PHYSIOL. PLANT. 63: 351-356. Copenhagen 1985 Respiration of crop species under CO2 enrichment Roger M. Cifford, Hans Lumbers and James L L. Morison Gifford, R. M., Lambers, H. and Mori.son. .1. I. L. 1985. Respiration of crop species inider CO, enrielinienl. - Physiol. Plant. 63: 351-356. Respiratory characteristics of wheat (Triticum aestivum L. evs Gabo and WW15), mung bean (Vigna radiata L. Wile/ek cv. Celera) and sunflower (Hclianlhus annmis L. CV. Sunfola) were studied in plants grown under a normal CO., concentration and in air eontaining an additional 340 (or 250) |il I ' CO,. Sueh an increase in global at- mospheric CO, concentration has been forecast for about the middle of the next cen- tury. The aim was to measure the effect of high CO, on respiration and ils com- ponenls. Polarographie and, with wheat, CO, exchange techniques were used. The capacity of the alternative pathway of respiration in roots was determined polar- ographically in the presence of 0.1 mM KCN. The actual rale of alternative pathway respiration was assessed by reduetion in oxygen consumption caused by 10 mM sali- eylhydroxamie aeid. Each species responded differently. In wheat, growth in high atmospherie CO, was a.s.soeiated with up to 45% reduction in respiration by both roots and whole plants. Use of respiratory inhibitors in polarographie measurements on wheat roots impli- cated reduetion in the degree of engagement of the alternative pathway as a major contributor to this redueed respiratory aetivity of high-CO, plants. No change was found in the total sugar content per unit wheat root dry weight as a result of high CO,. In none of the species was there an increase in the absolute, or relative, contri- bution by the alternative pathway to total respiration of the root systems. Thus the improved pluHosynthetie assiniila'te supply of plants grown in high CO, d\d not lead to inerea.sed diversion of earbon through the non-phosphorylating alternative path- way of respiration in the root. On the eontrary, in wheat grown in high CO, the re- duced loss of carbon through that route must have contributed to their larger dry weight. Additional key words - Atmospherie earbon dioxide, cyanide-resistant respiralion, gi'owth, Helianthus aniuius, Triticum aestivum, Vigna radiata. R. M. Gifford (tcpiint rcqucst.s) and./. /. L. Morison, CSIRO, Division of Plant In- dustry, G.P.O. Box 1600, Canberra, A.C.T. 2601, Australia: II. Lumbers, Univ. of Grotiingcn, Dept of Plant Physiology, PO. Box 14, 9750AA llarcn (GN). The Netherlands. ting ehange in erop eatbon balance. For exatnple, if sink Introduction ^^.^^^1,^ ^^^^.^ ^^^^^ ,^,, inctease suffieiently to utilize all the Atmospherie earbon dioxide eoneetitration is rising ovv- extra photosynthate, some of the excess ntiglit be lost by ing to fossil fuel burning and is likely to be double the an energetically wasteftil tespiratoty pathway. Lambers pre-industrial value around the middle of the next een- (1982) has gathered evidence that the cyanide-tesistant tury (Seidel and Keyes 198,3). This is expeeted to have alternative pathway of lespiration operating in roots direct effects on field crop yield. While the primary ba- niight funetion to dissipate excess photosynthetate (the sis for expecting crop yield to increase under elevated "energy ovetflow hypothesis"), but that work was tiot in CO, is stinuilalion of net photosynthetie CO, fixation ihe eontext of CO,-stinuilated net photosynthesis, (e.g. Giflotd 1982) involving, in part, decreased photo- The objeetive of the present work was to seek any respiration, any seeondary response of dark respiration change in respitation rates of wheat plants grown under to the ehange may need to be considered when predie- high CO, coticentration, and to examine root respira- Rcccived 22 August, 1984; revised 23 November, 1984 351 24 Pliysiiil. I'liinl. (>.1. I'lX.'i