Date Published: February 8, 2018
Publisher: Public Library of Science
Author(s): Dafeng Hui, Chih-Li Yu, Qi Deng, E. Kudjo Dzantor, Suping Zhou, Sam Dennis, Roger Sauve, Terrance L. Johnson, Philip A. Fay, Weijun Shen, Yiqi Luo, Ben Bond-Lamberty.
Climate changes, including chronic changes in precipitation amounts, will influence plant physiology and growth. However, such precipitation effects on switchgrass, a major bioenergy crop, have not been well investigated. We conducted a two-year precipitation simulation experiment using large pots (95 L) in an environmentally controlled greenhouse in Nashville, TN. Five precipitation treatments (ambient precipitation, and -50%, -33%, +33%, and +50% of ambient) were applied in a randomized complete block design with lowland “Alamo” switchgrass plants one year after they were established from tillers. The growing season progression of leaf physiology, tiller number, height, and aboveground biomass were determined each growing season. Precipitation treatments significantly affected leaf physiology, growth, and aboveground biomass. The photosynthetic rates in the wet (+50% and +33%) treatments were significantly enhanced by 15.9% and 8.1%, respectively, than the ambient treatment. Both leaf biomass and plant height were largely increased, resulting in dramatically increases in aboveground biomass by 56.5% and 49.6% in the +50% and +33% treatments, respectively. Compared to the ambient treatment, the drought (-33% and -50%) treatments did not influence leaf physiology, but the -50% treatment significantly reduced leaf biomass by 37.8%, plant height by 16.3%, and aboveground biomass by 38.9%. This study demonstrated that while switchgrass in general is a drought tolerant grass, severe drought significantly reduces Alamo’s growth and biomass, and that high precipitation stimulates its photosynthesis and growth.
Due to fossil fuel combustion and land-use change, global climate change has been accelerating over the past decades . Global land surface temperature is expected to increase 1.1–6.4°C by the end of the century. The increase in temperature affects the hydrological cycle, and causes more extreme precipitation events . For example, Easterling et al.  reported that intensity of precipitation during the growing seasons could increase, resulting in more droughts and flooding in the United States (US). Intense droughts and excessive flooding in California are projected to increase by at least 50% by the end of the 21st century . Changes in precipitation intensities will alter soil water availability and influence ecosystem productivity and biomass [5–11].
The primary objective of this study was to understand the physiological variables and growth responses of switchgrass to precipitation changes. We found that increased precipitations significantly enhanced plant aboveground biomass by stimulating leaf photosynthesis, but mostly, by increasing leaf biomass, stem biomass and height, compared to the ambient precipitation. The significantly reduced aboveground biomass in the severe drought (-50%) treatment was caused by the reduced growth of leaf and stem, not by leaf photosynthesis changes. Among all precipitation treatments, leaf photosynthesis and transpiration were significantly correlated and influenced by temperature. Plant growth and biomass were influenced by soil moisture and water use efficiency. This study provided direct evidence that sustained precipitation changes could have significant impacts on switchgrass physiology and growth, and severe drought would significantly reduce switchgrass growth and biomass. It is worth noting that biomass yield of switchgrass Alamo could be negatively affected by water logging due to heavy precipitation in poorly drained fields [48,49].
To conclude, we found significant effects of precipitation changes on leaf physiology and plant growth and biomass of switchgrass. Under the reduced precipitation treatments, leaf maximum photosynthetic rate and transpiration were not significantly influenced. But the severe drought treatment significantly reduced leaf biomass, stem biomass, and plant height, suggesting that lower water availability had more influences on leaf and stem development than physiology, and caused the reduced aboveground biomass. To develop high producing switchgrass cultivars for drier environments, attention should be paid to the traits related to leaf initiation and development. While switchgrass can tolerate drought conditions, precipitation increases could significantly enhance leaf photosynthesis, transpiration, and more on leaf and stem growth, and increase aboveground biomass. Thus, adequate irrigation under the drought condition could improve switchgrass growth and biomass. To verify whether the results from this mesocosm study could be applied in the field condition, more field experiments with multiple levels of precipitation treatment with switchgrass need to be conducted.