Assessing stomatal and non-stomatal limitations to carbon assimilation under progressive drought in peanut (Arachis hypogaea L.)

Authors: PILON, CRISTIANE - University Of Georgia; SNIDER, JOHN - University Of Georgia; Sobolev, Victor; CHASTAIN, DARYL - Mississippi State University; Sorensen, Ronald - Ron; MEEKS, CALVIN - University Of Georgia; Massa, Alicia; Walk, Travis; SINGH, BHUPINDER - Mississippi State University

Submitted to: Plant Physiology Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2018
Publication Date: 9/20/2018
Citation: Pilon, C., Snider, J.L., Sobolev, V., Chastain, D.R., Sorensen, R.B., Meeks, C.D., Massa, A.N., Walk, T., Singh, B. 2018. Assessing stomatal and non-stomatal limitations to carbon assimilation under progressive drought in peanut (Arachis hypogaea L.). Plant Physiology Journal. 231:124-134. https://doi.org/10.1016/j.jplph.2018.09.007.
DOI: https://doi.org/10.1016/j.jplph.2018.09.007

Interpretive Summary: One of the mechanisms by which drought limits productivity in field crops is by decreasing source strength, which is the product of leaf area and average photosynthetic efficiency of all leaves in the canopy. Furthermore, there has been a long-running debate on whether photosynthesis is primarily inhibited under drought by diffusional limitations to CO2 availability at the carboxylation site of rubisco (stomatal limitation) or impairment of biochemical processes required for carbon assimilation (non-stomatal limitation). Quantifying the response of net photosynthesis (AN) to leaf internal CO2 (Ci) concentration has provided valuable insight into the underlying mechanisms contributing to photosynthetic limitations under a range of experimental conditions. A relatively novel method to rapidly assess A-Ci responses (termed RACiR) exploits the short response time and the high frequency data collection of the LI-6800 portable photosynthesis system (LI-COR Inc., Lincoln NE) to generate A-Ci response curves in as little as 5 min with comparable results to traditional A-Ci methods, thereby increasing the number of samples that can be measured in a given window of time. While the aforementioned approach would lend itself to measurement of water deficit stress responses, there have been no studies published to date that have utilized RACiR analysis to quantify photosynthetic responses to drought in field-grown plants. Another consequence of drought stress is an increase in leaf temperatures due to low stomatal conductance and limited transpirational cooling. Chlorophyll fluorescence-based methods have been used successfully to quantify thermotolerance of multiple plant species under field conditions. These approaches typically involve collecting leaf samples from the field, exposing them to increasing incubation temperature in the laboratory, and quantifying high temperature thresholds for fluorescence-based responses.We hypothesize that (i) the impairment of photosynthetic process of peanut plants would be driven by stomatal and non-stomatal limitations, and as drought progresses, non-stomatal limitations would play a more relevant role in reducing photosynthesis, (ii) primary photochemistry of PSII will remain stable and become more heat tolerant in response to drought, and (iii) gs will serve as a broadly applicable indicator of water deficit in peanut, regardless of the underlying processes actually limiting AN. Therefore, the first objective of this study was to assess the underlying limitations to photosynthesis in peanuts grown under progressive drought using survey measures of gas exchange and fluorescence, PSII thermotolerance assessments, and pigment analysis. The second objective was to utilize rapid A-Ci response curves to document the underlying limitations to AN in peanut. All plants were grown under rainout controlled shelters (5.5 m × 12.2 m), which are equipped with sensors to close at the first drop of rain. Underneath the frame of the shelter, an irrigation system was installed to irrigate all or partial areas under the shelter. The drought treatment was imposed on July 15 in 2016 and July 7 in 2017 when the rainout shelter, which moves back and forth along rails to cover the plots, was programed to close in response to rainfall. Water was withheld from the drought plots until August 4 and 7 in 2016 and 2017, respectively. After that date, the drought plots were irrigated to fill the soil profile and then received full irrigation for the rest of the season. The fully irrigated plots received irrigation as needed using soil water potential sensors installed at 10 and 20 cm soil depth. An irrigation event occurred when the average soil water potential of the two sensors average -40 kPa. Physiological measurements were conducted on the uppermost, fully-expanded, mainstem, tetrafoliate leaf (second unfurled leaf node belo

Technical Abstract: Drought is known to limit carbon assimilation in plants. However, it has been debated whether photosynthesis is primarily inhibited by stomatal or non-stomatal factors. The objective of this research was to assess the underlying limitations to photosynthesis in peanuts (Arachis hypogaea L.) grown under progressive drought. To this end, field-grown peanut plants were exposed to either well-watered or drought-stressed conditions during flowering. Measurements included survey measurements of gas exchange, chlorophyll fluorescence, PSII thermotolerance, pigment content, and rapid A-Ci response (RACiR) assessments. Drought significantly decreased stomatal conductance with consequent declines in photosynthesis (AN), actual quantum yield of PSII, and electron transport rate (ETR). Chlorophyll A was generally stable between treatments, while chlorophyll B was either increased or decreased by drought, depending on the severity of stress. Carotenoids were unaffected by drought. Stomatal closure on stressed plants resulted in higher leaf temperatures, but PSII thermotolerance was only affected by drought on one date. A strong, hyperbolic relationship was observed between stomatal conductance, AN, and ETR. However, when RACiR analysis was conducted, drought significantly decreased AN at Ci values comparable to drought-stressed plants, indicating non-stomatal limitations to AN. The maximum rate of carboxylation and maximum electron transport rate were severely limited by drought. Thus, while stomatal conductance may be a viable reference indicator of water deficit stress in peanut, we conclude that declines in AN were largely due to non-stomatal limitations. Additionally, this is the first study to apply the rapid A-Ci response method to peanut, with comparable results to traditional A-Ci methods.