Addressing the challenge of combined heat and drought stress for cereal production

Drought currently restricts global cereal production by c.10% and is projected to worsen with climate change1. Researchers have recently identified agrochemical compounds that can reduce the impacts of drought by modifying plant physiology2. These compounds, typically anti-transpirants, encourage stomatal closure which reduces water loss from plant leaves and increases the efficiency of water use - enabling "more crop per drop". Our CASE partner, Syngenta, a leading science-based agricultural technology company, wants to advance this research by screening c. 200 candidate compounds.
A downside of existing 'drought avoidance by stomatal closure' approaches is that drought often co-occurs with heat waves3.
Plants with enough water can normally keep themselves cool;
they release water when air temperature is above optimal, the water evaporates, and plant internal temperature is reduced below ambient (Fig 1, left side of curves). Plants without adequate water, or plants with disrupted stomatal functioning, cannot cool themselves, with negative consequences for yield (Fig 1, right side, also see 4). In our rapidly warming world, crop producers cannot rely on stomatal closure or other drought tolerance mechanisms that sacrifice cooling ability.

Our proposed project has two main objectives: in broad terms we want to increase understanding, and eventually predict, the effects of combined drought and high temperature on crop yield. Second, we want to use this biological understanding to identify novel compounds that confer drought tolerance while avoiding increased susceptibility to heat stress.

Hypotheses
1) Drought increases plant susceptibility to heat stress and vice-versa
2) Stomatal closure compounds, antitranspirants, while reducing drought stress, increase risk of heat stress

How
Our PhD student will use a method we have developed for quantifying effects of heat and drought separately and in combination (Fig. 1). They will be able to take advantage of advanced plant phenotyping infrastructure at Reading, Aberystwyth and Syngenta. We withhold water for different durations, allowing the soil to dry to specific moisture levels before exposing plants to temperature regimes in controlled environments. Plant responses are measured using three key variables: plant growth rate via multispectral 3D scans to provide detailed insights into effects of stress and different compounds on development; transpiration rate to understand effects of stress and compounds on stomatal closure; and leaf temperature (particularly the temperature difference between plants and surrounding air) to quantify stress. By following this approach the student will be able to robustly quantify the effect of different compounds on plant stress tolerance.

The PhD student will conduct most of their work on crop seedlings. The short timescales involved allow multiple trials and rapid screening across Syngenta's large number of candidate compounds. We will subject plants to two subtly different types of drought treatment; i) withdrawal of water, to understand real world differences in stress experience (and real world benefits of compounds), ii) plants held at constant levels of soil moisture (e.g. via re-watering) to understand differences in physiology between compound treatments under equal levels of drought and heat. Once we have identified promising compounds we will conduct a trial on plants during reproductive development, which represents a realistic use case of the compounds in the field and brings us closer to the farm.