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While walking in the north-west Highlands of Scotland in late May, I came across several small, shallow, clear-water pools. The range of terrestrial plants growing in these pools indicated that they were temporary, probably resulting from snow melt. Interestingly, while the grasses appeared similar to specimens growing on drier ground, some of the submerged dicots had elongated petioles, bringing the leaf-blades to the water surface. It is this phenomenon that has been investigated by Chen et al. (Nijmegen and Utrecht, pp. 1057–1067). They were particularly interested to know the extent of genetic variation within populations of Rumex palustris. Eight genotypes were selected from each of 12 populations; the populations were from either river flood plains subject to periodic and often deep flooding or from stagnant water. At equivalent developmental stages, plants were completely submerged for 17 days; control plants were not flooded. The major response in all populations was petiole elongation (up to 6-fold longer); laminas also elongated but to a much lesser extent. Leaves of flooded plants were thus longer and narrower. Laminas were also thinner and less dense. Comparison of the 3rd, 4th and 5th oldest leaves showed that the most recently formed leaves elongated the most. In both extent and timing of this response, there was considerable variation between genotypes within each population. Thus, maximum flooding-induced petiole elongation varied between 50 and 90 mm; most genotypes completed the majority of their elongation growth in the first 7 days of flooding but some showed the greatest elongation in the last 10 days. However, contrary to the authors' expectations, there was no correlation between habitat and the extent or timing of the response. The difference between habitats in the selective pressure operating on these traits is thus not large. Indeed, it may well be masked by other selective pressures imposed by a range of different environmental factors.
Many orchids achieve pollination by sexual deception. Male insects, often of a single species, are attracted to the flower as if to a female insect and attempt to copulate with it. Several Ophrys species provide good examples of this, with flowers apparently providing a strong visual cue for the visiting insect. However, although the visual cues are indeed spectacular they are far from being the whole story: scent is also very important and may involve emission of female pheromone-like signalling molecules. The relative importance of these two types of attraction has been studied in Ophrys arachnitiformis by Vereecken and Schiestl (Brussels and Zurich, pp. 1077–1084). This orchid, native to the Mediterranean regions of western Europe, is early-flowering and is pollinated by male plasterer bees (Colletes cunicularius). In respect of visual cues, it is interesting that there are two colour morphs, one with a green perianth and one with a white perianth. The relative frequency of the two morphs varies considerably between different populations. The authors analysed floral scents by GC-MS, showing that the scent is attributable to a series of straight-chain alkanes and associated alkenes. There was no difference between colour morphs in respect of total amounts or relative abundance of these compounds. Further, pollinators were attracted specifically to beads coated with these compounds but not to control beads. ‘Decorating’ the beads with either green or white perianths did not result in any greater frequency of pollinator visits to the beads. Finally, flowers from which the floral scent compounds had been removed by washing with hexane did not attract pollinators. These results thus lead clearly to the conclusion that it is floral scent that determines pollinator specificity in O. arachnitiformis. The pressures that have driven the selection of other floral traits remain unknown, as do the interactions of traits and ‘their changes in composition over evolutionary time’.
In the early days of my botanical career, phloem transport of sucrose was a controversial topic. Discussions often became heated; argument and counter-argument were exchanged with some vehemence. The situation is much calmer now. At least three phloem-loading mechanisms have been identified and there is general consensus about sucrose transport in the sieve tubes. Of those phloem-loading mechanisms, that involving the sucrose/protein symporter (the SUC2 protein) is the major one. However, Srivastava et al., at Denton, Texas (pp. 1121–1128) have shown that Arabidopsis thaliana plants with a T-DNA insertion in intron-2 of the SUC-2 gene can still complete their life cycle. Mutant plants contained transcripts of the first two exons of SUC-2 but no functional protein was produced. Indeed, protein produced from the truncated mRNA was unable to catalyse sucrose uptake into yeast cells, again indicating that this mutation (SUC2-4) is a null mutation. Plants harbouring the mutation were extremely stunted and ‘debilitated’ with much smaller cells than wild-type. They took twice as long as wild-type to come into flower, although flowering took place at the same developmental phase as in wild-type. Not all plants produced seed and even in those that did, the number of seeds was only a small fraction of the number produced by wild-type plants. Nevertheless, many of the seeds set on mutant plants were viable (69 % mean percentage germination, c.f. 98 % in wild-type). These results raise several questions. Why have not plants harbouring other mutant alleles of SUC-2 produced seeds? The authors suggest differences in growth conditions as an explanation. How, in the absence of this major sucrose symporter, did the plants achieve any significant growth and, having done so, how did they channel fixed carbon to the developing seed? The authors discuss these questions at some length. I simply comment that perhaps the phloem story is not as clear as I implied earlier.
The flooding theme is continued here but the focus shifts to a forage crop species, Lotus tenuis. As described by Manzur et al., at Buenos Aires (pp. 1163–1169), this perennial legume is flood-tolerant and the authors have investigated its responses to different degrees of waterlogging and submergence. Six-month-old plants of a commercial cultivar were subjected to four treatments: control, totally waterlogged soil (2–5 mm standing water), partial submergence (60 mm water) and total submergence; plant responses were monitored over a period of 30 days. The results were very clear: at all levels from biochemistry to growth patterns, the plant can alter its responses according how bad the situation actually is. For example, in waterlogged soil the porosity of roots, and to a lesser extent of shoots, increased, presumably because of aerenchyma formation. Shoot number, shoot growth and plant biomass accumulation showed little or no effect of this treatment. In partially submerged plants, root and shoot porosity increased further. Overall biomass accumulation and shoot numbers were lower than in controls but shoots were on average 30 % longer, thus bringing leaves above the water surface. This might lead us to think that shoot elongation would occur to an even greater extent in completely submerged plants. However, in these plants, there was no shoot elongation; shoot numbers did not increase and there was an overall decrease in biomass; shoot and root porosities were similar to or less than waterlogged plants. Further, unlike plants in the other three treatments, submerged plants showed significant loss of starch and soluble carbohydrates. Thus plants respond to total submergence by ‘sitting it out’ in a non-growing state, utilizing the carbohydrates stored in the crown and surviving on limited energy available from mainly anaerobic respiration. I end with two questions: how do plants perceive the extent of submergence and how does that perception lead to the responses discussed here?