Jl. Araus et al., IMMUNOCYTOCHEMICAL LOCALIZATION OF PHOSPHOENOLPYRUVATE CARBOXYLASE AND PHOTOSYNTHETIC GAS-EXCHANGE CHARACTERISTICS IN EARS OF TRITICUM-DURUM DESF, Planta, 191(4), 1993, pp. 507-514
The presence and distribution of phosphoenolpyruvate carboxylase (PEPC
ase) in the glumes and immature grains of durum wheat (Triticum durum
Desf.) were studied by electron-microscopical immunolabeling of PEPCas
e with polyclonal antibodies followed by protein A-gold. Plants were g
rown under mediterranean field conditions and samples were obtained tw
o weeks after anthesis. In the kernels, high gold label was associated
with the unstained areas of the protein bodies of aleurone cells, whe
reas labeling in the cytoplasm and chloroplasts of the pericarp was sl
ight, although significantly above the background. In the glumes, high
gold label was only located in cytoplasmic granules (vesicles) of the
mesophyll cells, although labeling in the cytoplasm and chloroplasts
was also significantly above the background. These observations in imm
ature kernels and glumes are in accordance with the anaplerotic role o
f this enzyme, as evidenced in C3 plants. Measurements of apparent pho
tosynthesis and its O2 dependence and CO2 compensation concentration w
ere made on ears and flag leaves of durum wheat. In addition, an analo
g of phosphoenolpyruvate, hloro-2-dihydroxy-phosphinoyl-methyl-2-prope
noate, was used to inhibit PEPCase and, thereby, to assess the contrib
ution of the PEPCase to photosynthesis in detached ears. There was no
effect of the inhibitor on the apparent photosynthesis of ears. Wherea
s inhibition of apparent photosynthesis by 210 mL . L-1 O2 in flag lea
ves was typical of C3 Species, inhibition in ears was even greater. Th
e CO2 compensation concentrations in different ear parts were similar
to or higher than in flag leaves and the O2 dependence was also compar
able (about 70%). Therefore, gas-exchange data give further support to
the assumption that a C4 cycle is absent or limited to very low rates
in ears of durum wheat.