Wheat flowering: keeping cool under stress

Frederick Thomas Steinmeyer - The University of Reading.


The vulnerability of wheat to heat stress is a function of ambient temperature (e.g. Craufurd & Wheeler, 2009) and the degree of transpirational cooling, which is determined by vapour pressure deficit (VPD) and water availability (Amani et al., 1996). Climate change will affect the degree of heat stress experienced by plants through changes in (i) humidity; (ii) water availability; (iii) light levels (e.g. through cloud cover); and (iv) carbon dioxide concentration (ICPP09). Therefore the vulnerability of UK wheat lines to climate change must be considered in the context of the overall changes expected in weather patterns in the UK. Indeed higher humidity and high temperatures are likely to have to have greater stress effect than equivalent temperatures at lower humidity because transpiration cooling is reduced (Reynolds et al, 1999).

The most vulnerable stage in the plant life cycle to environmental stress is around flowering with pollen development demonstrating a greater vulnerability than ovule or stigma formation (Saini & Aspinall, 1982). Recent work has shown that flowering over a longer period has the potential to increase crop resilience through physiological escape (Lukac et al, 2012). Another mechanism linked to crop stress resilience is tolerance; more resistant CIMMYT wheat lines are able to keep canopy tissues cooler with the dominant mechanism potentially being evaporative cooling through the transpiration loss of water from the leaves (Reynolds et al, 1999; Pinto et al., 2010). For pollen development however, it is the temperature of the ear which is relevant for reducing heat damage. Ayeneh et al (2002) showed that glumes surrounding wheat florets are able to transpire, which indicates that a similar cooling mechanism may regulate floret temperature.

Recent work by Steinmeyer (unpublished) has identified significant variations in spike temperature between stress tolerant and sensitive varieties in CIMMYT germplasm. Further variation in spike temperature has been identified in the flower developmental stage (Steinmeyer, unpublished). Until now, the ability of wheat genotypes to keep the flowers cool has been overlooked in the selection of novel breeding lines. In the UK, avoidance/escape is the dominant strategy used to select earlier flowering varieties which avoid late season abiotic stress. However, the drought in the UK in Spring 2011 demonstrated sterility (empty grain sites) of early flowering lines was greater than in late flowering varieties (Jones & Lukac, 2011). As a consequence there is an urgent need to breed varieties with a tolerance to heat stress, as well as avoidance mechanism through adjustment of flowering time. Research is needed in the field to dissect the relationship between floret temperature, evaporative cooling and gross physiological adaptive traits including rooting depth which can be applied to future selection of UK breeding germplasm.

The studentship benefits from links with CIMMYT Global Wheat Programme; to work in extreme environmental conditions for genotype screening, and training in breeding selection for stress tolerance that can be applied to the UK. The University of Reading is an established centre for food security research with expertise in wheat flowering phenology, and climate change research.

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