Background

Earth is currently undergoing global warming and “atmospheric drying” as a result of the increase in atmospheric water vapor pressure deficit (VPD, [1]). Rising heat and VPD are exposing natural and agricultural systems to two problems – water [2] and temperature stress [3] - that lead to amplified tree mortality [4] and crop yield penalties [5]. Stomata, microscopically small pores on the surface of plant leaves that regulate plant gas exchange, are at the forefront of dealing with water- and temperature stress. Through closing, stomata prevent excessive water loss, even in wet soil, when the VPD is high to protect their hydraulic integrity [6]. Conversely, through opening, stomata avoid overheating as bio-chemical processes like photosynthesis have a temperature optimum, below and above which these enzymatic processes slow down [7]. As high temperatures increase VPD, there is an emergent trade-off between water saving and latent cooling [8]. Using soil-plant hydraulic models of plant water demand and supply, we can predict the ‘critical VPD’ (VPD_crit.) upon which stomata are expected to close during atmospheric drying at a given soil moisture status and the relative contribution of hydraulic traits [9]. However, we are currently unable to predict the absence or delay of stomatal closure in response to increasing VPD if the rise in VPD is driven by a significant temperature increase [10]. In nature, VPD inherently covaries with both temperature and relative humidity. The resulting net effect on the degree of stomata openness/closedness is challenging to predict when not accounting for their interaction. 

Objectives

The overall objectives of the joint project are:

1. To systematically unravel the interdependencies between (i) the heat sensitivity (T_crit.) to VPD and (ii) the atmospheric drought sensitivity (VPD_crit.) to temperature.

2. To infer the hydraulic-physiological underlying drivers and traits allowing plants to avoid thermal damages vs. dehydration.

3. To assess how edaphic factors, specifically soil texture and soil moisture, modulate (i) the relative importance of high VPD vs. high temperature in driving stomatal responses and (ii) stomatal sensitivity to underlying hydraulic-physiological traits.

We will conduct experiments with crops and trees in both controlled environments and manipulative field settings, addressing a spectrum of focuses: from agrophysiology to ecophysiology in collaboration with the Plant Ecology Research Laboratory at the École Polytechnique Fédérale de Lausanne (EPFL), and soil-plant hydraulics. Mechanistic insights will be leveraged to incorporate the temperature sensitivity of plant water use under drought stress into a soil-plant hydraulic model, which will then be upscaled to the agroecosystem level using a crop model and an ecosystem model.

Group members

Yanqiao Li (PhD candidate)

Hegarty Philip (PhD candidate)

Dikshya Maharjan (Master’s student)

Funding

International Graduate School of Science and Engineering (IGSSE), TUM

Fig. 1: Disentangling mechanisms of plant water use in response to drought and heat across scales through: (A) Misting system at the VPDrought experimental site in the Pfynwald forest/ Switzerland (by T. Koehler), (B) Cavicam installations in the canopy of mature beech trees at the KROOF experimental site in Kranzberger forest/ Germany (by T. Koehler), (B) Lysimeter experiment with beech seedlings at the TUMmesa climate chamber facilities/ Germany in collaboration with the Tree Growth and Wood Physiology group at TUM (by T. Koehler), (C) Measuring gas exchange in beech seedlings at TUMmesa climate chamber facilities/ Germany (by T. Koehler with permission of Hegarty Philip). 

Background

Plant productivity is tightly linked to plant water loss, and it is increasingly threatened by escalating drought. Crop physiologists and breeders are urgently seeking plant water use strategies and traits that can improve crop yield under drought. Certain plant traits and water use strategies can enhance crop performance under drought [11]. For example, smaller root xylem vessels (hollow tubes that conduct water from the roots to the shoot) have been shown to reduce vegetative biomass loss under water stress in maize [12] and provide a yield advantage of 3-11% in wheat [13]. In contrast, recent studies suggest a larger xylem area could also increase pearl millet yield under drought [14].

The impact of traits like xylem morphology on crop water use regulation is expected to vary depending on the specific drought scenario a plant encounters, a nuance that is often overlooked in scientific literature. Drought can manifest belowground due to soil water limitations (soil drought), or aboveground when the air is hot and dry (atmospheric drought). Traits that enhance water transport and use under high evaporative demand may differ from those advantageous as the soil dries [9]. Additionally, the relative importance of plant hydraulic traits during drought varies between soil textures [15]. 

Recognizing the complex spatiotemporally dependent trait-environment interactions, the resulting vast performance landscape is challenging to empirically assess during phenotyping and breeding [16]. Crop models are a valuable tool for evaluating combinations of various functional traits in a range of possible environments in silico [17]. While these models may aid breeding decisions, they currently do not incorporate plant/root hydraulic traits, which are increasingly recognized as essential for evaluating plant responses to drought across different agroecosystems [18]. Integrating soil-plant hydraulics into crop models could significantly enhance their ability to predict drought and water-use-related effects on yield.

Objectives

Our first objective is to experimentally quantify which role xylem morphology plays in the water use regulation during increasing VPD (atmospheric drought) and during decreasing soil moisture (soil drought) across contrasting soil textures in pearl millet in collaboration with the Diversity-Adaptation-Development of Plants group at the Institut de Recherche pour le Développement (IRD) Montpellier. 

The second objective is to use the data generated to parameterize a soil-plant hydraulic model to predict water use under varying drought regimes [9]. 

Ultimately, the aim is to integrate soil-plant hydraulics into crop models. 

Group members

TBA (Master’s student)

Funding

BayFrance (Bayerisch-Französisches Hochschulzentrum)

Fig. 2: Plant water use in responses to drought in crops: (A) Greenhouse experiment with pearl millet, sorghum, and maize at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) Hyderabad/ India in collaboration with the crop physiology group at ICRISAT (by T. Koehler), (B) Cavicam installations in maize in the greenhouse facilities at the University of Tasmania/ Australia (UTAS) in collaboration with the plant physiology group at UTAS (by T. Koehler), (C) Sheltering field-experiment with maize at the RhizoTraits experimental site in Schönburg/ Germany (by N. Tyborski).