Fruit Biology and Germination of Two Tropical Potamogeton Species

Summary The drupes or achenes of Potamogeton schweinfurthii A. Benn. and P. richardi Solms. can float for long periods and germination only occurs naturally once the fruits have sunk. The factors controlling their floating, sinking and germination have been investigated. After brief reference to water depths at which these species occur in Uganda, fruit structure is described. The epicarp and mesocarp are initially green in both species and remain green in ripe, floating fruits of P. schweinfurthii. The sheathed embryo lies along the rim of the stony endocarp beneath a lid which can be detached by surgery. Of the factors relating to buoyancy and sinking in P. schweinfurthii, daylight compared with almost total darkness decreased the rate of discoloration of the green floating fruits, the rate at which the pericarp disintegrated and the rate of sinking. Measurements of respiratory oxygen uptake, net photosynthesis rates and density changes, in a series of unripe to just‐sunk fruits of P. richardi, suggest that this light‐dependence of floating fruits is connected with the maintenance of constant volume and density through gaseous relations within an intact cuticle. During prolonged darkness, fruit density increased at constant mass by shrinkage of the soft parts, caused by degenerative changes and absence of photosynthesis. Neither species germinated in the dark, compared with 100% of light‐stored, sunken fruits or with similarly stored fruit from which epicarp and mesocarp had been scraped. In the light, substantial germination also occurred in both freshly shed and older floating fruits after surgical exposure of the embryo. No after‐ripening period is required. Presumptive evidence was obtained for the promoting effect of red light on germination in P. schweinfurthii. Using a calibrated Schwarz thermopile and narrow‐band interference filters, the promoting effect of 0.05 J/cm2 of red light was established for P. richardi fruits with exposed embryos. Photoreversibility was established by similar doses of far‐red.These irradiation values are characteristic of the phytochrome system. Embryos within the intact stony endocarp require an irradiation of 8–10 J/cm2 (652 nm) for promotion and an equivalent dose of far‐red for reversal. These findings are discussed in relation to the ecology and distribution of the species.