Comparative photosynthesis physiology of cultivated and wild lettuce under control and low-water stress

Authors: Eriksen, Renee; Adhikari, Neil; Mou, Beiquan

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2020
Publication Date: 9/15/2020
Citation: Eriksen, R.L., Adhikari, N.D., Mou, B. 2020. Comparative photosynthesis physiology of cultivated and wild lettuce under control and low-water stress. Crop Science. 60(5):2511–2526. https://doi.org/10.1002/csc2.20184.
DOI: https://doi.org/10.1002/csc2.20184

Interpretive Summary: Lettuce (Lactuca sativa L.) is an important vegetable crop in the United States, but increased demand and reduced recharge to the aquifer systems in the major lettuce growing regions are making irrigation requirements more difficult and costly to meet. If lettuce production is to be sustainable in these regions, there is need for varieties with higher water use efficiency. The wild progenitor of cultivated lettuce, Lactuca serriola, is a common weed that is considered more drought-tolerant than its cultivated relative. We found higher water use efficiency in L. serriola accession US23UC96 was due to its deep tap roots and greater ability to convert carbon dioxide into sugar. We find that this increased ability to convert carbon dioxide to sugar is due to an enhanced ability to move carbon dioxide from the air to the sites of photosynthesis inside the leaf. We propose that breeders concentrate on improving water use efficiency of cultivated lettuce by increasing tap root depth and increasing the efficiency of carbon dioxide movement through the leaf.

Technical Abstract: Lettuce (Lactuca sativa L.) is an important vegetable crop in the United States, but increased demand and reduced recharge to the aquifer systems in the major lettuce growing regions are making irrigation requirements more difficult and costly to meet. If lettuce production is to be sustainable in these regions, there is a need to breed varieties with higher water use efficiency. The wild progenitor of cultivated lettuce, Lactuca serriola, is a common, ruderal weed that is generally considered more drought-tolerant than its cultivated relative. We compared the photosynthetic physiology of two representative accessions of these species under drought for 10 d, and found that limiting access to deeper soil moisture by limiting root depth makes L. serriola US96UC23 less drought-tolerant. Stomatal conductance in US96UC23 dropped earlier and appeared more responsive to drought than in L. sativa, ‘Salinas,’ and therefore water use efficiency was higher under moderate drought. Under prolonged and severe drought, however, stomatal conductance was more likely to reach levels indicative of biochemical limitations to photosynthesis (< 0.05 µmol CO2 m-2 s-1) in US96UC23. Carbon assimilation and carboxylating efficiency was generally higher in US96UC23, which we find is due to higher mesophyll conductance. There are no differences among species in concentrations of the carboxylating enzyme RuBisCO. Based on these data, we propose that breeders concentrate on improving tap root depth and mesophyll conductance in cultivated lettuce through study of enzymatic enhancers of mesophyll conductance, such as the activity of carbonic anhydrases or aquaporins.