Physiological Aspects of Sugar Exchange between the Gametophyte and the Sporophyte of Polytrichum formosum

The sporophyte of bryophytes is dependent on the gametophyte for its carbon nutrition. This is especially true of the sporophytes of Polytrichum species, and it was generally thought that sucrose was the main form of sugar for long distance transport in the leptom. In Polytrichum formosum, sucrose was the main soluble sugar of the sporophyte and gametophyte tissues, and the highest concentration (about 230 mm) was found in the haustorium. In contrast, sugars collected from the vaginula apoplast were mainly hexoses, with traces of sucrose and trehalose. p-Chloromercuribenzene sulfonate, a nonpermeant inhibitor of the cell wall invertase, strongly reduced the hexose to sucrose ratio. The highest cell wall invertase activity (pH 4.5) was located in the vaginula, whereas the highest activity of a soluble invertase (pH 7.0) was found in both the vaginula and the haustorium. Glucose uptake was carrier-mediated but only weakly dependent on the external pH and the transmembrane electrical gradient, in contrast to amino acid uptake (S. Renault, C. Despeghel-Caussin, J.L. Bonnemain, S. Delrot 1989] Plant Physiol 90: 913-920). Furthermore, addition of 5 or 50 mm glucose to the incubation medium induced a marginal depolarization of the transmembrane potential difference of the transfer cells and had no effect on the pH of this medium. Glucose was converted to sucrose after its absorption into the haustorium. These results demonstrate the noncontinuity of sucrose at the gametophyte/sporophyte interface. They suggest that its conversion to glucose and fructose at this interface, and the subsequent reconversion to sucrose after hexose absorption by haustorium cells, mainly governs sugar accumulation in this latter organ.