GENETIC-VARIANTS OF PANICUM-MAXIMUM (JACQ) IN C-4 PHOTOSYNTHETIC TRAITS

During the evolution of the C-4 type of photosynthesis, the key C-4 en zymes levels have become increased, together with the development of t he typical C-4 <<Kranz>>-anatomy. The existence in nature of C-3-C-4 i ntermediate species suggest that C-4 plant variants hindered in their capacity to fix CO2 may exist. Seeds of the C-4 plant Panicum maximum (Jacq.) were mutagenized using ethylmethanesulfonate (EMS) and putativ e variants were isolated in the M(2) generation by visual inspection, considering morphology and pigmentation of the plant as criteria for v ariant selection. Variant plants and their progenies, when available, were characterized by photosynthetic, photorespiratory and leaf morpho logical parameters. The variants isolated were midribless (mbl), large interveinal space-1 and -2 (lis1, lis2), abnormal bundle sheath (abs) , high number of small veins-1 and -2 (hsv1, hsv2) and variegated leaf -1 (var1). The variant lis1 was a short plant with leaves smaller than in the wildtype, and had a leaf lamina with a crinkly surface and a l ighter green color. Photosynthetically, lis1 indicates a clear, but no t extreme, regression from C-4 to C-3 photosynthesis represented by a CO2 compensation value around 8 mu L.L(-1), which was correlated in th e leaf lamina with an increase in the distance between small veins. Th e leaf anatomy of the variant abs showed several alterations, includin g doublets of veins, veins without bundle sheath (BS cells), additiona l BS cells outside the veins or large BS cells participating in two bu ndle sheaths. The variant var1 had a variegated phenotype with stripes of yellow-green and white tissues alternating in the leaf laminae. In yellow-green sectors the chloroplasts were absent only in BS cells, a nd the adjacent mesophyll cells were less pigmented than similar ones present in non-variant sectors. Based on anatomical alterations induce d by the mutations isolated and characterized in this paper, it is con cluded that in P. maximum the establishment of the vascular system is under complex genetic control: all variants interfere with shape, orga nisation and distribution of midvein, lateral and small veins.