Rony Wallach

Improving plant stress tolerance and yield production: is the tonoplast aquaporin SlTIP2;2 a key to isohydric to anisohydric conversion?

Anisohydric plants are thought to be more drought tolerant than isohydric plants. However, the molecular mechanism determining whether the plant water potential during the day remains constant or not regardless of the evaporative demand (isohydric vs anisohydric plant) is not known. Here, it was hypothesized that aquaporins take part in this molecular mechanism determining the plant isohydric threshold. Using computational mining a key tonoplast aquaporin, tonoplast intrinsic protein 2;2 (SlTIP2;2), was selected within the large multifunctional gene family of tomato (Solanum lycopersicum) aquaporins based on its induction in response to abiotic stresses. SlTIP2;2-transformed plants (TOM-SlTIP2;2) were compared with controls in physiological assays at cellular and whole-plant levels. Constitutive expression of SlTIP2;2 increased the osmotic water permeability of the cell and whole-plant transpiration. Under drought, these plants transpired more and for longer periods than control plants, reaching a lower relative water content, a behavior characterizing anisohydric plants. In 3-yr consecutive commercial glasshouse trials, TOM-SlTIP2;2 showed significant increases in fruit yield, harvest index and plant mass relative to the control under both normal and water-stress conditions. In conclusion, it is proposed that the regulation mechanism controlling tonoplast water permeability might have a role in determining the whole-plant ishohydric threshold, and thus its abiotic stress tolerance.

The Role of Tobacco Aquaporin1 in Improving Water Use Efficiency, Hydraulic Conductivity, and Yield Production Under Salt Stress

Tobacco (Nicotiana tabacum; C3) plants increase their water use efficiency (WUE) under abiotic stress and are suggested to show characteristics of C4 photosynthesis in stems, petioles, and transmitting tract cells. The tobacco stress-induced Aquaporin1 (NtAQP1) functions as both water and CO2 channel. In tobacco plants, overexpression of NtAQP1 increases leaf net photosynthesis (AN), mesophyll CO2 conductance, and stomatal conductance, whereas its silencing reduces root hydraulic conductivity (Lp). Nevertheless, interaction between NtAQP1 leaf and root activities and its impact on plant WUE and productivity under normal and stress conditions have never been suggested. Thus, the aim of this study was to suggest a role for NtAQP1 in plant WUE, stress resistance, and productivity. Expressing NtAQP1 in tomato (Solanum lycopersicum) plants (TOM-NtAQP1) resulted in higher stomatal conductance, whole-plant transpiration, and AN under all conditions tested. In contrast to controls, where, under salt stress, Lp decreased more than 3-fold, TOM-NtAQP1 plants, similar to maize (Zea mays; C4) plants, did not reduce Lp dramatically (only by approximately 40%). Reciprocal grafting provided novel evidence for NtAQP1s role in preventing hydraulic failure and maintaining the whole-plant transpiration rate. Our results revealed independent, albeit closely related, NtAQP1 activities in roots and leaves. This dual activity, which increases the plants water use and AN under optimal and stress conditions, resulted in improved WUE. Consequently, it contributed to the plants stress resistance in terms of yield production under all tested conditions, as demonstrated in both tomato and Arabidopsis (Arabidopsis thaliana) plants constitutively expressing NtAQP1. The putative involvement of NtAQP1 in tobaccos C4-like photosynthesis characteristics is discussed.

High fertigation frequency: the effects on uptake of nutrients, water and plant growth

The objective of the present research was to explore the effects of combined irrigation and fertilization (fertigation) frequency on growth, yield and uptake of water and nutritional elements by plants. Lettuce (Lactuca sativa L., cv. Iceberg) was used as the model plant. Two experiments were conducted in a screen-house: compound fertilizer at a constant N:P:K ratio at different concentrations was used in the first, while in the second the concentration of P varied solely while the concentration of the other nutritional elements was kept constant. The lettuce was planted in pots filled with perlite and irrigated daily with a constant volume of nutrient solution at different frequencies. The major finding in the two experiments was that high fertigation frequency induced a significant increase in yield, mainly at low nutrients concentration level. Yield improvement was primarily related to enhancement of nutrient uptake, especially P. It was suggested that the yield reduction obtained at low frequency resulted from nutrient deficiency, rather than water shortage, and that high irrigation frequency can compensate for nutrient deficiency. Frequent fertigation improved the uptake of nutrients through two main mechanisms: continuous replenishment of nutrients in the depletion zone at the vicinity of root interface and enhanced transport of dissolved nutrients by mass flow, due to the higher averaged water content in the medium. As such, an increase in fertigation frequency enables to reduce the concentrations of immobile elements such as P, K and trace metals in irrigation water, and to lessen the environment pollution by discharge.

Hexokinase mediates stomatal closure

Stomata, composed of two guard cells, are the gates whose controlled movement allows the plant to balance the demand for CO2 for photosynthesis with the loss of water through transpiration. Increased guard-cell osmolarity leads to the opening of the stomata and decreased osmolarity causes the stomata to close. The role of sugars in the regulation of stomata is not yet clear. In this study, we examined the role of hexokinase (HXK), a sugar-phosphorylating enzyme involved in sugar-sensing, in guard cells and its effect on stomatal aperture. We show here that increased expression of HXK in guard cells accelerates stomatal closure. We further show that this closure is induced by sugar and is mediated by abscisic acid. These findings support the existence of a feedback-inhibition mechanism that is mediated by a product of photosynthesis, namely sucrose. When the rate of sucrose production exceeds the rate at which sucrose is loaded into the phloem, the surplus sucrose is carried toward the stomata by the transpiration stream and stimulates stomatal closure via HXK, thereby preventing the loss of precious water.

Role of aquaporins in determining transpiration and photosynthesis in water-stressed plants: crop water-use efficiency, growth and yield

The global shortage of fresh water is one of our most severe agricultural problems, leading to dry and saline lands that reduce plant growth and crop yield. Here we review recent work highlighting the molecular mechanisms allowing some plant species and genotypes to maintain productivity under water stress conditions, and suggest molecular modifications to equip plants for greater production in water-limited environments. Aquaporins (AQPs) are thought to be the main transporters of water, small and uncharged solutes, and CO2 through plant cell membranes, thus linking leaf CO2 uptake from the intercellular airspaces to the chloroplast with water loss pathways. AQPs appear to play a role in regulating dynamic changes of root, stem and leaf hydraulic conductivity, especially in response to environmental changes, opening the door to using AQP expression to regulate plant water-use efficiency. We highlight the role of vascular AQPs in regulating leaf hydraulic conductivity and raise questions regarding their role (as well as tonoplast AQPs) in determining the plant isohydric threshold, growth rate, fruit yield production and harvest index. The tissue- or cell-specific expression of AQPs is discussed as a tool to increase yield relative to control plants under both normal and water-stressed conditions.

Role of aquaporins in determining transpiration and photosynthesis in water-stressed plants

The global shortage of fresh water is one of our most severe agricultural problems, leading to dry and saline lands that reduce plant growth and crop yield. Here we review recent work highlighting the molecular mechanisms allowing some plant species and genotypes to maintain productivity under water stress conditions, and suggest molecular modifications to equip plants for greater production in water-limited environments. Aquaporins (AQPs) are thought to be the main transporters of water, small and uncharged solutes, and CO2 through plant cell membranes, thus linking leaf CO2 uptake from the intercellular airspaces to the chloroplast with water loss pathways. AQPs appear to play a role in regulating dynamic changes of root, stem and leaf hydraulic conductivity, especially in response to environmental changes, opening the door to using AQP expression to regulate plant water-use efficiency. We highlight the role of vascular AQPs in regulating leaf hydraulic conductivity and raise questions regarding their role (as well as tonoplast AQPs) in determining the plant isohydric threshold, growth rate, fruit yield production and harvest index. The tissue- or cell-specific expression of AQPs is discussed as a tool to increase yield relative to control plants under both normal and water-stressed conditions.

Hydraulic properties of sphagnum peat moss and tuff (scoria) and their potential effects on water availability

The potential rate of water and nutrient supply to plant roots depends on the hydraulic properties of the container medium (growth medium, substrate), primarily on its unsaturated hydraulic conductivity, which is a measure of the mediums resistance to water flow. Water availability to plants grown in containers is usually being evaluated using criteria based exclusively on water characteristic curves of the medium in which the plant is grown. This approach is challenged in the present paper. We hypothise that the coarse structure of peat moss as well as of other container media may result in a sharp decrease in hydraulic conductivity, as the water content of peat is reduced. Transient changes in unsaturated hydraulic conductivity may result in reduced water uptake by plant roots. The objectives of this research were to determine the hydraulic properties of sphagnum peat moss and to evaluate their potential effects on water availability. Tuff (granulated volcanic ash) and its mix with peat were also tested for comparison. Water characteristic curves (drying and wetting cycles) and saturated hydraulic conductivity were measured. A predictive mathematical model was used to calculate the unsaturated hydraulic conductivity of the media. Measured water retention and saturated hydraulic conductivity data were used to estimate model parameters by a nonlinear least-squares curve-fitting technique. Model predictions of unsaturated hydraulic conductivity were validated by direct measurements. Results showed that sharp variations in hydraulic conductivity occur in a very narrow suction range (0–2.5 kPa). In this range a decrease of more than three orders of magnitude in the unsaturated hydraulic conductivity was observed for peat. A similar trend was observed for the other media tested. This suggests that the approach that has been commonly used for determinations of water availability and for irrigation scheduling in container media may provide inaccurate predictions as to potential plant response.

The errors in surface runoff prediction by neglecting the relationship between infiltration rate and overland flow depth

Abstract Many simplifications are used in modeling surface runoff over a uniform slope. A very common simplification is to determine the infiltration rate independent of the overland flow depth and to combine it afterward with the kinematic-wave equation to determine the overland flow depth. Another simplication is to replace the spatially variable infiltration rates along the slope i ( x , t ) due to the water depth variations h ( x , t ) with an infiltration rate that is determined at a certain location along the slope. The aim of this study is to evaluate the errors induced by these simplications on predicted infiltration rates, overland flow depths, and total runoff volume. The error analysis is accomplished by comparing a simplified model with a model where the interaction between the overland flow depth and infiltration rate is counted. In this model, the infiltration rate is assumed to vary along the slope with the overland flow depth, even for homogeneous soil profiles. The kinematic-wave equation with interactive infiltration rate, calculated along the slopy by Richards equation, are then solved by a finite difference scheme for a 100-m-long uniform slope. In the first error analysis, we study the effect of combining an ‘exact’ and ‘approximate’ one-dimensional infiltration rate with the kinematic-wave equation for three different soil surface roughness coefficients. The terms ‘exact’ and ‘approximate’ stand for the solution of Richards equation with and without using the overland flow depth in the boundary condition, respectively. The simulations showed that higher infiltration rates and lower overland flow depths are obtained during the rising stage of the hydrograph when overland flow depth is used in the upper boundary condition of the one-dimensional Richards equation. During the recession period, the simplified model predicts lower infiltration rates and higher overland flow depths. The absolute relative errors between the ‘exact’ and ‘approximate’ solutions are positively correlated to the overland flow depths which increase with the soil surface roughness coefficient. For this error analysis, the relative errors in surface runoff volume per unit slope width throughout the storm are much smaller than the relative errors in momentary overland flow depths and discharges due to the alternate signs of the deviations along the rising and falling stages. In the second error analysis, when the spatially variable infiltration rate along the slope i ( x , t ) is replaced in the kinematic-wave equation by i ( t ), calculated at the slope outlet, the overland flow depth is underestimated during the rising stage of the hydrograph and overestimated during the falling stage. The deviations during the rising stage are much smaller than the deviations during the falling stage, but they are of a longer duration. This occurs because the solution with i ( x , t ) recognizes that part of the slope becomes dry after rainfall stops, while overland flow still exists with i ( t ) determined at the slope outlet. As obtained for the first error analysis, the relative errors in surface runoff volume per unit slope width are also much smaller than the relative errors in momentary overland flow depths and discharges. The relation between the errors in overland flow depth and discharge to different mathematical simplifications enables to evaluate whether certain simplifications are justified or more computational efforts should be used.

UNSATURATED HYDRAULIC CHARACTERISTICS OF COMPOSTED AGRICULTURAL WASTES, TUFF, AND THEIR MIXTURES

For effective management of irrigation and fertilization in container media, a complete understanding of their hydraulic properties is essential. This study was conducted to test the applicability of an existing predictive hydraulic model to the calculation of a reliable unsaturated hydraulic conductivity function for compost grape marc, tuff, and their mixtures. These materials are widely used in Israel and in other countries as container media for plants in greenhouses. Water retention curves (main drying and primary wetting scanning branches) were measured in the 0–120-cm range, and the model parameters were determined by curve-fitting of the measured data. The K(h) relationship was obtained by multiplying the measured saturated hydraulic conductivity, Ks, by the calculated K(h) curve. A direct validation of calculated K(h) was performed using the steady-state flux control method. Comparison of simulated to measured suction profiles also provided an indirect validation of the model. The model was found to provide a useful tool for the prediction of the unsaturated hydraulic conductivity of container media of horticultural importance, consisting of composted agricultural wastes, tuff, and their mixtures.

A comprehensive mathematical model for transport of soil-dissolved chemicals by overland flow

The model developed in this study simulates the contamination of overland flow by soil chemicals that reside near its surface during a surface runoff event. The model includes mass-balance equations for both water flow and chemical transport in the soil profile and surface runoff. A rate-limited mass transfer through an overland-flow boundary layer at the soil overland flow interface controls the dissolved chemical transfer from soil solution to overland flow, once formed. The model predicts water flow and chemical transport in the soil profile prior to the rainfall ponding (when overland flow starts) and during the surface runoff event. The predictions of these variables, together with the total load to the surface runoff, were successfully compared with the measured data of Hubbard et al. [Trans. ASAE, 32(4) (1989) 1239]. Being physically based, the model was used to investigate the dependence of surface runoff pollution and its extent on the system hydrological parameters. A key factor on the availability of soil chemicals to pollute the overland flow is their displacement by infiltrating water prior to runoff initiation. Being dependent on soil moisture prior to rainfall initiation and on rainfall intensity, a lower chemical concentration and a lower load in surface runoff are obtained for longer ponding times, ones that are associated with lower rainfall rates and initially drier soil profiles. During the surface runoff flow, the chemical concentration in overland flow at the slope outlet is affected by the contact time of an overland flow parcel with the soil surface. Thus, it increases for higher values of equilibrium time — tE, lower rainfall rates, slope gradients, and higher soil-surface roughness coefficients. These parameters have an inverse effect on the surface runoff concentration by affecting the transfer coefficient of soil chemical to overland flow. A different insight into the relationship between the relevant dynamic processes throughout the storm event is achieved by studying the transient variation of soil chemical flux to overland flow, the chemical flux at the slope outlet, and the change of chemical mass in the overland flow.