Trait - climate relationships of Sorghum crop wild relatives grown in a common environment

The C4 Andropogoneae grass, Sorghum bicolor is an important crop for food, forage and biofuel in tropical and subtropical, arid to semi-arid regions worldwide. There are 25 species in Sorghum, providing an important source of genes and traits to guide breeding programs for "future-proofing" sorghum against expected increasing frequency of droughts and heatwaves in coming decades.

We grew nine Sorghum species native to the northern Australian tropics under controlled glasshouse conditions, measuring traits related to plant performance and tolerances to heat and drought. First, we quantified relationships between traits and climate-at-origin (climate at the location from where each species was sourced), teasing apart the associations with site precipitation and temperature. Second, we quantified relationships between traits and climate niche width (climate range across a species' entire distribution), testing for putative trade-offs between performance and generality. Third, we quantified relationships among the measured traits.

Key findings included: (i) Species from drier sites exhibited higher intrinsic water use efficiency; (ii) Species from warmer climates had less negative turgor loss point; (iii) Photosynthetic temperature tolerance, Tcrit, showed quadratic trends with precipitation- and temperature-of-origin. (iv) Species with broader climate niches displayed lower photosynthetic rates under standard conditions yet strong photosynthetic performance across a wider range of leaf temperatures.

Physiological adaptations related to water use and tolerance to water stress were clearly evident in Australian Sorghum. Dry-site species seemingly economise on photosynthetic water use; hot-site species - subject to periods of high evaporative demand - display a drought avoidance strategy (early onset of stomatal closure), protecting tissues against drought-related damage. Heat tolerance traits showed few relationships to climate, or non-linear trends that were difficult to interpret. This study provides valuable insights for understanding climate adaptation among C4 grasses, potentially informing agricultural breeding programs in the context of climate change.