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Fakultät für Biologie, Chemie und Geowissenschaften

Bodenphysik

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Masterarbeit

A stomatal regulation algorithm consistent with abscisic acid (ABA) signaling predicts the concomitance of hydraulic loss and transpiration reduction during drought

Fabian Wankmüller

Betreuer: Andrea Carminati, Mohsen Zare

The thesis of Fabian Wankmüller presents a new model and concept of stomata regulation that links soil and plant hydrological principles ( soil-plant hydraulic conductance) to plant physiological evidence of stomata regulation (chemical signaling -i.e., abscisic acid, photosynthesis rate). The model developed here includes simulations of leaf water potential based on plant transpirational demand, soil water potential, and variable hydraulic conductances of the soil-plant-atmosphere continuum, and a function that links stomatal conductance to photosynthesis rate. This model reproduces a flexible stomatal behavior in response to various air moisture and soil-hydraulic conditions and casts light on new aspects of stomata regulation under variable environmental conditions such as soil drying, increasing vapor pressure deficit in the air, and day and night cycles.

Predicting plant responses to drought has been a research goal for a long time. Negative im- pacts on terrestrial productivity in the face of climate change driven drought intensification add urgency to it. Since stomata have control over plants’ gas-exchange, understanding their behaviour is essential but challenging. Current predictions of stomatal response to drought mainly rely on empirical models. Alternatively, stomatal optimization theory offers a more mechanistic framework to predict plants’ gas-exchange when water is scarce. It posits that stomata would maximize the carbon gain in relation to a penalty of the inevitable water loss. To quantify the water penalty, soil-plant hydraulics is employed which points out the role of stomata to prevent vascular damage by cavitation. To fulfil this function, stomata should not exceed the water supply determined by soil-plant hydraulics. But until now, a mecha- nism that enables stomata to do so is not known despite increasing physiological insights into stomatal regulation at the cellular level. It has been shown, for instance, that stomatal closure during drought involves abscisic acid (ABA) signaling. Nonetheless, a linkage be- tween stomatal regulation at the cellular level and the ’macroscopic’ modeling approach of soil-plant hydraulics is still missing. As a consequence, this thesis hypothesizes a ‘stomatal regulation algorithm’ (SRA) consistent with ABA signaling which incorporates the underly- ing regulation and explains the emerging response of stomata during drought: the timely closure to avoid excessive cavitation of the vascular system resulting in the concomitance of hydraulic loss and transpiration reduction. To explore how stomata, according to the SRA, respond to various atmospheric and soil hydraulic conditions, the model was tested under different environmental scenarios. Moreover, the SRA was compared to an empirical model (EM). The modelling showed that the EM was unable to respond to the different en- vironmental conditions. By contrast, the SRA enabled plants owing to their flexible stomatal response to exploit but not to exceed the given water supply under any environmental sce- nario. Hence, this thesis outlines a promising approach to predict stomatal behaviour and plant performance during drought but calls for experimental validation.

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