LACK OF C-4 PHOTOSYNTHETIC METABOLISM IN EARS OF C-3 CEREALS

There is continuing controversy over whether a degree of C-4 photosynt hetic metabolism exists in ears of C-3 cereals. In this context, CO2 e xchange and the initial products of photosynthesis were examined in fl ag leaf blades and various ear parts of two durum wheat (Triticum duru m Desf,) and two six-rowed barley (Hordeum vulgare L,) cultivars, Thre e weeks after anthesis, the CO2 compensation concentration at 210 mmol mol(-1) O-2 in durum wheat and barley ear parts was similar to or gre ater than that in flag leaves. The O-2 dependence of the CO2 compensat ion concentration in durum wheat ear parts, as well as in the flag lea f blade, was linear, as expected for C-3 photosynthesis. In a compleme ntary experiment, intact and attached ears and flag leaf blades of bar ley and durum wheat were radio-labelled with (CO2)-C-14 during a 10s p ulse, and the initial products of fixation were studied in various par ts of the ears (awns, glumes, inner bracts and grains) and in the flag leaf blade, All tissues assimilated CO2 mainly by the Calvin (C-3) cy cle, with little fixation of (CO2)-C-14 into the C-4 acids malate and aspartate (about 10% or less). These collective data support the concl usion that in the ear parts of these C-3 cereals C-4 photosynthetic me tabolism is nil.