rhizospheric salinity is expected. It is concluded that root hairs may be very efficient with respect to improve plant water supply from saline soils and crop salt-tolerance.


Fig. 4: Water uptake rates by roots of onions and rape [based on root dry matter] from
saline soil solutions following a water application with 50meq/l NaCl
(Schleiff 1983, unpublished data)

Empirical support of the hypothesis that root morphology may be relevant for water uptake from saline soils is rare, but a primary result obtained from a pot experiment with onions (very short root hairs, small rhizocylinder) and rape (long root hairs, large rhizocylinder) confirm this hypothesis. In this experiment rape and onion were chosen as their root systems are known to differ significantly in their morphology, but shoots of both plants may adapt to saline conditions to a similar degree. From experiments with nutrients (P, K) it was concluded earlier that roots of rape form a rhizocylindric soil volume that is about 20- to 30-fold larger than the volume of onion roots (Jungk 2002). This factor explained significant differences in the efficiency of nutrient uptake, especially when relatively immobile nutrients are considered.

Comparable results were obtained when water uptake rates by roots of onion and rape from saline soils were analysed as presented in Fig. 4. Water uptake rates of rape roots (25-30 ml/(g root-DM*h) were 5 to 6-fold higher than for onion roots (5-7 ml/[g root-DM*h]) at a rooting density of 0.3 to 0.4 g/l root-DM for both plants, when soil osmotic water potential ranged around -0.35 to -0.6 MPa and soil matric water potential around field capacity and slightly lower (Schleiff 1983c). Similar results were obtained, when water uptake rates by roots of leek and rape were compared (Schleiff 1986, unpublished data). Even when the presented results have to be discussed from other aspects than root morphology only (e.g. stomatal characteristics) , there is much evidence that root morphology plays an important role for crop salt tolerance and water acquisition from saline soils. In the past the aspect of root morphology was not investigated, which is explainable from the lack of appropriate investigation techniques. However it is urgently recommended to start research in this field, as it will be extremely promising to understand these factors at the soil-root interface. To strengthen this field of research will make more sense than to add another millions of data on the K/Na- or other ratios to the already existing millions of data from fractions of shoots, even when due to the complexity of the subject results will be difficult to achieve and sometimes less clear.

5. Effects of Soil Osmotic and Matric Water Potential on Plant Water Supply

There is no doubt, generally crop water supply is good, when soil moisture is close to field capacity and soil salinity is low. Decreasing soil water contents reduce water uptake by roots. This reduction of root water uptake is even higher when soil salinity is involved. Since Wadleigh and Ayers (1945) and Ayers and Westcot (1985, p.20) it is postulated for saline soils that plant water supply and plant available soil water are closely related to the total soil water potential (ΨT), which is the sum of the soil matric (ΨM) and soil osmotic (ΨO) water potential (ΨT = ΨM + ΨO). In the context of this paper it will be not the question that both types of soil water potentials usually (with the exception of some halophytes!) tend to reduce water uptake rates by roots, but it is doubted that both components of ΨT will affect root water uptake to the same degree, at least when root water uptake from the rhizospheric soil is evaluated.

Root water uptake of young wheat from a loamy soil dropped drastically, when the ΨT-value of the non-saline rhizospheric soil ranged around -0.2 to -0.3 MPa (ΨT = ΨM), but was just slightly affected when salinity (ΨO) was involved (Schleiff & Schaffer, 1984). Differences between soil osmotic and matric potentials on root water uptake seem to be even more distinct in sandy soils. Water uptake by barley roots from a non-saline sandy loam ceased at -0.2 MPa, but was still high at around -1.0 MPa, when ΨO dominated (Schleiff 1986). Same results were obtained with young rape plants cultivated in a sandy soil under well controlled conditions in a growth chamber as presented in Fig. 5 (Schleiff 1987). Under non-saline conditions root water uptake nearly ceased when ΨT dropped to -0.2 MPa, but decreased at the same ΨT only for 30-40% of the optimum, when salinity was involved. Working with a saline silt soil, Schmidhalter & Oertli (1991) confirmed for young carrots that decreasing ΨM affected plant growth much stronger than decreasing ΨO.

Using more sophisticated instruments such as microtensiometers for measuring soil matric water potentials in the rhizosphere (Vetterlein et al. 1993) and micro-cups to determine the osmotic water potential of the soil solution, Vetterlein & Jahn (2004) only recently checked the classical concept, too. They also doubt that ‘soil solution osmotic and matric potential are really additive in respect to plant water availability’. However there is one major shortcoming of their experimental approach, as the measuring range ceases at relatively high matric potentials (-0.07 to -0.08 kPa). Consequently data from the range of -0.08 to -0.5 MPa, where water uptake by roots is still significant, are missing. Unfortunately due to technical limitations it is not expected that this interesting approach will give a full and clear picture on water availability from saline soils.


Fig. 5: Root water uptake rate (ml/[g root-DM*h) of young rape plants in relation to the
total soil water potential of sandy soil

6. Discussion and Conclusions

There are certainly many cases where salt-induced lack of various nutrients (Ca, K, N, P and micronutrients) limit plant growth as reported by Schleiff & Finck (1976) and recently summarized by Bernstein and Kafkafi (2002). Fortunately technical tools to identify (soil chemical analysis combined with plant tissue analysis) and to compensate nutrient deficiencies are available (e.g. fertilizer applications). So it is not expected that the scientific progress in this field will bring significant contributions with respect to practical recommendations for managing saline irrigation.

On the other side our knowledge on processes happening in the contact zone between soil and plant under saline irrigation is very limited. There is certainly a justified feeling that the analysis of rapidly changing processes in the root environment might be extremely complex. Research in this field certainly faces severe problems with appropriate experimental set-ups, the direct measurement of important parameters (ions, soil water potential, root properties) and their evaluation with respect to crop salt tolerance. Nevertheless there is the expectation that the scientific analysis will bring large progress on our knowledge on crop salt tolerance, practical recommendations for saline irrigation and breeding of salt tolerant crops (Lieth et al. 1997).

As long as direct measurements of basic parameters are not able to cover the required range, indirect investigation methods such as the vegetation technique developed by Schleiff (1987a) are most promising to improve our knowledge on processes and effects in the rhizosphere. As presented under chapter 4 and 5 of this paper, the potential of this technique has given already clear and quantitative answers on the effect of soil matric and soil osmotic water potential on root water uptake and plant growth. From results presented in Fig.4 it is expected that the applied vegetation technique may also give quantitative data on root properties that are supposed to be relevant for crop salt tolerance. Modifications of the vegetation technique have to give answers on the contribution of interactions between the rhizospheric and non-rhizospheric soil volume on root water uptake. Following Helal & Schnug et al. (1996) the dynamic turnover of developing root systems, which affects nutrient uptake, should not be neglected for root water uptake from saline soils either.

The practical relevance of the concept on vertical salt distribution around roots for saline crop production, especially under high frequency irrigation, can be explained at hand of one interesting field experiment carried out by Pasternak & Malach (1986) with tomatoes. Yields of tomatoes under drip irrigation with water of 10dS/m were 20% higher in the 2-days irrigation cycle as compared to the daily cycle. This result is only surprising, when the average salinity of a soil profile is considered in the 1-layer and 4-layer concept as presented earlier in this paper (equation 3 and 5). Following this concept yields should be highest under daily irrigation, as frequent irrigations are generally expected to lower salinity stress to roots.

Obviously concepts based on the vertical salt distribution under saline irrigation were not appropriate to explain the higher salinity stress under frequent irrigation. In pot experiments with maize irrigated with water of 5.3dS/m at daily, 2- and 3-days cycle Schleiff (1983a) found similar results. The higher salinity stress for plants under daily irrigation was even confirmed by Chloride-contents of older and younger leaves, which were always highest under daily irrigation.

Remains the question why salinity stress seems to be higher under frequent irrigation, which is usually not the case at lower soil salinity. The explanation is that the lower water application rates given under frequent intervals are probably less effective to leach the very high salinity in the rhizospheric soil, whereas higher application rates at longer intervals may be more effective in this respect.

There is no doubt root environment under saline irrigation is complex and rapidly changing. Research in this field causes difficulties in experimental set-ups, measurement of parameters, making diagnosis and in predicting consequences for treatments. However there is also a real chance for a better understanding of plant growth under saline conditions, to improve irrigation management and to give ideas for breeding plants of higher salt tolerance.

7. References

Abrol I.P., Yadav J.S.P. and Massoud F.I., 1988: Salt-affected soils and their management. FAO Soils Bulletin 39, Rome

Ayers R.S. and Westcot D.W., 1985: Water quality for agriculture. Irrigation and drainage paper 29, FAO, Rome

Bernstein N. and Kafkafi U., 2002: Root growth under salinity stress. In: Plant roots – the hidden half. Ed. Waisel Y., Eshel A., Kafkafi U.; 3rd edition, Marcel Dekker, N.Y. – Basel - Hongkong

Boeer B. and Al Ghais S., 1999: A contribution towards the successful propagation of halophytes in harsh environmental conditions. In: Proceedings ‘Saline irrigation: halophyte production and utilization’; UNESCO/CIHEAM-IAMB, Agadir, Morocco

Dichtl N., 2002: Wiederverwertung von Abwasser. ATV-DVWK-Bundestagung 2002;
ISBN 3-936514-06-2; 239-253.

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