OBJECTIVE: Improved understanding of root water uptake from saline soils as
a base to optimize growth of crops irrigated with saline waters
INTRODUCTION: Experience with the application of
crop salt tolerance models developed for irrigated agriculture shows, that these models often may not predict crop growth satisfactory, especially when sophisticated irrigation technologies or frequent irrigation procedures are applied. Basically all models presently in use only consider the salinity of the soil solution in the average of one or several defined soil layers. They also take into consideration a vertical increase of the average soil salinity, but they do not consider that there is especially at arid growth conditions also a horizontal differentiation of soil solution salinization. As a result of physiological processes such as leaf transpiration, root water uptake and ion exclusion by roots there may occur a rapid separation between a highly saline soil solution close to the roots and a less saline solution at a distance from the root surface. We assume that a proper understanding of processes related to salt distribution around roots within irrigation cycles will essentially contribute to an improved mangement of saline waters resp. saline soils for crop production. Furthermore the concept will also give some stimulation for breeders to improve the salt tolerance of crops.
This concept will primarily consider those aspects of plant salt tolerance that occur below the soil surface in the rooted soil and thus are extremely difficult to verify. A further complication derives from the pretention to understand processes in the contact zone between roots and soil as one of the most important ‚missing links' to extend our present knowledge in the field of salt tolerance of irrigated plants.
PRINCIPLES ASPECTS OF THE CONCEPT:
The principle aspects of the presented concept of salt-dynamic processes in the rhizosphere of irrigated crops and their effects on the shoot water supply are outlined in the following Fig.1 entitled:
SALT-DYNAMIC PROCESSES IN THE RHIZOSPHERE OF IRRIGATED
CROPS AND THEIR EFFECTS ON THE SHOOT WATER SUPPLY
The left hand side of the sketched plant indicates the principal situation of a plant immediately after a water application, when the salt stress is relatively low:
the soil water content is high up to field capacity/saturation
the average salt concentration of the soil solution is relatively low
the soil solution close to the soil surface is less saline than in lower soil layers
the gradient between the salt concentrations of the soil solution in close vicinity of the roots and at a distance is relatively small
the roots/root hairs are well supplied with water thus having a large diameter
consequently the root water uptake rate is relatively high
as a result of a high root water uptake the water supply of the shoot/leaves is good
the water content of leaves and their turgor pressure are high
the salt concentration of the plant sap of leaves is relatively low, in older leaves often higher than in younger leaves
the conditions for high leaf assimilation/growth rates of the plants are good
The right hand side of the sketch indicates the principal situation of a plant at the end of a water depletion period resp. before the following water application, when the salt stress is relatively high:
the average soil water content is low at the end of a soil water depletion period
the average salt concentration of the soil solution is relatively high
there may exist a saline crust on the soil surface coming from evaporation
there exists a significant gradient between the salt concentrations of the soil solution in close vicinity of the roots (rhizosperic soil solution) and at a distance (nonrhizospheric soil solution)
the rooted soil volume has increased due to root growth
the water content of roots/root hairs is low and thus they have a smaller diameter
the root water uptake rate is low due to low soil water contents and very high salt concentrations of the rhizospheric soil solution
as a result of the low water uptake rates by the roots the water supply of the shoot/ leaves is critical
the water content of leaves and their turgor pressure are low, plants are wilting
younger leaves often wilt stronger than older leaves
the salt concentration of the plant sap of leaves is high
the growth conditions for the plant are unsatisfactory due to low water uptake
THE RHIZOCYLINDER IN RELATION TO THE SALT
TOLERANCE OF IRRIGATED CROPS
The following graphic will show properties of the root and the root surrounding soil that are assumed to be of great significance for differences in the salt tolerance of irrigated plants. The rhizocylinder is defined as that part of the rooted soil volume that is directly contacting the surface of the root or the root hairs.
The graphic considers the following properties at the soil/root-contact zone:
The following properties support the SALT-TOLERANCE of a crop:
The rhizocylinder of a root system is large, as the active root mass exists of many fine roots which are equipped with long root hairs. So a large part of a rooted soil layer is in direct contact with active roots.
Plants supplied with a deep rooting system enlarge their rooted soil volume and will exploit water also from deep soil layers.
During a water depletion period roots of salt tolerant plants may accumulate salts in their rhizosperic soil solution up to very high salt concentrations, which correspond osmotic potentials e.g. up to -2,0 to -2,5 Mpa (equals 20 to 25 bar). Their roots may resist such concentrations and still absorb enough water for survival. High water uptake rates are still obtained from rhizosperic soil solution with osmotic potentials in the range of -0,5 to -1,0 Mpa (5 - 10 bar), when roots of salt sensitive plants are already inactive.
The following properties support the SALT-SENSITIVITY of a crop:
The rhizocylinder of a root system is small, as the active root mass exists of few thick roots which are nearly free of root hairs. So just a small part of a rooted soil layer is in direct contact with active roots.
Plants supplied with a flat rooting system dispose of a smaller rooted soil volume and will only exploit water from the top soil layer directly.
During a water depletion period roots of salt sensitive plants may accumulate salts in their rhizosperic soil solution up to relatively low salt concentrations only, which correspond osmotic potentials e.g. up to -0,6 to -0,8 Mpa (equals 6 to 8 bar) only. Their roots may resist only low salt concentrations and not survive higher concentrations. High water uptake rates are only obtained from rhizosperic soil solution with osmotic potentials in the range of -0,1 to -0,3 Mpa (1 - 3 bar), when roots of salt sensitive plants are already inactive.
There is doubtless a need for further discussion and research to develop and verify the presented concept. In case of further interest please contact the author.