Plasmodesmata density in vascular bundles in leaves of C4 grasses grown at different light conditions in respect to photosynthesis and photosynthate export efficiency

The study was conducted with young plants of four species of C4 grasses of three photosynthetic sub-types: Panicum miliaceum (NAD-ME), Panicum maximum (PEP-CK), and Zea mays and Digitaria sanguinalis (both NADP-ME) with the aim to verify the hypothesis that light growth conditions affect the density of plasmodesmata connecting the photosynthetically active chlorenchymatous Kranz mesophyll (KMS) and bundle sheath (BS), as well as vascular parenchyma (VP) cells, and that the density of the plasmodesmata limits the efficiency of photosynthesis and photosynthate export from the leaf.The most important ultrastructural difference between the LL (grown at 50 μmol quanta m−2 s−1), ML (grown at 200 μmol quanta m−2 s−1) and HL (grown at 1000 μmol quanta m−2 s−1) plants was the increase, with increasing growth illumination, in the density of plasmodesmata connecting KMS and BS cells and, to a lesser extent, of those between BS and VP cells. This tendency was observed for all C4 grasses tested, although the magnitude of the reaction was species-specific, with the weakest and highest increase noted for P. miliaceum and D. sanguinalis, respectively. The maximum net photosynthesis rate (measured at saturating photon flux) was much higher in HL than in ML plants (except for P. miliaceum) and it was correlated very well with the density of the KMS/BS plasmodesmata. The photosynthate export capacity of the plants, as characterized by the time of 14C-photosynthate transfer into the transport path, the fraction of newly synthesized photosynthates exported from the leaf, and the transport speed in the leaf blade showed that ML plants were source-limited and HL plants were sink-limited.Apparently, C4 grasses are able to adjust their photosynthetic apparatus to light growth conditions by changing the number of plasmodesmata connecting cells of Kranz mesophyll, bundle sheath and vascular parenchyma in proportion to the intensity of illumination, so that for plant grown at high illumination the transport of photosynthates ceases to be a bottleneck limiting the efficiency of photosynthesis.