Effects of mesophyll conductance and nitrogen content on carbon assimilation during low-water stress in leaf lettuce cultivars

Authors: Eriksen, Renee; KUMAR, PAWAN - Bayer Cropscience; SANDOYA, GERMAN - University Of Florida; ADHIKARI, NEIL - California Department Of Public Health; Mou, Beiquan

Submitted to: Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/6/2025
Publication Date: 4/13/2025
Citation: Eriksen, R.L., Kumar, P., Sandoya, G., Adhikari, N.D., Mou, B. 2025. Effects of mesophyll conductance and nitrogen content on carbon assimilation during low-water stress in leaf lettuce cultivars. Horticulturae. 11(4). Article 414. https://doi.org/10.3390/horticulturae11040414.
DOI: https://doi.org/10.3390/horticulturae11040414

Interpretive Summary: Breeding for low-water stress tolerance is challenging, but it is particularly challenging for crops such as lettuce for which biomass is the primary agricultural product rather than seed crops, such as corn or wheat. This is because plants in natural populations have evolved mechanisms to continue reproducing and producing seeds under low-water stress, and these mechanisms can be harnessed by breeders to develop low-water stress tolerant cultivars. In previous field trials, low-water stress almost always reduced leafy biomass production in lettuce, even in the crop wild relative; therefore, it is very difficult to harness the traits of wild relatives to breed for low-water stress tolerance in lettuce. The primary factor in determining biomass is photosynthesis, and the crop wild relative of lettuce has higher rates of photosynthesis than cultivated lettuce. This study tested the hypothesis that improved biomass production in certain cultivars of low-water stress tolerant lettuce cultivars is due to higher photosynthesis under low-water stress. The study further tested the hypothesis that higher photosynthesis is due to higher conductance of carbon dioxide through the leaf to the chloroplasts, and higher nitrogen content which can be used to produce the machinery in photosynthesis. The study measured photosynthesis, transpiration, mesophyll conductance (passage of carbon dioxide through the leaf), expression of genes related to photosynthesis and mesophyll conductance, and nitrogen content of the leaf. The study finds evidence of a mechanism of concentrating carbon dioxide in the leaf for improved photosynthesis that has not been previously described in lettuce. The study concludes that photosynthesis, mesophyll conductance, and nitrogen content is higher in at least one low-water stress tolerant cultivar, however a multitude of factors affect low-water stress tolerance in other cultivars.

Technical Abstract: Plants have evolved many mechanisms to acclimate to deficit soil moisture conditions, and breeders can use these mechanisms to develop crops with improved abiotic stress tolerance in irrigated agriculture. However, many of these mechanisms are not compatible with crops for which leafy biomass is the primary agricultural product, such as lettuce. Improving biomass production in lettuce under conditions that induce stomatal closure involves understanding traits that compensate for stomatal limitations during photosynthesis. We tested the hypothesis that cultivars with tolerance to stomatal limitations during low-water stress have higher carbon assimilation, which might result from higher mesophyll conductance or higher total nitrogen content. We found higher carbon assimilation in the tolerant cv. Slobolt and higher mesophyll conductance and nitrogen content in the tolerant cv. Australian. We sequenced the transcriptomes, and found an increased expression of transcripts involved in carbon assimilation during stomatal limitations in tolerant cultivars, including a carbonic anhydrase that may be involved in mesophyll conductance. We propose that breeding for improved and consistent biomass production in lettuce should focus on stacking traits of small effect, including improved nitrogen uptake and mesophyll conductance.