Colour blind
If I buy mauve, yellow and white cut freesias, the mauve ones always die first, then the yellow ones and finally the white ones. This happens regardless of the time of year. Why?
鈥� Senescence of flowers 鈥� fading and withering, followed by petal shedding 鈥� is a genetically programmed event. It is common among plants with flowers pollinated by animals or insects.
Senescence generally occurs either after fertilisation has taken place, and seeds begin to form, or if there is no fertilisation at all and the flowers begin to deteriorate. There are major differences in the speed and conditions under which senescence occurs in different flower types, and between different varieties of the same species. Flowers like orchids can persist for months without senescing.
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The advantage of senescence and shedding is that after pollination has been achieved, the plant conserves its energy and nutrients and removes a potential entry point for infection. Variety, age, plant hormones and environmental conditions 鈥� light, temperature, water, air quality 鈥� all influence senescence. In addition, the characteristics of senescence and plants鈥� responses to the environment are greatly influenced by plant breeding and genetics.
Because there is no clear relationship between flower colour and senescence 鈥� some red, white, yellow or blue flowers last for one or a few days, whereas others persist for weeks 鈥� the answer to the question must be in the breeding. I suspect there are small varietal differences between the freesias with different flower colours. In addition to having different flower colour genes, there are probably unseen genetic differences that affect the senescence process.
There are two ways you could test this theory. First, you could study the behaviour of freesia flowers obtained from different geographical regions or different seed companies, which would be likely to reveal varietal differences. Secondly, you could use genetic modification to produce a range of plants derived from genetically identical parents but with different flower colours. With such a range, you could be certain that the colour genes would be the only ones that varied, and you would expect all these flowers then to senesce at the same speed.
If they still showed differences we would have to consider a more complex theory, for example that somehow the biochemical pathway determining flower colour could also influence the speed of senescence, perhaps by influencing production of a hormone. This is possible, because there are biochemical relationships between the compounds that generate flower colour and biosynthetic pathways that lead to hormone production.
Don Grierson
Plant Sciences
University of Nottingham
Loughborough, Leicestershire, UK
鈥� There is no clear and unambiguous answer. In many species there is a clear variation in flower longevity between differently coloured varieties of the same species. For example, in the amaryllis Alstroemeria, yellow varieties have a much shorter vase life as cut flowers than do white or pink varieties. Treatments to extend their lifespan in the vase are effective on all varieties, but the extension is similar for each type, so the yellow varieties still last for less time than other colours.
Perhaps one should consider the function of a flower in the natural environment. Brightly coloured flowers have evolved to attract insect pollinators, and so if one colour is more effective at attracting pollinators then it does not need to have a long life. Other colours, perhaps attracting different species of insects that may be less abundant, may have evolved to last longer to increase their chance of pollination. In species where flowers change colour during their lives there is very good evidence to support the hypothesis that different insects are preferentially attracted to flowers of particular colours.
The alternative view is to consider the 鈥渃ost鈥� of a flower and the continuing cost of keeping it for longer than is actually needed to achieve pollination. Studies have suggested three main costs. The first is the waste of pollen when pollinators continue to visit a flower after pollination. The second cost is the considerable loss of water, and maybe nutrients, via transpiration. The final cost is that large attractive flowers, while inviting pollinators, may also attract herbivores. Any one of these costs may exert an evolutionary pressure on a species to produce flowers with a lifespan precisely tailored to the insects that pollinate them.
Tony Stead
School of Biological Sciences
Royal Holloway, University of London
Egham, Surrey, UK
In-flight meal
During migration the ruby-throated hummingbird (Archilochus colubris) tanks up with a few drops of nectar for the last time on the northern shores of the Gulf of Mexico. It then flies non-stop for at least 800 kilometres to reach the shores of the southern Gulf. Can anyone calculate the metabolic fuel efficiency of these birds that fly so far on so little, and how does this compare to a human?
This question may involve some erroneous assumptions. Before departing the Yucatan peninsula, a hummingbird spends weeks gorging on arthropods and does not merely consume 鈥渁 few drops of nectar.鈥� It puts on enough fat to nearly double its weight: a female can grow from 3.2 grams to around 6 grams, and can barely get airborne. When, after anything up to 22 hours, it reaches its destination, it will weigh around 2.7 grams, having consumed the fat and often some muscle tissue. Many do not complete the trip.
The average metabolic rate for the black-chinned hummingbird is 29.1 卤 6.3 kilojoules per day. A man, metabolising energy at the same rate, would have to consume twice his weight in meat a day, or 45 kilograms of glucose, and his body temperature would rise to over 400 掳C.
Lanny Chambers
St. Louis, Missouri, US
This week鈥檚 questions
Ant killer
Instead of resorting to toxic ant powder to deal with a plague of ants, is there any reason why you could not tame an anteater to do the job for you, providing a wholly natural solution to the problem? Would a single suburban garden be enough to sustain the anteater or would you have to hire it out to neighbours and friends?
David Byrne
Farnborough, Hampshire, UK
Slipping up
What is in a banana peel, but apparently no other fruit skin, that makes it so slippery?
Vivek Jain
Gujarat, India