Blooming bunches

How can a bunch of flowers bloom, when they've been cut from their roots?
14 April 2020

Interview with 

Alice Fairnie, Cambridge University


flowering lillies


Julie wanted to know how lilies can flower when they've been cut away from their roots, and placed in a vase. Plant scientist Alice Fairnie from Cambridge University's Sainsbury Laboratory got on the case...

Alice Fairnie - Flowers - they're the reproductive structures of a plant and they open for pollination and to produce seeds. Most flowers won't produce seeds and that's because they lack the energy that comes from their now detached roots. But cut flowers, do open. The answer to the incredible act of blooming is in the mechanisms of flower opening.

Katie - and we'll come onto those mechanisms shortly, but there are already a few clues in Julie's question. The lilies are in a vase of water and the bouquet was accompanied by what she referred to as the little salt sachet or packet. Alice says that for most flower species, opening and movement depends primarily on cell growth, elongation and expansion. And it's influenced by internal as well as external environments. Petal cell growth, elongation and expansion in turn depend on inputs, mainly water and heat. Remember Julie's vase was filled with water. Well, how about trying an experiment at home, split your cut flowers between vases with and without water and with and without the sun. Does this change how they open?

Alice Fairnie - You might also notice differences between flower species and their response to the different experimental conditions. This is because although cell expansion and growth can explain much of flower opening, there are internal and external influences that also influence opening and that vary between flowers. Different flowers for example, have different flower forms and the cell in different petal regions - for example, front versus back, the centre or the middle versus the edge - might expand at different rates and this will create different patterns of uncurling and opening. Different flowers are also found in different environments. Julie has lilies and I have tulips on my desk. I also work with a hibiscus.

Katie - Lilies, Alice explains, remain open after blooming. Tulips repeatedly open and close with fluctuations in their external environment, and the hibiscus flower that Alice works with is affectionately and perhaps aptly named flower of an hour because each flower opens only in the right environmental conditions and only for a day or sometimes an hour.

Alice Fairnie - But interestingly they do open when the flower has been cut and when that flower that is cut is without water. And this is because they're adapted to their environment in which they experience water shortages. Cotton stems, which are relatives of hibiscus, share the ability to open their flowers when they've been cut and without water. And this is because they are resistant to stem blockages. These stem blockages prevent water from reaching their flowers. They also store water in their cells if their flowers too, which means they can open even when the stem is without water. Cut roses, on the other hand, constantly need trimming to prevent water blockages and to let the water flow to their flower cells. Rose food, for example, will often include stem unpluggers - chemicals that stop the stem from blocking and ensure your flowers open. And this brings me to those little salt packages you mentioned. These packages do look like salt, but they're in fact plant food. A mixture typically of sugar and bleach, but sometimes these stem unpluggers. Why don't the packages contain salt? While salt is dehydrating, if we have salt in our vase or in our water, we draw water away from the cells and the stem and prevent flower cells from expanding.

Katie - So why is sugar in these packets? Well, sugar is a food for plants to promote cell expansion and growth. Just like the sugars and energy needed to fuel legs through a marathon, or my legs, a light jog. To increase the life of your cut lilies, Alice suggests, Julie, that you try removing a few of the dying flowers from a still blooming bud. This will help new buds access the limited sugar supplies rather than feeding the old dying ones. So in the packets of so-called plant food you get with a bunch of flowers there's sugar, chemicals to prevent the stems getting plugged up and bleach.

Alice Fairnie - Why is there bleach? Well bleach also helps with floral longevity by keeping the cut stems clean and preventing bacterial growth. You could experiment at home, add salt or sugar to your vase. Are there other nutrients that plants need to grow? Do they need hormones like us? You can even visualize water movement within the stems, just add a couple of droplets of food coloring to your vase, either with freshly cut flowers, old flowers stems, or stems that have been sealed at the base with vaseline that mimics the stem blocking.


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