«Analysis of Water Supply of Plants under Saline Soils Conditions and Conclusions for. Analysis of Water Supply of Plants under Saline Soils ...»
Schleiff U, 2006: Analysis of Water Supply of Plants under Saline Soils Conditions and Conclusions for….
Analysis of Water Supply of Plants under Saline Soils Conditions
and Conclusions for Research on Crop Salt Tolerance
Independent Expert for Irrigation&Salinity – Fertilizers&Crops – Soils&Environment
http://www.salinity.de; D-38289 Wolfenbuettel; P.O.Box 1934; firstname.lastname@example.org
Abstract In the dry areas of the world there is an increasing pressure to apply low quality brackish waters for plant irrigation (agriculture, horticulture, landscape greening). Consequently there is a demand to improve salt tolerance of conventional crops and to develop adequate irrigation techniques too. The efforts in the past decades to approach the understanding of salt stress mechanism by focussing on biochemical and physiological research were disappointing with respect to progress for crop growth and yields under saline soil conditions. Meanwhile it is generally agreed by all disciplines involved in research for crop salt tolerance that under saline soils conditions the reduced water supply of crops is the most critical growth factor. The paper presents some model calculations and field investigations that demonstrate the effect of root water uptake on the salinity of the root surrounding soil fraction (rhizospheric soil). It is shown that root hair length and rhizospheric soil volumes are factors most relevant for understanding crop salt tolerance, when growing in soils. It is postulated that short root hairs contribute to a lower salt tolerance (onions), whereas long root hairs improve the water uptake from saline soils and crop salt tolerance (rape). As interactions between roots and soil contribute to the salt tolerance of crops under field conditions, it is doubtful that selection for salt tolerant varieties and breeding for salt tolerance under conditions of water and flow culture experiments is very efficient. Breeding for more salt tolerant crops should consider root morphology too.
Key words: brackish irrigation, root morphology, rhizosphere, water uptake, breeding for crop salt tolerance, soil salinity Zusammenfassung Analyse der Wasserversorgung von Pflanzen aus versalzten Böden und Schlussfolgerungnen für die Forschung zur Salztoleranz In den Trockenzonen der Erde nimmt der Zwang zu, auch Brackwasser für die Bewässerung in Landwirtschaft, Garten- und Landschaftsbau zu verwenden. Damit ist auch der Bedarf vorhanden, die Salztoleranz von Kulturpflanzen zu verbessern und angepasste Bewässerungsverfahren zu entwickeln. Die Anstrengungen der vergangenen Jahre, durch Konzentration der Forschung auf biochemische und physiologische Prozesse zu einem vertieften Verständnisses der Salztoleranz zu gelangen, das auch zu Fortschritten unter Feldbedingungen führt, waren enttäuschend. Inzwischen gibt esKonsens zwischen allen Fachgebieten, die sich mit der Salztoleranz von Pflanzen befassen, dass in versalzten Böden die Wasserversorgung der Pflanzen der kritischste Wachstumsfaktor ist. Es werden Modellrechnungen und Felduntersuchungen vorgestellt, die die Wirkung der Wasseraufnahme durch Wurzeln auf die Versalzung des Wurzel umgebenden Bodens und der Bodenlösung zeigen. Es wird gezeigt, dass Wurzelhaarlänge und Volumen des Wurzelhaarzylinders Faktoren sind, die sehr bedeutend sein können für ein vertieftes Verständnis der Salztoleranz von Pflanzen unter Feldbedingungen. Es wird postuliert, dass kurze Wurzelhaare zu einer geringen Salztoleranz Intern. Confer. on ‘Sustainable Crop Production on Salt-Affected Land’; Dec.2006, Faisalabad/Pakistan von Pflanzen beitragen (Zwiebeln), während lange Wurzelhaare (Raps) die Voraussetzungen für die Wasseraufnahme von Wurzeln aus versalzten Böden verbessern und die Salztoleranz erhöhen. Da Wechselwirkungen zwischen Wurzel und Boden wesentlich zur Salztoleranz unter Feldbedingungen beitragen können, erscheint es fragwürdig, wenn die Selektion und Züchtung salztoleranter Pflanzen in Wasserkulturen und ähnlichen Techniken erfolgt. Die Züchtung salztoleranter Pflanzen sollte den Aspekt der Wurzelmorphologie nicht vernachlässigen.
Schlüsselwörter: Bewässerung mit Brackwasser, Wurzelmorphologie, Rhizosphäre, Wasseraufnahme, Züchtung salztoleranter Pflanzen, Bodenversalzung
1. Introduction In the dry areas of the world there is an increasing pressure on agriculture, horticulture and landscape greening to replace good quality irrigation waters by waters of lower qualities such as brackish, drainage or treated wastewaters. The application of lower qualities water often causes salinization of soils, which may affect growth of many crops depending from there salt tolerance (Rhoades et al, 1992). Consequently there is a general demand to improve crop salt tolerance in order to maintain yield levels and plant growth even at increasing levels of soil salinity.
In the past decades the focus of research to elevate salt tolerance of plants mainly referred to biochemical and physiological aspects (Koyro and Huchzermeyer, 1999). Genes responsible for salt tolerance of some crops (e.g. soybean, tomatoes, grasses, rice) have been identified (Jaradat, 1999). However, in spite of the large efforts put into the understanding of biochemical and physiological processes in plants grown under saline conditions results are disappointing with respect to their relevance for crop yields under brackish or saline agriculture (Jones 2006; Mengel and Kirkby 2001).
The presented concept for further progress of research on crop salt tolerance under irrigation analysis the internationally applied standard procedure for rating of crop salt tolerance, which was developed by USDA and proposed by FAO (Ayers and Westcot, 1985). This concept considers the upward and downward movements of water and salts in irrigated soils. No attention has been given to the fact that within irrigation cycles between two water applications the lateral movement of water and salts from the bulk soil to the root surface dominates.
Based on the assumption that under saline soils conditions usually plant water supply is the most critical growth factor the conditions for water uptake by roots is given special attention.
2. Present Concept for Rating of Crop Salt Tolerance in Agriculture In agronomy the term ‘crop salt tolerance’ is defined as the relative crop yield (Y in %) at a given soil salinity as compared to the yield of a non-saline soil. Soil salinity refers to the average salinity of the rooted soil layer and is expressed as the electrical conductivity (EC) of the soil saturation extract, the ECe. For a specific crop the relative crop yield is calculated
according to the following equation (Maas and Hoffmann, 1977):
(1) Y = 100 – b (ECe – a)
‘b’ = the yield loss in % per unit increase of soil salinity ECe (1 dSm-1) (ranging from 3%/[dS/m] for salt tolerant to 30%/[dS/m] for sensitive crops) Schleiff U, 2006: Analysis of Water Supply of Plants under Saline Soils Conditions and Conclusions for….
‘a’ = soil salinity threshold value: ECe, where yield decrease starts (ranging from 1.5 dS/m for sensitive to 10 dS/m for salt tolerant crops) Both values ‘a’ and ‘b’ are crop specific and were published by Maas & Hoffmann (1977) and Ayers & Westcot (1985). This approach is often helpful to compare the relative advantage of crops or cropping patterns.
Based on the equation (1) crops are classified into groups of different salt tolerance as presented in Fig.1: sensitive (S), moderately sensitive (MS), moderately tolerant (MT), tolerant (T) and very tolerant (VT) halophytes. When the ECe exceeds a range of 40 to 45 dS/m, soil salinity is even too high for halophytes.
The principles of this soil salinity based concept for rating the crop salt tolerance is presented on the left hand side of Fig.2. According to this concept the relative root zone is divided into 4 layers of equal depth (0-25%, 25-50%, 50-75%, 75-100%). The different layers contribute different amounts of water to the water requirement of the crop (ET = evapotranspiration): the top layer (0-25% soil depth) contributes 40% (= 0.4 ET), the 2nd layer (25-50%) 30% (= 0.3 ET), the 3rd layer (50-75%) 20% (= 0.2 ET) and the bottom layer (75-100%) 10% (= 0.1 ET) to the total evapotranspiration of the crop. Consequently the amount of soil water available for leaching of salts (LF = leaching fraction) from the upper layers downwards the soil profile into the non rooted subsoil decreases with soil depth (LF = ECi/ECe: LF0 LF1 LF2 LF3 LF4). The decreasing amounts of leaching water result in a soil water salinity (ECsw) that continuously increases with soil depth (ECsw0 ECsw1 ECsw2 ECsw3 ECsw4). The rating of salt tolerance for all crops considers the average salinity (ECe) of the total soil layer.
Intern. Confer. on ‘Sustainable Crop Production on Salt-Affected Land’; Dec.2006, Faisalabad/Pakistan More details on the relevant calculation procedures are already published (Ayers et al 1985;
Fig. 2: Principles of the soil based concept of FAO for rating of crop salt tolerance and development of this concept (see last page) The right hand side of Fig.2 indicates an aspect that is neglected by this concept, but is assumed to be of great importance for our understanding of crop salt tolerance. The FAOconcept does not take into consideration that the lateral movement of salts and water in soils that dominates between 2 succeeding water applications, could play a prominent role for plant growth conditions in conjunction with morphologic properties of roots (Schleiff 1982a).
Driven from the transpiration of the shoot saline soil solution is moving from the bulk soil to the root surface, where water uptake occurs, but most ions are excluded from root uptake. As this aspect of ion dynamic in the root surrounding soil during irrigation cycles has been neglected in the past, we will return to the principles of these dynamic processes between roots and soil under brackish irrigation in chapter 4 after summarizing recent research activities related to crop salt tolerance.
3 Overview on Main Research Activities Related to Crop Salt Tolerance Research activities related to crop salt tolerance focus on a deepened understanding of plant and soil factors that may be relevant for the limitations of plant growth under saline conditions. Based on results from this research it is our hope to find strategies that finally will elevate yields of crops growing on salt affected fields. As plant salt tolerance under field conditions is recognized as an extremely complex affair, many disciplines are involved in salinity research and contribute many interesting details.
Table 1 lists some scientific topics that have contributed in the past decades to achieve a better understanding of salt tolerance. The overview is sub-divided into for research activities focussing on the shoot, the root and the bulk soil. Research on shoots has focussed at the level of the whole plant, the organelles and at molecular level (Hu and Schmidhalter, 2004; Ashraf, 2004). Many interesting aspects were investigated, only some are mentioned here. Interesting results have been obtained on the contribution of roots, the reduction of nutrient and water uptake, the exclusion of toxic ions and many other details.
Research on soils has been mostly orientated to farmers need such as relation between soil solution salinity and crop growth, ion specific effects on soils and plants, long term effects of water qualities on soil fertility parameters and other aspects. In this context it should be stressed that there is no need for a further debate on the antiquated concept of Wadleigh and Ayers (1946) concerning the additive effect of the soil matric and osmotic water potentials on water supply of plants. Experiments with various crops and soils have clearly demonstrated that the additivity of the water potentials on the plant water supply and plant growth cannot be considered as a general rule, but there are specific combinations of soils and plants, where the matric and osmotic water potential may affect plant growth additive (Schleiff, 1984 – 1987, 2005).
The approach generally applied by agronomists, irrigation engineers or plant breeders to calculate the effective soil solution salinity roots are exposed to at decreasing soil water contents between water applications is based on the average soil salinity and water contents of a specific soil layer. This approach does not consider aspects concerning interactions between Schleiff U, 2006: Analysis of Water Supply of Plants under Saline Soils Conditions and Conclusions for….
root and soil at the soil/root interface. Riley and Barber (1970) were the first, who observed in greenhouse experiments with soybeans that after a period of water depletion salinity of a Table 1: Research topics of various disciplines related to soil salinity and crop salt tolerance
Intern. Confer. on ‘Sustainable Crop Production on Salt-Affected Land’; Dec.2006, Faisalabad/Pakistan rooted soil layer is far from being a relatively homogeneous affair. They found that there had developed a steep salinity gradient between the bulk soil (far from roots) and the soil close to the roots (rhizospheric soil). The soil fraction close to the root surface was up to 15times more saline than the bulk soil. It is hardly to explain, why during the past 3 decades the international scientific community involved into research concepts for brackish irrigation or breeding of more salt tolerant crops did not appreciate this basic observation and overlooked the potential of this significant innovation.
Cl-contents of soils in meq/100g