Frugal Euphorbia saves water

Euphorbia milii Crassulacean acid meta­bol­ism (CAM) is a car­bon fix­a­tion path­way that evolved in some plants as an adapt­a­tion to arid con­di­tions. In a plant using full CAM, the sto­mata in the leaves remain shut dur­ing the day to reduce evapo­tran­spir­a­tion, but open at night to col­lect car­bon diox­ide (CO2). The CO2 is stored as the four-carbon acid mal­ate, and then used dur­ing pho­to­syn­thesis dur­ing the day. Clever stuff.

C4 car­bon fix­a­tion is one of three bio­chem­ical mech­an­isms, along with C3 and CAM pho­to­syn­thesis, used in car­bon fix­a­tion. It is named for the 4-carbon molecule present in the first product of car­bon fix­a­tion in the small sub­set of plants known as C4 plants, in con­trast to the 3-carbon molecule products in C3 plants. Plants use CAM to dif­fer­ent degrees. Some are “oblig­ate CAM plants”, i.e. they use only CAM in pho­to­syn­thesis, although they vary in the amount of CO2 they are able to store as organic acids. Other plants show “indu­cible CAM”, in which they are able to switch between using either the C3 or C4 mech­an­ism and CAM depend­ing on envir­on­mental con­di­tions. Another group of plants employ “CAM-cycling”, in which their sto­mata do not open at night; the plants instead recycle CO2 pro­duced by res­pir­a­tion as well as stor­ing some CO2 dur­ing the day.

In the genus Euphorbia, a vari­ety of car­bon fix­a­tion path­wats are used. A new paper in AoB PLANTS exam­ines Euphorbia milii and demon­strates that this spe­cies can be con­sidered as CAM-cycling spe­cies, primar­ily inter­ested in water con­ser­va­tion rather than car­bon acquis­i­tion. The pos­sible occur­rence of C2 pho­to­syn­thesis mer­its research.


Crassulacean acid metabolism-cycling in Euphorbia milii. AoB PLANTS (2013) 5: plt014 doi: 10.1093/aobpla/plt014
Crassulacean acid meta­bol­ism (CAM) occurs in many Euphorbiaceae, par­tic­u­larly Euphorbia, a genus with C3 and C4 spe­cies as well. With the aim of con­trib­ut­ing to our know­ledge of the evol­u­tion of CAM in this genus, this study examined the pos­sible occur­rence of CAM in Euphorbia milii, a spe­cies with leaf suc­cu­lence and drought tol­er­ance sug­gest­ive of this car­bon fix­a­tion path­way. Leaf ana­tomy con­sisted of a pal­is­ade par­en­chyma, a spongy par­en­chyma and a bundle sheath with chloro­plasts, which indic­ates the pos­sible func­tion­ing of C2 pho­to­syn­thesis. No evid­ence of noc­turnal CO2 fix­a­tion was found in plants of E. milii either watered or under drought; watered plants had a low noc­turnal res­pir­a­tion rate ®. After 12 days without water­ing, the pho­to­syn­thetic rate (PN) decreased 85 % and noc­turnal R was nearly zero. Nocturnal H+ accu­mu­la­tion (ΔH+) in watered plants was 18 ± 2 (cor­res­pond­ing to mal­ate) and 18 ± 4 (cit­rate) μmol H+ (g fresh mass)−1. Respiratory CO2 recyc­ling through acid syn­thesis con­trib­uted to a night-time water sav­ing of 2 and 86 % in watered plants and plants under drought, respect­ively. Carbon iso­topic com­pos­i­tion (δ13C) was −25.2 ± 0.7 ‰ in leaves and −24.7 ± 0.1 ‰ in stems. Evidence was found for the oper­a­tion of weak CAM in E. milii, with stat­ist­ic­ally sig­ni­fic­ant ΔH+, no noc­turnal CO2 uptake and val­ues of δ13C inter­me­di­ate between C3 and con­stitutive CAM plants; ΔH+ was appar­ently attrib­ut­able to both mal­ate and cit­rate. The res­ults sug­gest that daily mal­ate accu­mu­la­tion res­ults from recyc­ling of part of the noc­turnal res­pir­at­ory CO2, which helps explain the occur­rence of an inter­me­di­ate value of leaf δ13C. Euphorbia milii can be con­sidered as a CAM-cycling spe­cies. The sig­ni­fic­ance of the oper­a­tion of CAM-cycling in E. milii lies in water con­ser­va­tion, rather than car­bon acquis­i­tion. The pos­sible occur­rence of C2 pho­to­syn­thesis mer­its research.


AJ Cann. ORCID 0000-0002-9014-3720

Alan Cann is a Senior Lecturer in the School of Biological Sciences at the University of Leicester and formerly Internet Consulting Editor for AoB.

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