Indicadores de humedad y salinidad del sustrato para mejorar la eficiencia del riego en cultivos en maceta

Abstract

This doctoral dissertation has been presented in the form of thesis by publication. Water scarcity is creating the need for more sustainable water use in arid and semi-arid regions. Due to lack of fresh water, or to high salinity in the available water, the agriculture of these regions has to deal with the effects of water stress and salt stress. This problem can be addressed from different approaches, and one of them is using sensor-based irrigation strategies. Soil sensors provide constant information on soil moisture and salinity, which allow an irrigation controller to optimize water consumption or control salt accumulations in the substrate. However, implementing a sensor-based irrigation system presents many challenges, such as using salinity indexes based on sensor data, analyzing the plant responses to water and/or salt stress, and establishing relationships between these responses and salinity indexes. This thesis focuses on the irrigation management with soil sensors of potted ornamental plants. The main objective was to evaluate the plant behavior at different stress levels and to assess several irrigation strategies to improve their water efficiency and adaptation to salinity. To achieve this purpose, the thesis was distributed into four secondary objectives that correspond to the four articles that comprise it. The article I evaluated the capacity of five salinity indexes, calculated from sensor measurements, to estimate the actual salinity of the substrate. This experiment consisted of saturating the substrate of pots without plants with solutions of different salt concentrations, and then letting them evaporate in a culture chamber monitored with soil sensors and analytical balances. The balances were used to determine the actual salinity, which was then compared with the indexes by correlation and linear regression analysis. In articles II, III and IV, a PLC executed the irrigation instructions by comparing the sensor values with the parameters specified for each irrigation strategy. In papers II and III, an irrigation without drainage was studied in Hebe andersonii cv. Variegata (Hebe) to improve its water use efficiency under water déficit and high salinity, respectively. The irrigation applied in article IV was similar to that of article III, but with drainage. In this last paper, three salinity indexes were proposed to control salt damage in Euphorbia x martinii Ascot Rainbow (Euphorbia) and to optimize water consumption in the flushes that are applied to reduce salt accumulation in the substrate. The stress tolerance of these species was evaluated by growing them under different levels of water or salt stress, and measuring their main physiological and morphological parameters: size, biomass, leaf mineral content, leaf area, leaf anatomy, leaf chlorophyll content, plant water potentials, gas exchange parameters, and chlorophyll fluorescence. All salinity indexes in the first paper were able to estimate the actual salinity of the substrate accurately, but they presented different limitations and sensitivities. In general, salinity indexes were more accurate at higher substrate moisture levels. Regarding Hebe, a deficit irrigation with no leaching improved its wáter use efficiency. Hebe also improved its water use efficiency in an irrigation with medium salinity water (3 Ds m-1) without drainage and at high substrate moisture. However, an irrigation with high salinity water (5 dS m-1) reduced plant size and the number of leaves. On the other hand, Euphorbia showed limited tolerance to salt stress since an irrigation with water of 5 dS m-1 visibly impaired its ornamental quality. However, an irrigation with 3 dS m-1 water affected neither its development nor its aesthetics. Out of the three indexes evaluated in this last article, the bulk electrical conductivity measured just after irrigation was the most suitable for scheduling salt flushing, as it was the most sensitive for detecting changes in salinity. In Euphorbia, flushing when this index reached 1.9 dS m-1 maintained salt stress under control and optimized water consumption.