Nirit Bernstein

Involvement of the plant antioxidative response in the differential growth sensitivity to salinity of leaves vs roots during cell development.

Sensitivity to salinity varies between plant organs and between cells of different developmental stages within a single organ. The physiological and molecular bases for the differential responses are not known. Exposure of plants to salinity is known to induce formation of reactive oxygen species (ROS), which are involved in damage mechanisms but also in cell growth processes. The objective of this study was to elucidate developmental-stage-specific and organ-specific involvement of oxidative defense in the plant response to salinity in maize (Zea mays L.). Plants were grown in nutrient solution containing 1mM NaCl (control) or 80mM NaCl. The oxidative stress response and damage symptoms along the cell developmental gradient in growing and mature tissue of leaves and roots were examined. Unlike leaves, roots did not suffer oxidative damage in either growing or mature cells and demonstrated reduced antioxidant response. This may reflect different requirements of ROS for growth mechanisms of leaf and root cells. In leaves, growing tissue demonstrated higher stimulation of superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity under salinity than mature tissue, whereas mature tissue demonstrated higher stimulation of catalase. These results indicate differential roles for these ROS-scavenging enzymes at different cell developmental stages. Because ROS are required for cell expansion, the higher increase in SOD and APX activities in the growing leaf cells that resulted in reduction of ROS content under salinity could lead to the inhibition of cell growth under salinity.

Involvement of calcium-mediated effects on ROS metabolism in the regulation of growth improvement under salinity.

Salinity reduces Ca(2+) availability, transport, and mobility to growing regions of the plant and supplemental Ca(2+) is known to reduce salinity damages. This study was undertaken to unravel some of the ameliorative mechanisms of Ca(2+) on salt stress at the cellular and tissue levels. Zea mays L. plants were grown in nutrient solution containing 1 or 80 mM NaCl with various Ca(2+) levels. Measurements of growth and physiological parameters, such as ion imbalance, indicated that the Ca(2+)-induced alleviation mechanisms differed between plant organs. Under salinity, H(2)O(2) levels increased in the leaf-growing tissue with increasing levels of supplemental Ca(2+) and reached the levels of control plants, whereas superoxide levels remained low at all Ca(2+) levels, indicating that Ca(2+) affected growth by increasing H(2)O(2) but not superoxide levels. Salinity completely abolished apoplastic peroxidase activity. Supplemental Ca(2+) increased its activity only slightly. However, under salinity, polyamine oxidase (PAO) activity was shifted toward the leaf base probably as an adaptive mechanism aimed at restoring normal levels of reactive oxygen species (ROS) at the expansion zone where NADPH oxidase could no longer provide the required ROS for growth. Interestingly, addition of Ca(2+) shifted the PAO-activity peak back to its original location in addition to its enhancement. The increase in PAO activity in conjunction with low levels of apoplastic peroxidase is supportive of cellular growth via nonenzymatic wall loosening derived by the increase in H(2)O(2) and less supportive of the peroxidase-mediated cross-linking of wall material. Thus extracellular Ca(2+) can modulate ROS levels at specific tissue localization and developmental stages thereby affecting cellular extension.

Evidence for internalization of Escherichia coli into the aerial parts of maize via the root system.

Escherichia coli introduced into the hydroponic growing medium of maize plants was detected 48 h later in the shoot. Decapitation of root tips or severing of the plant root system at the root-shoot junction enhanced bacterial internalization. The density of the bacteria in shoots of plants with damaged roots or removed root systems was 27.8 and 23.9 times higher than that in plants with intact roots, respectively. The concentration of viable cells in the hydroponic solution decreased over time from 9.3 x 10(6) CFU/ml at the time of inoculation to 8.5 x 10(1) CFU/ml 4 days thereafter. The number of E. coli cells associated with the roots also decreased with time, but a significant decline appeared only at 4 days postinoculation. At the time of sampling for E. coli presence in the shoot, 10(2) CFU/ml was present in the nutrient solution and 8 x 10(3) CFU/g was associated with the roots. The present study is the first to demonstrate internalization of E. coli via the root in a monocotyledonous plant.

SALINITY-INDUCED CHANGES IN THE NUTRITIONAL STATUS OF EXPANDING CELLS MAY IMPACT LEAF GROWTH INHIBITION IN MAIZE

Salinity-induced excess or deficiency of specific nutrients are often hypothesised to operate as causes of growth inhibition and to trigger primary responses, which directly affect growth. Information concerning salinity effects on microelement nutrition in the growing cells is limited. In this study, salinity-(80 mm NaCl) inflicted alterations in spatial profiles of essential elements (N, P, K, S, Ca, Mg, Fe, Zn, Mn, Cu) and the salinity source (Na and Cl) were studied along the growing zone of leaf 4 of maize (Zea mays L.). Correlations between spatial profiles of growth and nutritional status of the tissue were tested for evaluation of the hypothesis that a disturbance of specific mineral nutritional factors in the growing cells might serve as causes of salt-induced growth inhibition. Examined nutritional elements exhibited unique distribution patterns, all of which were disturbed by salinity. With the exception of Na, Cl and Fe, the deposition rates of all the studied mineral elements were reduced by salinity throughout the elongating tissue. Localised contents of Ca, K and Fe in the growing tissue of the salt-stressed leaf were highly correlated with the intensity of localised tissue volumetric expansion, suggesting reduced levels of Ca and K, and toxic levels of Fe as possible causes of growth inhibition. Na and Cl accumulation were not correlated with growth inhibition under salinity.

Assessment of Contamination Potential of Lettuce by Salmonella enterica Serovar Newport Added to the Plant Growing Medium

The capacity of Salmonella enterica serovar Newport to contaminate Romaine lettuce (Lactuca sativa L. cv. Nogal) via the root system was evaluated in 17-, 20-, and 33-day-old plants. Apparent internalization of Salmonella via the root to the above-ground parts was identified in 33- but not 17- or 20-day-old plants and was stimulated by root decapitation. Leaves of lettuce plants with intact and damaged roots harbored Salmonella at 500 +/- 120 and 5,130 +/- 440 CFU/g of leaf, respectively, at 2 days postinoculation but not 5 days later. These findings are first to suggest that Salmonella Newport can translocate from contaminated roots to the aerial parts of lettuce seedlings and propose that the process is dependent on the developmental stage of the plant.

Salt-stress effects on avocado rootstock growth. I. Establishing criteria for determination of shoot growth sensitivity to the stress

The mechanisms of shoot growth inhibition under salt-stress are not yet understood. Comparative evaluation of growth tolerance level of plant material used for mechanistic studies into stress effect on growth processes is a critical step towards establishing a plant model system for studies of stress effects on growth.In the present study, numerous vegetative shoot-growth parameters were evaluated as criteria for determination of shoot growth sensitivity of avocado rootstocks (Persea americana Mill.) to salt-stress. Seven different West-Indian rootstocks were investigated for their growth response to the stress. The plants, grown in 50 L soil-filled containers, were treated with 4 mM Na and 6 mM Cl (control) or 18 mM Na and 20 mM Cl (salt) for a period of 3 years. Changes in trunk diameter, branch diameter, number of new branches, branch fresh weight (FW) and dry weight (DW), leaf length, leaf area, leaf number, leaf FW and DW and distribution of the leaf population into size classes, were quantified during the season of rapid shoot growth, and used for grading growth sensitivity.The seven rootstocks studied were ranked according to the sensitivity of their vegetative growth to the stress. The order of sensitivity was found to be similar when the ranking was based on changes in DW and FW deposition per branch, cumulative leaf area and leaf weight per branch, or branch stem weight. Stress effects on leaf FW deposition were similar to the effect on total branch FW. Stress effects on branch stem FW were closely related to total branch DW deposition. Inhibition of cumulative leaf area per branch was less sensitive to the stress than deposition of FW or DW biomass. Biomass production per branch, or alternatively leaf biomass production per branch, is suggested as a practical parameter for evaluation of avocado rootstock shoot growth sensitivity to salt-stress.

Root internalization, transport and in‐planta survival of Salmonella enterica serovar Newport in sweet basil

It is now acknowledged that food-borne pathogens present in the irrigation water or soil can become associated with crop plants in the field, penetrate internal plant tissues via the root, translocate and survive inside plants. Only little information is available concerning interaction between enteric pathogens and plants. The present study evaluated the potential for contamination of the aromatic plant, sweet basil during cultivation, by Salmonella enterica serovar Newport. Root internalization was plant-age-dependent, with the highest susceptibility occurring at the beginning of the rapid growth phase of the root. Higher incidence of internalization was detected in vegetative than reproductive plant organs, pointing at bacterial transport in the transpiration stream. Internalized Salmonella survived only < 30 h in the phyllosphere. In contrast, survival of Salmonella on the leaf surface was much pronounced (at least 8 days), and the initial decay rate was lower at the abaxial (lower) compared with the adaxial (upper) side of the leaf. Although the experiments were conducted with high concentration of Salmonella unlikely to happen in the field, internalization occurred at a low frequency and in-planta survival was limited to less than 30 h. These findings imply that leaf surface contamination, rather than root internalization, may pose higher risk for human infection following consumption of contaminated basil.

Utilization of reclaimed wastewater for irrigation of field-grown melons by surface and subsurface drip irrigation

Shortage of water in arid and semiarid areas throughout the world makes utilization of marginal water for agricultural irrigation a necessity. The marginal water most used for irrigation in Israel is secondary-treated urban effluents. In spite of the water treatment process, these waters often contain higher levels of bacterial human pathogens than the potable water from which they were derived. Utilization of the treated effluents for irrigation in Israel is strictly regulated according to the water quality and the irrigated crop. Due to health concerns, and a lack of experimental data, the treated effluents are not yet used for irrigation of vegetables. In the present study we have evaluated safety and agronomic issues involved in irrigation of summer melon with secondary-treated urban effluents, administered to the production field by surface and sub-surface drip irrigation according to the national regulations. Two water qualities were compared, secondary-treated wastewater and potable water. The effluents contained higher levels of EC, pH, Na and Cl, N, P, K, microelements, and heavy metals than the potable water. Potable water was applied by surface drip irrigation, and three irrigation regimes were compared for the treated effluents. These included surface irrigation, and subsurface irrigation at 20 or 40 cm below the soil surface. No differences in yield quantity and quality were found between treatments. Na concentrations and SAR levels of the soil were higher under irrigation with the effluent. Contamination by E. coli, fecal coliforms, and total coliform bacteria were found on the melon peel of all treatments, and the quantity and quality of the contamination did not vary significantly between treatments. E. coli and fecal coliforms were found in the surface 0-2 cm soil samples of treatments irrigated with both water qualities by surface drippers, but no contamination was found in the treatments irrigated by subsurface irrigation. The fact that the microbial contamination of the fruit was not prevented by subsurface drip irrigation or by irrigation with fresh water suggests that environmental factors, rather than an irrigation treatment affect, were the cause for the microbial spread. Further analysis is required concerning effects of environmental factors, such as the interaction between weather conditions and distance from the effluent oxidation ponds on temporal geospatial distribution of the bacterial human pathogens and the potential for subsequent contamination of fresh produce in the field.

Potential for contamination of crops by microbial human pathogens introduced into the soil by irrigation with treated effluent

In arid and semiarid regions throughout the world, shortage of water necessitates utilization of marginal water for agricultural irrigation. Because of its availability and relatively low cost, treated wastewater is commonly considered as an alternative water source for agricultural needs. Application of treated wastewater for agricultural irrigation may result in exposure of soil to pathogens, creating potential public health problems. Raw sewage water is known to contain a variety of human pathogens. Although their concentrations decrease during the wastewater reclamation process, the secondary treated effluents most commonly used for irrigation today still contain bacterial human pathogens. Therefore, irrigation with treated effluents introduces bacterial human pathogens to the soil. Although not in their natural host, human pathogenic bacteria are capable of surviving long periods of time in soil and water and thereby have the potential to contaminate crops in the field. Therefore, there is a risk of direct contamination of crops by human pathogens from the treated effluents used for irrigation, as well as a risk of indirect contamination of the crops from contaminated soil at the agricultural site. Bacterial human pathogens were recently demonstrated to have the ability to enter plants through their roots and translocate and survive in aerial plant tissues. The practical implications of these findings for food safety no doubt depend on the ability of bacterial pathogenic microorganisms to survive and multiply in the irrigated soil, in the water, and in the crop.

Integrating High Resolution Water Footprint and GIS for Promoting Water Efficiency in the Agricultural Sector: A Case Study of Plantation Crops in the Jordan Valley

Addressing the global challenges to water security requires a better understanding of humanitys use of water, especially the agricultural sector that accounts for 70% of global withdrawals. This study combined high resolution-data with a GIS system to analyze the impact of agricultural practices, crop type, and spatial factors such as drainage basins, climate, and soil type on the Water Footprint (WF) of agricultural crops. The area of the study, the northern Lower Jordan Valley, covers 1121 ha in which three main plantation crops are grown: banana (cultivated in open-fields or net-houses), avocado and palm-dates. High-resolution data sources included GIS layers of the cultivated crops and a drainage pipe-system installed in the study area; meteorological data (2000–2013); and crop parameters (yield and irrigation recommendations). First, the study compared the WF of the different crops on the basis of yield and energy produced as well as a comparison to global values and local irrigation recommendations. The results showed that net-house banana has the lowest WF based on all different criteria. However, while palm-dates showed the highest WF for the yield criteria, it had the second lowest WF for energy produced, emphasizing the importance of using multiple parameters for low and high yield crop comparisons. Next, the regional WF of each drainage basin in the study area was calculated, demonstrating the strong influence of the Gray WF, an indication of the amount of freshwater required for pollution assimilation. Finally, the benefits of integrating GIS and WF were demonstrated by computing the effect of adopting net-house cultivation throughout the area of study with a result a reduction of 1.3 MCM irrigation water per year. Integrating the WF methodology and local high-resolution data using GIS can therefore promote and help quantify the benefits of adopting site-appropriate crops and agricultural practices that lower the WF by increasing yield, reducing water consumption, and minimizing negative environmental impacts.