UC Davis

This thesis explores the broad physiological responses of tomato (Solanum lycopersicum) in solution culture to various spatial potassium (K) distributions under heterogeneous rootzone salinity (NaCl). Chapter 1 is a collaborative review of heterogeneous soil salinity, introducing how management and environment influence salt distribution patterns, and reviews ensuing physiological responses. The review also summarizes the limited research on interactions between heterogeneous salinity and nutrient distribution, particularly split-root experiments, a line of inquiry which this research seeks to enrich. Chapter 2 outlines an original experiment where tomato plants were grown in solution culture with roots evenly divided between two compartments. Except for a salt-free control group (Treatment 0), the same overall amount of salt (NaCl) was either provided to the plant uniformly across the entire root zone (treatment 1) or provided to only one half of the root system (treatments 2, 3, and 4). Treatments 2, 3, and 4 feature an increase in the share of the K budget which is supplemented in the saline compartment compared to the non-saline compartment. Treatment 2 provides nutrients including K to one side and NaCl to the other. Treatments 3 and 4 increase K in the saline compartment to 40% and 80% of the K budget, respectively. The impacts on biomass accumulation, biomass partitioning, water uptake, sodium uptake, and potassium uptake were measured and analyzed for statistical significance. There was no difference in the total biomass, overall water uptake rate, or root distribution between root halves across uniform treatment groups (0 and 1) despite a major difference in overall solution NaCl concentration (0 mM and 20 mM average, respectively). In all treatments where supplemented sodium (40 mM) was confined to half of the root zone (2, 3, and 4), plant water uptake was restricted almost completely to the non-saline compartment demonstrating a remarkable plasticity of root response to local saline conditions. Whole plant water uptake rates were generally comparable irrespective of saline distribution. Saline compartments of treatments 2, 3, and 4 did not show sodium or potassium uptake, regardless of potassium richness. Across all treatments, there was a strong tendency for water, and potassium uptake, as well as root growth, to occur in the Na-free compartment. The only instance of plants utilizing solution K in the presence of NaCl occurred in treatment 1, where K was supplemented along with all other nutrients uniformly in an overall saline root environment. Interestingly, this treatment was also the only clear instance of sodium uptake to occur among any treatment groups.

The results of these experiments suggest a salt-avoidant response, whereby the presence of any salt-free and nutrient-rich root zone will result in preferential water uptake from that zone. Research (summarized in Chapter 1) also demonstrates that the provision of a full nutrient supply exclusively to the saline side of a split root system will result in considerable water uptake from the saline compartment and increase whole plant salt uptake. This research was conducted to determine which of the nutrients in the nutrient-rich zone was responsible for plant activity in the saline zone that may otherwise have been avoided. The research performed here demonstrates that the driving dynamic for this plant response is not K alone. The goals of minimizing the incidence of salinity stress and maximizing nutrient use efficiency are inextricable in the agronomic system. Understanding the relationship between nutrient use and salt localization is important if we are to optimize management systems under the heterogeneous ion distributions that are commonplace in irrigated agriculture.