Occurrence of distinctive cells and effects of irradiance on vascular formation in leaves of shade-tolerant C grass Paspalum conjugatum.

The denser leaf vasculature of C plants than of C plants may suit rapid export of assimilates associated with their higher photosynthetic rate. However, some C grasses have partially reduced leaf vasculature with vascular bundle (VB)-free bundle-sheath cells called distinctive cells (DCs). The shade-tolerant C grass Paspalum conjugatum has such a reduced leaf vascular system with DCs. We examined whether irradiance during growth affects vascular formation in leaves of P. conjugatum grown under 100%, 30%, or 14% sunlight for 1 month alongside the C grass maize. Under all conditions, P. conjugatum leaves had partially reduced vasculature: DCs and incomplete small VBs without phloem occurred between VBs with a normal structure consisting of both xylem and phloem. Shaded plants had less phloem in the small VBs than the full-sunlit plants. In maize, however, all VBs always had both xylem and phloem under all light conditions. The net photosynthetic rate of both grasses was reduced under shade; that of P. conjugatum was always lower than that of maize under all light conditions, but was reduced less by shade than that of maize. The light compensation point was lower in P. conjugatum than in maize, indicating that P. conjugatum acclimatizes better to low light. The reduction of phloem in VBs of P. conjugatum may be an acclimatization to shade, because dense vasculature may be expensive for C plants growing in environments where the higher photosynthetic rate is not realized.