Other studies also indicated that bacterial inactivation in the presence of antimicrobial compounds increases with increasing temperature due to the higher metabolism rate and cellular activity. Due to the higher degree of cellular activity and thus more metabolic burden at a higher temperature, bacteria are more susceptible to antimicrobial agents. Several possible reasons could be attributed to the ability of light-activated curcumin at 4 ◦C to inactivate bacteria, including: light activation of curcumin is independent from bacterial metabolism; increased proportion of unsaturated fatty acids in bacterial membrane at lower temperatures;entering some bacterial species such as V. parahaemolyticus to a viable but not culturable state at lower temperatures. Thus, the antimicrobial properties of light-activated curcumin at refrigerated temperatures indicate that photodynamic inactivation using curcumin could be a promising approach for sanitizing seafood, live organisms , and water to maximize the bacterial inactivation and shelf life of the products. The world’s population is expected to increase to 9.9 billion by 2050 , prompting a need to double annual food production within the next 30 years . To achieve this production goal, agriculture will inevitably expand further into marginal lands ,grow bag for tomato which often suffer from poor soil structure and low fertility . Soil salinity, both naturally occurring and as a consequence of human activities , is a threat to agriculture and a major limitation to food production.
The salinization of agricultural land commonly occurs as a result of using irrigation water containing elevated levels of ions without adequate, periodic leaching of the accumulated salts from the soil. Secondary dry-land salinization can also occur in non-irrigated areas due to changes in the hydrological balance of a landscape and rising water tables . The problem is often exacerbated by decreases in soil permeability caused by sodicity and over-exploitation of groundwater, which exhaust high-quality water resources, resulting in water extraction from less favourable groundwater that may be brackish or saline . Increases in salinity can also be expected in low-lying coastal areas associated with sea-level rise due to climate change and salinization of groundwater due to salt water intrusion into depleted aquifers . While the vast majority of research on crop responses to salinity has been conducted under homogeneous saline conditions, root-zones of plants in both natural and managed environments can commonly experience spatial and temporal heterogeneity in soil salinity . The nature of soils and irrigation practice, crop type and phenology, climate-type and seasonal weather, and the duration of crop exposure, together deter-mine the extent and impact of salinity and its heterogeneity on plant growth and crop productivity . Despite this complexity, most experiments on the impacts of salinity on plants have imposed homogeneous root-zone salinity, which does not represent saline agricultural settings. The extent of the temporal heterogeneity in soil salinity in the field is illustrated within an irrigated wheat trial in China and a rain-fed wheat trial in Western Australia . These patterns of saline heterogeneity can be contrasted with the near homogeneous conditions commonly imposed in controlled-environment research trials . Since complex multi-faceted traits are involved in plant tolerance of salinity, this raises the question of whether trials conducted under near-uniform soil salinity are indeed optimal for identifying and selecting traits of most value to increasing plant tolerance to the common reality of heterogeneous salinity.
The generic guidelines used to predict crop response to soil and water salinity, produced under near homogeneous conditions, are generally described by crop yield curves that consist of a threshold value at which salinity induced damage first occurs, and a linear percentage yield reduction with every increment in the electrical conductivity of the saturated soil extracts thereafter . Such static diagnostic criteria do not reflect soil salinity under realistic field conditions that are highly spatially and temporally heterogeneous . Thus, the prevailing standard of describing plant response to salinity is both inadequate and generally overestimates crop response , is not relevant nor easily interpreted under field reality, and may not adequately inform irrigation practice, crop selection or salinity mitigation strategies . In the following, we summarize the pattern of occurrence of salinity heterogeneity in rain-fed and irrigated systems, and discuss results from studies of plant responses to heterogeneous root-zone salinity. We then discuss the mechanistic understanding of root physiological and morphological adaptations to heterogeneous conditions, and place these findings in the context of defining future research priorities and possible management and crop breeding opportunities to improve productivity in saline lands.In naturally saline environments, within the rooting zone of a single plant, non-saline patches can coexist with nearby saline ones, ranging from a few millimolar to several times seawater . The magnitude of this heterogeneity varies in time and space depending upon soil parent material, landscape position , soil physical and chemical characteristics , surface runoff and subsurface lateral flow of water, intrusion by saline ground waters or seawater , and root water extraction . Climatic conditions also affect the temporal and spatial heterogeneity of soil salinity, with rainfall leading to soil leaching events while droughts and heatwaves concentrate solutes depending on soil features and topography, and differences in radiation and resulting evaporation due to aspect and slope . Irrigation also dramatically influences soil salinity . Irrigation-induced heterogeneity can commonly result in differences in soil ECe greater than 10-fold . An extremely heterogeneous distribution of salinity in irrigated systems makes it difficult to design a soil sampling regime to determine the truly effective root-zone salinity .
This effect is highly relevant to modern drip and micro-irrigated agriculture in arid regions , which are the most common irrigation strategies in many areas of the world. While micro-irrigation is generally considered a valuable way to improve water use efficiency and allows con-trolled fertigation strategies, these systems may complicate salinity management, generating highly non-uniform salt and disparate nutrient deposition patterns below the irrigation emitter . These salt/nutrient deposition pat-terns below the micro-irrigation emitter directly impact root growth, root activity, and nutrient and salt movement in the soil within the root zone, with effects strongly determined by crop placement, soil preparation, irrigation design and management. Bar-Yosef further discussed the risk of salt accumulation in the root-zone under drip irrigation, suggesting that salts are not efficiently displaced to the periphery of the wetted soil volume as might occur under a full surface irrigation system. Under drip irrigation, salts can accumulate in the wetting front after several irrigation cycles but this wetting front will shrink and swell with subsequent irrigation events and root water consumption. Varying the frequency and volumes of irrigation events can manipulate this salt displacement and represents a management strategy. Salts can also accumulate at the upper margin of the wetted soil volume due to capillarity driven by soil evaporation. This effect is particularly marked with buried drip irrigation systems where the depth of irrigation tubes, shape of the furrow,grow bag for blueberry plants and environmental water use patterns influence the ultimate salt distribution. Based upon current understanding, irrigation system placement and operation could theoretically be managed to ensure that the deposition of salinity is largely restricted to the outer margins of the wetted root-zone, thereby providing a zone of lower inner salinity with abundant plant root activity. A better understanding of soil processes and plant responses under heterogeneous conditions may therefore allow us to mitigate the adverse effects of salinity . Although very few field experiments have tested this theory that drip irrigation can be optimized to minimize the impacts of salinity, several studies of split-root plants showed greater plant growth under heterogeneous salinities than uniform salinity, at the same average root-zone salinity . This suggests that further developing these irrigation strategies should be fruitful. Understanding the nature of plant responses to heterogeneous salinity is therefore essential to develop and implement improved irrigation practices for saline systems. In particular, this opens an excellent opportunity to improve production by manipulating the heterogeneity in the salinity of the soil solution, thereby harnessing the abilities of plants to make optimum use of less-saline patches within root-zones.Under naturally occurring and agriculturally induced salinity, plant growth is affected by the salinity of the soil solution or the ratio of salt and the water content of the soil. Soil salinities vary on spatial scales of micro-metres to metres, and on temporal scales ranging from seconds to seasonal changes . Thus, roots of a single plant will be ex-posed to a range of soil water salinity levels that vary temporally and spatially, with differential effects depending upon the stage of plant growth. Nevertheless, while heterogeneous salinities typically occur in salt-affected soils, experiments have almost exclusively imposed homogeneous salinity or highly manipulated experimental conditions such as split-root systems, which expose a portion of a root system to salinity while the remainder receives non-saline conditions.
Although split-root experiments may not adequately mimic a complex field condition, these have provided valuable insights by demonstrating how plant responses to heterogeneous conditions differ markedly from those of homogeneous saline conditions. Split-root experiments indicate a more nuanced plant response to saline environments than commonly recognized. As summarized in Bazihizina et al. , key features of plants exposed to heterogeneous salinities are: shoot water potentials are determined by the salinity level of the low-salinity zone, water uptake occurs predominantly from the low-salinity medium and greater maintenance of shoot growth even when a large proportion of the root system is exposed to high NaCl concentrations that would greatly inhibit growth if applied uniformly to the roots. Transcriptome profiling of plants exposed for 6–9 h to heterogeneous salinities indicated that improved performance under heterogeneous conditions compared to uni-form salinities is related to the rapid activation of salt resistance genes and crosstalk between the non-saline and high-saline root sides . This suggests that roots operate as the central hub that control: how stress is perceived, long-distance communication with the shoots and the integration of long-distance systemic signals with local root-based ones. Furthermore, salinity heterogeneity is inevitably linked to temporal and spatial variation in the distribution and biological availability of water, essential nutrients and soil pH , with the latter having a major impact on root membrane potential, thus affecting both a plant’s ability to acquire essential nutrients and exclude toxic Na+ and Cl− ions . Thus, responses at the root level also play a critical role in: how plants compensate for water/nutrient deprivation and limit salt stress by optimizing root-foraging in the most favourable part of the soil. Understanding how roots respond to heterogeneous salinities is therefore of utmost importance and is needed to develop management strategies to optimize resource use and crop productivity in saline soils. Different processes enable roots to integrate fluctuating soil conditions into appropriate developmental and physiological responses that ultimately determine how efficiently resources are captured. These are fundamentally controlled at variable spatial scales, from the single cell to the entire organ.Local patches of high salinity are sensed in individual cells, and then integrated into organ-scale processes. After salinity increases, plants experience multiple constraints ranging from reduced water availability, disturbance to cytosolic ion homeostasis, and dramatic increases in reactive oxygen species accumulation. The emerging picture suggests that more than one sensory mechanism may operate in the same cell at the same time, with some common downstream signalling pathway . Calcium and ROS signals are amongst the first signals commonly evoked upon biotic and abiotic stressors. Ca2+ and ROS signals are established second messengers involved in most stress responses, and increasing evidence suggests that these act in tandem, interacting and amplifying each other during root salt sensing . Several molecular components underlying Ca2+ and ROS signalling have been identified and are currently being considered as potential salt sensors . Interestingly, local salt stress at the root apex triggers immediate cytosolic Ca2+ in-creases at the point of application, leading to propagation of a TPC1– dependent Ca2+ wave to distal shoot tissues, passing through cortical and endodermal cell layers . By combining experimental analyses and mathematical modelling, Evans et al. also clearly linked the [Ca2+]cyt wave triggered by a localized salt application with systemic ROS waves. Additional salt sensors include: cell walls, and in particular the salt-induced alterations in cell wall integrity and composition that are sensed by the receptor-like kinase, FERONIA ; mechanosensory channels and transporters that sense the mechanical force exerted on the plasma membrane due to the osmotic component of salinity and trans-late hydraulic cues into chemical signals ; and Na+ transport systems and proteins with regulatory Na+ binding sites .