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Understanding how plants overcome drought by controlling stomatal function: applicability and impacts on agriculture

Periodic Reporting for period 2 - AgroPHYS (Understanding how plants overcome drought by controlling stomatal function: applicability and impacts on agriculture)

Reporting period: 2019-08-01 to 2020-07-31

We live currently under a global crisis where the increase of the world population and therefore, the food demand, is placing agriculture under a context of urgency because it will need to produce this food without wasting the water necessary for that production. In addition to that, the climate crisis and the increase in water scarcity, considering that agricultural water use can be up to 80% of available freshwater, make essential the improvement of agricultural practices and the development of more efficient irrigation strategies. Drought events, more and more frequent and severe, cause plant water stress, a functional and structural plant response to low water availability, that reduces the productivity of a crop. Thus, an understanding of the impact, mechanisms, and traits underlying drought tolerance in agricultural plant species is essential to increase the efficiency of irrigation strategies and to improve productivity. The impacts of this on society are direct, representing major European priorities: optimizing agriculture in a changing climate with reduced water availability and a growing population.
The AgroPHYS project aims to combine three important fields of research (diagram attached) to deal with this urgent need: a fundamental understanding of the physiological mechanisms of plant response to drought, the use of plant sensors to monitor these responses in real-time, and the implementation of physiological-based models to predicting the impacts of global change on plants and providing new hypotheses to be tested.
The conclusions of the action can be summarized as: (1) the use of the optical technique to visualize in vivo the air blockage formation within the vascular system as olive seedlings dehydrated, allowed us to demonstrate that roots were the most resistant organs to hydraulic dysfunction; (2) the results obtained from this optical technique, which is easy to use and low-cost, agree with most common, hydraulic techniques and with highly-resolution, synchrotron-based techniques; (3) stomatal opening limitations appear to be related with a decrease in soil-root hydraulic conductance under moderate levels of water stress in olive; (4) leaf abscisic acid production is crucial for protecting vessels from air blockage formation by playing a key role on triggering stomatal closure; and (5) with a combination of mechanistic models and leaf turgor pressure sensors, the automatic and continuous monitoring of stomatal conductance is possible in fruit tree species, which will improve the water used by these fruit orchards.
The very first results obtained during the Outgoing Phase were: 1) the optical technique was used for the first time to monitor simultaneously xylem embolisms formation in roots, stems, and leaves in entire, intact olive seedlings; (2) roots were identified as the most resistant organs to these embolisms formation, in contrast with previous studies that pointed at the roots as more vulnerable tissues than core tissues (stems); and (3) a relatively high variation in resistance was found between individuals, between tissues, and within tissues. As this work did not assess the impact of plant hydraulics on stomatal functioning, a specific experiment was conducted also in olive aiming at determining whether the hydraulic pathway from the soil to the leaf is dynamic or static during stomatal closure. Results included: (1) the development of a new hydraulic method to partition the hydraulic pathways from the soil to the leaves, and (2) an observed decrease in the root water transport capacity under moderate drought conditions that worked as a hydraulic signal to limit stomatal opening, helping the plant to conserve water and isolating it from the drying soil.
Directly related to AgroPHYS, we expanded our knowledge on how hydraulic traits can be decisive for protecting plants against drought negative effects by working with different olive genotypes. Moreover, the application perspective of AgroPHYS was also addressed by developing robust, physiologically based tools for irrigation management, demonstrating that combining the three main fields of research of AgroPHYS is not only possible but fundamental for progressing on optimizing water use in agriculture.
In addition to these direct outcomes from AgroPHYS, and thanks to the participation in Synchrotron-based campaigns, we demonstrated that the optical technique was equally effective as hydraulic and micro-tomography techniques for measuring hydraulic resistance thresholds of water stress.
During the entire Incoming Phase, experiments were mainly focused on applying the new plant physiological knowledge acquired during the Outgoing Phase on the experimental orchard ‘La Hampa’ from the IRNAS-CSIC. Six fruit tree species were physiologically, extensively characterized, and monitored with meteorological, soil, and plant sensors.
As an overview of the overall results of AgroPHYS we can say that (1) knowing the resistance to water stress of agricultural species is pivotal under a global change context; (2) however, these levels of water stress should not be reached in fruit tree orchards and, in fact, plants avoid that by closing stomata; (3) limitations on productivity derived from this stomatal closure appear to be related with the belowground capacity to water uptake; and (4) we thus demonstrated that stomatal conductance should be our target to monitor water stress in fruit orchards.
Exploitation and dissemination of these results have been addressed by presenting them in several Xylem International Meetings, in seminars in both University of Tasmania and Researcher Centres in Spain, starting a collaboration with soil scientist at the University of Bayreuth (Germany), and publishing several papers in highly ranked journals, giving extra support to our conclusions.
Drought can cause important economic losses due to a decline in productivity. Therefore, new insights on how plants overcome drought resulted from AgroPHYS outcomes will have great impacts on reducing these economic losses by optimizing water use in agriculture and even improve the quality of the final product. For instance, in agriculture there is an irrigation strategy called Regulated Deficit Irrigation based on applying exactly the water that our crop needs at each of its phenological stage, because it is known that depending on which stage the crop is, it will be less or more resistant to drought. These strategies can be highly optimized by improving the physiological knowledge behind these different resistances at different phenological stages. Moreover, this knowledge will allow to place plant sensor outputs in a physiological context, so we will be able to monitor these physiological variables over time and to use them as inputs for improving predictions of climate change on plants through mechanistic models and for providing new hypotheses to be tested. However, this will be only possible if there is good communication between physiologist and agronomist to improve the irrigation strategies, between them and engineers and companies to minimize the costs of technologies in agriculture, and between them and mathematicians and modellers to translate physiological mechanisms into equations.
Diagram of the interaction of the three main approaches of AgroPHYS.