Uri Shani

A highly conductive drainage extension to control the lower boundary condition of lysimeters

Lysimeters are used to study and monitor water, fertilizers, salts and other contaminants and are particularly valuable in transpiration and evapotranspiration research. Saturation at the soil bottom boundary in a lysimeter is inherent to its design. A drainage extension made of porous media with high hydraulic conductivity and substantial water holding capacity was devised to extend the lysimeter in order to produce soil moisture conditions mimicking those in the field. Design criteria that assure equal discharge in the soil and in the highly conductive drain (HCD) were established and formulated. Desired matric head at the lysimeter base is determined by HCD extension length. Its value can be manipulated and can range between saturation and the soils field capacity. Conditions where the HCD is not limiting to flow are obtained through selection of the appropriate cross sectional area ratio between the soil in the lysimeter and the HCD. The validity of these criteria was confirmed with 200 l working lysimeters in the field, with and without plants, and with detailed flow tests utilizing smaller (15 l) lysimeters. Comparison of computed and measured matric head and leachate volume indicates that the proposed method can serve to maintain conditions similar to those in the field.

Numerical investigation of irrigation scheduling based on soil water status

Improving the sustainability of irrigation systems requires the optimization of operational parameters such as irrigation threshold and irrigation amount. Numerical modeling is a fast and accurate means to optimize such operational parameters. However, little work has been carried out to investigate the relationship between irrigation scheduling, irrigation threshold, and irrigation amount. Herein, we compare the results of HYDRUS 2D/3D simulations with experimental data from triggered drip irrigation, and optimize operational parameters. Two field experiments were conducted, one on loamy sand soil and one on sandy loam soil, to evaluate the overall effects of different potential transpiration rates and irrigation management strategies, on the triggered irrigation system. In both experiments, irrigation was controlled by a closed loop irrigation system linked to tensiometers. Collected experimental data were analyzed and compared with HYDRUS 2D/3D simulations. A system-dependant boundary condition, which initiates irrigation whenever the matric head at a predetermined location drops below a certain threshold, was implemented into the code. The experimental model was used to evaluate collected experimental data, and then to optimize the operational parameters for two hypothetical soils. The results show that HYDRUS 2D/3D predictions of irrigation events and matric heads are in good agreement with experimental data, and that the code can be used to optimize irrigation thresholds and water amounts applied in an irrigation episode to increase the efficiency of water use.

Subsurface Drip Irrigation in Gravel-Filled Cavities

Subsurface drip irrigation (SDI) is regularly used to provide water and nutrients to plants while maintaining a dry soil surface. Problems associated with the practice of SDI are spatially dependent reductions in dripper discharge and possible surfacing of water resulting from positive pressure at the emitter–soil interface. These can be resolved either through prudent care in matching dripper flow rates to soil hydraulic properties or by otherwise providing conditions under which positive pressure cannot arise. We present a method where water is applied to the soil within a gravel-filled cavity. The necessary volume of gravel is determined by the contact area between the cavity and the soil and is a function of irrigation rates, dripper spacing, and soil hydraulic properties. A theoretical solution for the radius of a gravel-filled cavity based on the perimeter of the saturated zone from a line source in the soil demonstrates that larger cavities are needed as soil hydraulic conductivity decreases. The method was tested using a numeric simulation model (HYDRUS-2D) and was used and tested in a vineyard of table grapes ( Vitis Vinifera L. cv. Sugraone) in a 7-yr study with SDI and gravel-trenched subsurface application of effluent and fertilizers.

Water use and yield of tomatoes under limited water and excess boron

Boron is essential to growth at low concentrations and limits growth and yield when in excess. Little is known about plant response to excess boron (B) and water stress occurring simultaneously. The influences of B and water supply on tomatoes (Lycopersicon esculentumMill.) were investigated in lysimeters. Water application levels were 30, 60, 100, 130 and 160% of potential evapotranspiration. Boron levels in irrigation water were 0.02, 0.37, and 0.74 mol m−3. Conditions of excess boron and of water deficits were found to decrease yield and transpiration of tomatoes. Simultaneous B and drought stresses did not result in a larger effect but rather, one or the other stress-causing factor was found to be dominant in plant response. Both irrigation water quantity and boron concentration influenced water use of the plants in the same manner as they influenced yield. A dominant-stress-factor model following the Liebig-Sprengel law of the minimum was assumed and validated. The model applies the principle that, when a plant is submitted to conditions of stress caused by B in conjunction with water and/or salinity stress, the most severe stress determines yield.

Sweet corn response to combined nitrogen and salinity environmental stresses

To define the nature of the combined response curve of sweet corn (Zea mays L.) plants to nitrogen and salinity, a lysimeter study was designed to follow water and solute budgets with combinations of the two variables over wide ranges of 0.5–7.5 dS m−1 and 0–150% of local N-fertilization recommendations. Patterns of water-use efficiency, N content, N uptake, and shoot dry-matter yield indicated the predominance of environmental interactions over Cl-nitrate physiological antagonism. At low salinities, the leaf N content, N uptake, and yield increased with increased N fertilization up to 45% of local N-fertilization recommendations, nitrogen was efficiently stripped from the percolating water and practically no nitrate was leached. At higher N fertilization the amount of leached N increased linearly with increased N input, and N uptake and yield were independent of N rates, levelling off at increased values for decreased salinities. The Liebig–Sprengel and Mitscherlich–Baule models were evaluated against measured data; both achieved similar values for the systems inherent N, the salinity level corresponding with zero-yield, and the predicted yields, which were highly correlated with the experimental data (R2 > 0.9). It is suggested that both models can be used successfully in mechanistic-based plant–soil solution models to predict yield, water and nutrient needs, and the resulted N leaching.

Temporal robustness of linear relationships between production and transpiration

Seasonal dependence of biomass production on transpiration has been previously reported for a number of crops under salinity and drought. Linear yield (Y) to transpiration (T) relationships have been utilized in plant-growth and water-uptake models to estimate yield based on predicted transpiration values. The relationship is often employed for time steps that are very small compared with the whole season measurements, even though no empirical validation exists for such application. This work tests the hypothesis that linear Y-T relationships are valid throughout the life span of crops under varied natural conditions and levels of environmental stress. Effects of salinity and water supply on growth, water use and yields of tomatoes (Lycopersicon esculentum Mill.) were studied for two distinct conditions of potential transpiration. Linear relationships between relative Y and relative ET were found to be consistent throughout the life span of the crops for both growing seasons. Water-use efficiency increased together with plant growth as a result of changes in the plants surface area to volume ratio. This empirical validation of linear Y-T relationships for short time periods is beneficial in confirming their usefulness in growth and water uptake models.

Modeling Plant Response to Drought and Salt Stress: Reformulation of the Root-Sink Term

Because transpiration is often the largest component of the water budget in arid systems, the efficacy of computer simulation models as predictors of water and salt movement is predicated on their ability to predict transpiration. The objective of this study was to improve the root-sink term for water extraction and thus improve predictions of transpiration. The root-sink terms often use either a transpiration-apportioning (TA) empirical function for the plant response to the soil matric and osmotic potential or a potential-flow (PF) function. Three root-sink terms, a TA formulation, a PF formulation, and a combination of the two (PFTA, a TA function for computing water uptake in response to salinity and a PF function for computing water uptake in response to water availability) were coded into a simulation model, and model predictions were compared with field-collected data. When predicted and measured relative yields (yield/yield max ) were compared, the PF produced the poorest agreement with data ( y = 0.8741 x + 0.0251), the TA gave better agreement ( y = 0.9283 x + 0.0476), and PFTA provided the best agreement ( y = 0.9909 x + 0.0430). The plant and plant–soil based formulation predicted the salinity profile at the end of the field experiment under conditions of high salinity (EC irrigation water = 6.0 dS m −1 ) and irrigation equal to potential evaporation. The plant–soil formulation was the better predictor under the same salinity condition with irrigation at 40% of potential evaporation. The simulated evolution of the water content and salinity profile across time was also examined.

Root Water Uptake Efficiency Under Ultra-High Irrigation Frequency

We investigated the hypothesis that continuous water application allows favorable and steady water content and hydraulic conductivity in the root zone, thus enabling higher water potential in the soil–root interface (ψroot). Elevated ψroot increases transpiration (T) and prevents yield loss due to stomatal closure or to low root osmotic potential that develops in response to low ψroot. We assume further, that the advantage of continuous water application is more pronounced for young plants, where water uptake per root length and competition on resources in the root system is higher. We investigated this hypothesis by examining the average water content of the root zone and T as a function of time for sunflowers grown under varied irrigation frequencies experimentally and in a modeled simulations, and by solving for the necessary effective root length and ψroot for each case. High frequency water application was shown to positively affect root water uptake efficiency and yield, especially when plants were young. Irrigation frequency affected growth through the water content in the bulk soil (θsoil) which in turn affects ψroot. A low θsoil and coupled low hydraulic conductivity decreased T and yield. Moreover, a decreased θsoil caused low ψroot, inefficient allocation of energy and carbohydrates and eventual yield loss. It was likely that these phenomena were more pronounced with young plants due to higher water uptake per root length.

Salt stress aggravates boron toxicity symptoms in banana leaves by impairing guttation

Boron (B) is known to accumulate in the leaf margins of different plant species, arguably a passive consequence of enhanced transpiration at the ends of the vascular system. However, transpiration rate is not the only factor affecting ion distribution. We examine an alternative hypothesis, suggesting the participation of the leaf bundle sheath in controlling radial water and solute transport from the xylem to the mesophyll in analogy to the root endodermis. In banana, excess B that remains confined to the vascular system is effectively disposed of via dissolution in the guttation fluid; therefore, impairing guttation should aggravate B damage to the leaf margins. Banana plants were subjected to increasing B concentrations. Guttation rates were manipulated by imposing a moderate osmotic stress. Guttation fluid was collected and analysed continuously. The distribution of ions across the lamina was determined. Impairing guttation indeed led to increased B damage to the leaf margins. The kinetics of ion concentration in guttation samples revealed major differences between ion species, corresponding to their distribution in the lamina dry matter. We provide evidence that the distribution pattern of B and other ions across banana leaves depends on active filtration of the transpiration stream and on guttation.

Response of Melon Plants to Salt: 2. Modulation by Root Growth Temperature — the Role of Root Membrane Properties

The proposition that the physical state of root membranes is critical in plant response to salinity was tested by utilizing temperature as a physical modulator of fluidity..Simultaneous studies of melon (Cucumis melo L.) seedling morphology and root membrane properties as a function of root growth temperature, alone and in combination with salinity, were carried out. Melon seedlings were grown hydroponically in the absence (control) and presence (salt-grown) of 100 mM NaCl. The growth media were set at various temperatures ranging from 20 to 30 o C. Growth of salt-grown seedlings was considerably inhibited at 20 o C. At higher temperatures growth was enhanced more in the salt-treatment than in the control. Consequently, the morphological differences between control and salt-grown seedlings, which were substantial at 20 o C, were diminished at 30 o C. In control seedlings, chemical analysis of isolated root membranes revealed that the growth temperature affected their composition, primarily the content of lipids. As a result, the weight ratio of lipid to protein (L/P) in the membranes was lower at higher temperatures. A parallel change in membrane fluidity (the inverse of viscosity) was measured by fluorescence de-polarization techniques, suggesting that the changes in membrane L/P Were related to the plant mechanism of «homeoviscous adaptation», i.e. maintenance of a constant membrane viscosity. In salt-grown seedlings, the effect of temperature was primarily on the membrane protein content. Membrane L/P decreased between 20 and 25 o C, but did not change significantly above 25 o C. A comparison between root membranes from control and salt-grorvn seedlings disclosed that at 20 and 25 o C L/P values were significantly higher in the control. However, the magnitude of the difference decreased as the growth temperature increased, and at 30 o C it was practically null. Thus, raising the temperature from 20 to 30 o C resulted in smaller effects of salinity on both growth and membrane composition. Further support for the role of membrane L/P in the plant response to salt came from experiments with control seedlings of different root-membrane L/P. The different seedling groups, produced by growth at various temperatures, were potted, transferred to the greenhouse (27/17 o C, day/night temperatures) and tested for their salt sensitivity by irrigation with salt containing medium. Seedlings pre-grown at the lowest temperature, thus, having the highest root membrane L/P, were the most tolerant to salt treatment. The results substantiate the prominent role of root membranes in the response of melon seedlings to the growth temperature alone and in combination with salinity. Furthermore, they support the proposition that membrane composition, in particular L/P, plays an important role in salt-tolerance, probably due to its effect on membrane physical properties