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

Abstract

AbstractLettuce (Lactuca sativa L.) is an important vegetable crop; however, increased demand and reduced recharge to the aquifer systems in the major field 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 (WUE) that produce similar biomass under reduced irrigation. The wild progenitor of lettuce, Lactuca serriola L., 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 progressive low‐water stress for 10 d and observed that leaf intrinsic WUE was higher under moderate low‐water stress in L. serriola acc. US96UC23 than lettuce cultivar ‘Salinas’. Carbon assimilation (A) and carboxylating efficiency was generally higher in US96UC23 under control water treatments, which we determine to be due to higher mesophyll conductance (gm). No differences were observed between the two species in concentrations of the carboxylating enzyme Rubisco. We find mesophyll limitations to be the most significant relative limiting factor in Salinas. The domestication of cultivated lettuce from this wild relative involved intensive morphological alteration of the leaf, which appears to have led to significantly reduced gm and, concomitantly, A. Breeding for improved gm in lettuce may lead to increases in A and WUE under moderate low‐water stress.