Plant Water Relations in Response to Drought and Different Cropping Systems

Abstract

Agriculture is one of the main contributors to climate change and is also severely threatened by the consequences of the changing climate. To reduce the environmental pressure from agriculture, sustainable practices are urgently in need of evaluation and implementation. In croplands, organic farming and conservation tillage can reduce energy consumption, greenhouse gas emissions, and pollutant production as well as increase carbon sequestration, hence are considered more environmental-friendly and sustainable than conventional farming and intensive tillage. Moreover, organic farming and conservation tillage are shown to improve soil health such as physical structure, chemical conditions, and microbial functionalities. Therefore, these practices are also recommended as beneficial to mitigate climate change effects, such as crop drought stress. However, if organic farming and conservation tillage are truly climate-smart adaptations for cropping systems is still to be tested in different climate conditions. Extreme weathers, increasing in frequency and severity due to climate change, are posing extra challenges on food security in addition to the growing population. Drought as a top threat causes intense damage in agroecosystems, especially in rainfed agriculture. Crop water relations are highly responsive to drought stress, and directly linked to growth and productivity. Therefore, understanding crop water relations in response to drought can provide insights on assessing cropping systems for future climate. As introduced in Chapter 1, this thesis aims to bring insights on the effects of cropping systems and drought on plant water relations along the soil-plant-atmosphere continuum. This includes root water uptake patterns, stem xylem vulnerability and anatomy, leaf water status and physiological processes, phenology, growth, and yield. Combining different management practices, four cropping systems are studied: conventional intensive tillage, conventional no-tillage, organic intensive tillage, and organic reduced tillage. Simulated drought periods with portable shelters were carried out in Swiss rainfed cropland under temperate climate. During the 2018 growing season pea-barley mixture, an important fodder crop, was studied. Winter wheat, a globally important food source, was studied during 2019. Root water uptake depths of crops provide important information on drought response and therefore may provide insight based on differential outcomes in cropping systems under drought. The foci in Chapter 2 are on root water uptake patterns of pea and barley grown in a mixture under the targeted cropping systems and different water availabilities. The water uptake patterns of winter wheat are included in Chapter 4. Stable water isotopes were used to estimate the water uptake patterns of these three species with a Bayesian framework. In all cropping systems, when subjected to the experimental drought, both pea and barley shifted their water uptake patterns to shallower depths without niche differentiation, whereas winter wheat went for deeper water uptake. Moreover, due to the natural drought period in summer 2018, we also observed responses of pea and barley to this more moderate drought compared to our treatment, where only barley shifted up in water uptake depths, but pea did not. Plant hydraulics greatly determines water transport, and it is shown to be affected by environments including soil conditions. The hydraulic traits of pea and barley in response to cropping systems as well as their links to growth and productivity are presented in Chapter 3. Xylem vulnerability to cavitation of both species was evaluated with the cavitron technique, and percentage loss of conductivity derived with xylem water potential in situ was used to assess the drought stress of these two species. Once again, different species showed inconsistent responses. Although grown in a mixture, cropping systems only affected the hydraulic traits of barley but not pea. Crop growth and productivity are shaped by various physiological processes with great complexities among regulation and compensation responses. Integrating crop traits along the entire continuum into a trait space helps to draw a clear picture on changes responding to the environment and managements. Winter wheat traits about crop water relations are presented in Chapter 4. In the trait space, cropping systems only changed growth trait space, which differed between organic and conventional systems but not between conversation and intensive tillage systems. Meanwhile, the drought treatment dominated water trait space, irrelevant to cropping systems. Other than the traits investigated on individual plant, visual changes in crop phenology can also help reveal cropping system effects. PhenoCams were used to track phenology of pea-barley mixture and winter wheat. Crop phenology was not affected by cropping systems for pea-barley mixture, but for winter wheat, showing differences consistent with the trait space (i.e., differed between organic and conventional systems). Because I contributed to data collection and analysis as well as revising the manuscript, this work is included in the Appendix. Our results show that cropping systems can affect certain hydraulic, physiological, and growth traits, as well as phenology of crops, which could link to yield traits. However, the responses and outcomes largely depend on species. These results emphasise that using different cropping systems to alleviate drought stress for plants might not be as effective as previously assumed, at least in temperate areas, hence compromising the expectation to counteract or compensate the aggravating drought threats under the changing climate.