Death and glory
When UK rock group The Rolling Stones sang ‘…you can send me dead flowers every morning…’, floral decoration was very far from their minds. However, dead and dried flowers used as decorations often last several years. One plant used in this way is Helichrysum bracteatum, one of the so-called ‘eternal’ flowers. In this species, the colour is provided by several rows of large, scarious (but otherwise corolla-like) bracts, as described by Nishikawa et al. (Kyoto, Japan, pp. 31–37). Bract water content is low, well below 50 % of the water content of growing tissues and much more typical of tissues containing high proportions of dead cells. So, at what stage do the bracts of H. bracteatum actually die? The authors used DAPI staining combined with fluorescence microscopy to examine cell nuclei; other aspects of cell structure and morphology were investigated with a range of microscopic techniques. Even in the unopened flower bud, loss of nuclei had started in the bract tip and this loss worked its way down towards the base of the bract as the bud expanded and then the flower opened. By the anthesis stage there were no nuclei in the upper half of the bracts (although cells at the base of the bract had retained their nuclei) and cells lacked all other organelles. Both the epidermal and the inner cells were secondarily thickened (in contrast to bracts and petals of a conventional flower) and, further, the cell walls showed birefringence, indicative of orientated cellulose microfibrils. The cells thus have some features of tracheary elements. However, in the bracts these features are present in epidermal and parenchymatous cells, leading the authors to postulate that they represent a new cell type. Here, then, is a specific form of programmed cell death associated with floral function and initiated very early in the life of the flower.
Once more unto the breach
The genus Cuscuta shows a strong evolutionary commitment to a particular lifestyle: all 145 species are holoparasitic vines and many of these are significant pests in agriculture. Further, along with other genera in the Convolvulaceae, at least some Cuscuta species exhibit physical dormancy of seeds, as discussed by Jayasuriya et al. (Lexington, Kentucky, USA and Taipei, Taiwan, pp. 39–48). A common feature of seeds with physical dormancy is the presence in the seed coat of a water gap, the initial point of entry of water into the seed, but it is not known whether this exists in Cuscuta. The authors have therefore investigated this and other features of dormancy in C. australis. The majority of seeds exhibited physical dormancy, which could be broken by scarifying the seed coat or by dipping the seeds into boiling water for 10 s or, more ‘naturally’, by incubating them wet in a temperature regime of 35/20 °C. Seeds were made sensitive to the latter dormancy-breaking treatment by dry storage at ambient laboratory temperature for 2 months. Breakage of dormancy was associated with opening of the hilar fissure; staining with an aqueous solution of aniline blue demonstrated that this was the route for water entry. No dye entered dormant seeds nor non-dormant seeds in which the hilar fissure had been sealed. The hilar fissure thus functions as a water gap, equivalent to the bulge adjacent to the micropyle in other members of the Convolvulaceae. Finally, there was evidence for sensitivity cycling, regarded as a strategy that enables dormant seeds to sense favourable conditions for germination and seedling growth. It is a relatively common phenomenon amongst physiologically dormant seeds but unusual in physically dormant seeds. Indeed, the authors state that this is the ‘second example of a species in Convolvulaceae that can undergo sensitivity cycling and the first demonstration in the only holoparasitic genus whose seeds have physical dormancy’.
Parasite prefers to do it on grass
Continuing the parasite theme, we now consider a hemi-parasite, Thesium chinensis an Asian member of the Santalaceae. Thesium is a widely distributed Old World genus, all members of which are parasitic or hemiparasitic. Several members of the genus have a nuisance value as agricultural weeds and thus it is important to gain knowledge of their ecology and host range. Suetsugu et al. (Kyoto, Japan, pp. 49–55) point out that for T. chinensis such knowledge is very fragmentary. Further, there is a world of difference in observing proximity in the field to potential host species and actually knowing that parasitism is occurring. There is also a difference between study of interactions with selected plants under controlled conditions and study of what happens in the wild. The authors first carried out an association analysis in the field. Of the 38 species recorded at the study site only two (Eragrostis curvula and Lespedeza juncea) were present at greater frequency than expected. When haustorial connections were examined by excavating around the plants, 22 of the 38 species were shown to be parasitized, indicating a broad host range such as is seen in many hemiparasites. However, there was clear evidence that some species were more effective hosts than others: estimation of haustorium numbers showed that for many host species numbers differed significantly from expected. On this basis, grass species (Poaceae) were very good hosts whilst members of the Rosaceae and Caryophyllaceae were poor hosts. Other families, such as the Fabaceae contained both good (L. juncea) and poor (Pueraria lobata) hosts. A slightly puzzling feature of the data is that haustoria on Fabaceae were larger than on other families. Nevertheless, the results indicate that T. chinensis has a clear preference for Poaceae whilst retaining the ability to utilize a range of other species when preferred hosts are not in reach.
Seed behaviour: can we blame the mothers?
In cool temperate regions, an unusually cool, wet summer often results in some cereal seeds germinating on the parent plant. This is an extreme case of a more general phenomenon, namely that the conditions experienced by the parent plant during flowering and seed set can affect the dormancy and subsequent performance of the seeds. Although there have been many investigations of this, one group that has not been studied are Australian native plants. To remedy this deficiency, Hoyle et al. (St Lucia, Queensland, Australia and Royal Botanic Gardens, Kew, UK, pp. 93–101) have studied the effects of parental environment on seed dormancy in Goodenia fascicularis, a species that has potential for use in re-vegetation programmes. In G. fascicularis dormancy is broken by stratification in warm and damp conditions. Germination itself has a strong requirement for light. Thus, seeds that are shed in spring and which lie on or near the soil surface delay germination until the more favourable autumn conditions develop. In investigations of the effects of parental growth conditions plants were grown under two temperature regimes, 39/21 °C and 26/13 °C. In both regimes, some plants were well-watered while others had a limited water supply. The effects of temperature on growth were readily apparent: plants in the cooler environment were taller, with greater above-ground biomass. They set seed later and produced fewer seeds than in the warmer environment. However, seeds from the plants in cooler conditions had a higher level of viability. Seeds from both sets of plants were, as expected, dormant at maturity; however, those from the plants grown under warmer conditions were less dormant in that it took shorter periods of warm stratification to break dormancy. Water supply also had effect: in both temperature regimes, the water-restricted plants produced fewer, less-dormant seeds. Seed biology is thus subtly affected by maternal environment, exhibiting a physiological fine-tuning rather than a dramatic change in behaviour.