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

There is continuing controversy over whether a degree of C-4 photosynthetic metabolism exists in ears of C-3 cereals. In this context, CO2 exchange and the initial products of photosynthesis were examined in flag leaf blades and various ear parts of two durum wheat (Triticum durum Desf,) and two six-rowed barley (Hordeum vulgare L,) cultivars, Three 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 greater than that in flag leaves. The O-2 dependence of the CO2 compensation concentration in durum wheat ear parts, as well as in the flag leaf blade, was linear, as expected for C-3 photosynthesis. In a complementary experiment, intact and attached ears and flag leaf blades of barley and durum wheat were radio-labelled with (CO2)-C-14 during a 10s pulse, and the initial products of fixation were studied in various parts 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) cycle, with little fixation of (CO2)-C-14 into the C-4 acids malate and aspartate (about 10% or less). These collective data support the conclusion that in the ear parts of these C-3 cereals C-4 photosynthetic metabolism is nil.