The role of ethylene in shoot elongation with respect to survival and seed output of flooded Rumex maritimus L. plants

R um ex m aritim us L. occurs in fre­ quently flooded sites of lowland flood plains. Upon submergence this species exhibited rapid elongation of shoots, but the response depended upon the developmental stage when flooding was initiated. In the rosette stage, petioles showed a fast and large response; during early stem elonga­ tion the response of petioles was less, but the extension of the lower internodes considerable; during flowering stem development, high internodes extended and the contribution to final stem length diminished. Ethylene production by an intact plant before, during and after submergence was measured with a laser-driven photoacoustic technique. Internal ethylene concentrations increased within 12 h of submergence. Ethylene accumulated in the submerged plant due to increased synthesis as well as a reduced diffusion from the plant to the water (instead of air). The elongation response could in part be mimicked by exogenous ethylene. Directly after submergence a further increase of ethylene synthesis was observed, which may be of vital importance in causing shoots to continue their rapid elongation even after the water surface is reached. The $ responses of shoots were related to fitness in experimental field plots. Survival of submerged R. m aritim us depended on its ability to emerge above the water surface, while seed production was positively correlated with shoot height above the water. Key-words: Ethylene, fitness, flooding, in te rnode elonga­ tion, petiole elongation, Rumex maritimus, shoot e lon­ gation Introduction Downstream river systems in The Netherlands are frequently flooded, not only during winter high waters, but in the growing season as well (Van de Steeg, 1984; Brock, Van de Velde & Van de Steeg, 1987). Plant performance in these river areas depends upon mechanisms to survive and repro­ duce during fluctuating water levels, including periods of complete submergence (Blom et al., 1990). Several R um ex species are distributed along the river flood plain according to the eleva­ tion gradient. R um ex m aritim us L. occurs in the most frequently flooded areas and encounters flooding in several stages of its life cycle. Flooding imposes a severe stress on plants. Complete submergence of the shoot will result in reduced aeration of the plant tissues and an impaired photosynthesis (Ridge, 1987). Survival and sexual reproduction may depend on adap­ tations which restore contact between shoot and air. A commonly observed mechanism is the rapid elongation response of petioles or internodes, which is stimulated by ethylene accumulation following submergence. The occurrence of this so-called ‘depth accommodation’ and the role of ethylene in the process have been studied for amphibious plants since 1970 (reviewed by Osborne, 1984; Jackson, 1985; Ridge, 1987) and have been recently demonstrated for several Rum ex species (Voesenek & Blom, 1989a,b; Voesenek et al., 1990a). Not many studies, however, have been undertaken to assess the impact of this response on fitness. Ridge (1987) discussed the survival value of several morpho­ logical and anatomical changes induced by flood­ ing, including elongation responses. The importance of petiole elongation for the survival of Rum ex species was demonstrated by Voesenek (1990) and Laan (1990). In this study we investigated the impact of rapid shoot elongation on the fitness of flooded R. maritimus plants. An outdoor experiment was designed to address this question in plants at different stages of the life cycle. Survival and seed production were used to determine fitness. To 305 Shoot elongation upon flooding elucidate the mechanism behind the observed rapid elongation growth, the role of ethylene was studied in several laboratory experiments. To monitor production and accumulation of ethylene in submerged plants we used the recently developed laser-driven photoacoustic detection technique (Harren, 1988; Harren et al., 1990) that has been used previously for the measurement of ethylene production of waterlogged R um ex plants (Voesenek, 1990; Voesenek et al., 1990b). Materials and methods