RCA improves drought tolerance
Drought stress is a primary limitation to crop production, and important agroecosystems may face increasing drought risk as the result of global climate change. It is estimated that by 2030, 47% of the human population will inhabit areas of high water stress, with the majority of these areas in Asia and sub-Saharan Africa11. In Africa alone, up to 250 million people may be exposed to increased water stress by 2020 due to climate change 12, with an estimated tripling of agricultural water demand by 202513. The identification and understanding of traits improving crop drought tolerance are essential for the development of more drought- tolerant crops and cropping systems.
The basic challenge in improving the drought tolerance of annual crops such as maize is that such crops must grow quickly and the carbon they require from photosynthesis is gained at the inevitable expense of water lost through open stomates. Researchers have identified a number of traits that might help plants use acquired water efficiently and tolerate dessication, as well as a smaller number of traits that may assist soil water acquisition14. Many of these traits have been researched for decades and elegant physiological and biochemical studies have provided a solid foundation for focused research at the molecular scale. We now propose that RCA reduces root metabolic costs, enhancing root growth and thereby water acquisition under drought.
Fig. 4. 4a: Greenhouse mesocosms set up. High RCA RILs had less total root system respiration per unit root length (4b), greater length of the primary root and its lateral (4c), and greater maximum root depth (4d) than low RCA RILs. Each bar is the mean of 2-4 replicates of 3 RILs harvested 5 weeks after planting + SE (Zhu et al., 2010).
We have employed soil mesocosms in a greenhouse and field studies in rainout shelters. The greenhouse mesocosms are cylinders of soil (1.5 m by 15.24 cm, Figure 3a) which are permitted to gradually dry as single maize seedlings grow in them over 5 to 6 weeks, whereas in the field, rainout shelters (plastic-covered greenhouse structures on rails) automatically cover the field during rainfall events and uncover the field when the rain stops, permitting realistic simulation of drought conditions. Over several years we have employed these systems to compare the performance of closely related maize Recombinant Inbred Lines (RILs) of OH43 x W64a (OWRI), and intermated B73 by Mo17 (IBM) differing in RCA under drought and well-watered conditions. RILs ranged from 0% to 35% root cross sectional area as aerenchyma. In the mesocosms RILs with more RCA had less in situ root respiration (Figure. 3b), greater root length (Figure. 3c), and greater rooting depth (Figure. 3d) than RILs with less RCA. Screening of 21 IBM RILs showed that genotypic variation in RCA was correlated with a 53% increase in stomatal conductance under drought (F=3.1, p=0.08). Under water stress conditions in the field, high-RCA RILs had deeper rooting (Figure 4a), better leaf water status (Figure 4b), and 800% greater yield (Figure 4c) than closely related low-RCA RILs6. These results are the first evidence that RCA can improve drought tolerance.
Fig. 5. Under water stress conditions in the field, high RCA RILs had deeper rooting (5a), better leaf water status (5b), and better yield (5c) than closely related low RCA RILs. WS= “water stress”; WW= “well water”. Each bar is the mean of 4 replicates + SE. Bars with different letters are significantly different at p<0.05 (Zhu et al., 2010).