The Partitioning of Photosynthetically Fixed Carbon in the Leaf Blade and Leaf Sheath ofPoa pratensisL.

Abstract

The partitioning of photosynthetically fixed carbon between carbohydrate fractions and the processes of export and storage were compared in mature leaf blades and sheaths of the grass Poa pratensis L. Most of the fixed carbon was destined for export from the leaf blade with only 1% of the carbon fixed during the photoperiod being stored after 24 h. Although most of the assimilates imported to the sheath from the blade were subsequently exported, there was some unloading and storage of assimilates. Autoradiography was used to compare the translocation of ¹⁴C-labelled assimilates through non-fed areas of leaf blade and sheath and revealed that the veins in the sheath showed a greater capacity for storage of assimilates compared to the leaf blade. Biphasic kinetics of sucrose and glucose uptake were observed in segments of leaf blade and sheath. Although similar carriers for each of the sugars appear to exist in the blade and sheath, the rate of uptake via these carriers was significantly lower in the sheath compared to the blade. Assuming that unloading proceeds via a symplastic pathway, it would appear that the conversion of sucrose to starch in the sheath could be an important means of regulating unloading and in determining sink strength of the sheath. It is concluded that although the net amount of sugars unloaded in the sheath is small, the storage of assimilates in the vein network could be an important means of buffering changes in sucrose concentration in the translocate during periods of fluctuating assimilation.