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The sunflower bed in the Understanding Plants area at the Botanic Garden is sporting young plants which are now growing at a rapid rate. In the first blog, we explained the creation of the new Understanding Plants area and how this particular bed will showcase the research on branching led by Sainsbury Laboratory Director, Professor Ottoline Leyser. Her group uses Arabidospis thaliana as a model plant. This is a brassica which won’t win a lot of prizes for beauty, and also has a very short lifecycle, so doesn’t make it as a botanic garden showstopper. As a result we chose to use sunflowers, (Helianthus), to represent the scientists’ research. The sunflowers should behave the same way – but in a far more dramatic manner.
Our Horticulturists initially planted double the numbers of sunflowers we needed, to ensure enough plants germinated to give uniform lines, and last week they thinned out the seedlings. The next step was to nip off the apical or top bud to encourage the plants to branch. However, we needed to be pretty certain the sunflowers would branch when we cut the apical buds off, rather than just stop growing, or die. This called for our own miniature trial.
Earlier this year, our head of Experimental section grew a small number of the dwarf Helianthus ’Big Smile’ in the reserve glass house behind the scenes at the Botanic Garden. He then nipped the apical buds off when some had produced, either, two, four or six nodes. A node is where the leaf stalks meet the main stem. Taking the tops off early, at two nodes, initiated branching, any later and the side buds were too slow to grow.
This week, to demonstrate science in action, sunflowers in the middle third of the bed were decapitated – i.e. the apical buds were removed. These buds make the plant hormone auxin. In order to grow, the apical buds have to get rid of this auxin and transport it down the main stem. However, as Professor Leyser explained in her talk for the Science on Sundays series last weekend, environmental conditions also play a significant role in influencing whether plants grow branches. For example, when plants can’t get all the nutrients from the soil, a less well-known plant hormone called strigolactone gets involved. Strigolactone is made in the roots and travels up the main stem. This then slows the downward flow of auxin, allowing side buds lower down the stem to get rid of their auxin, and transport it down the main stem. This means these side buds can then grow and form branches. Side buds compete with each other to do this. The bud that is most successful at getting auxin out and down the main stem, grows to form a branch. We are quietly confident that the dwarf sunflowers in the Garden will branch like the dwarf Helianthus ’Big Smile’ did in the Glasshouses. It will certainly be exciting to see the results.
However, different cultivars branch in different ways. We know from RHS Wisley’s sunflower trial last year – along with many a garden sunflower growing competition – when tall single stem sunflowers have their apical buds taken off, they tend to produce only one ‘leader’ branch rather than two, like we saw with the test dwarf plants.
In our Understanding Plants display, the tall sunflowers represent the competition between side buds as they jostle with each other to export their auxin and so form branches. Could strigolactone be also playing a role in these yellow giants of the Asteraceae family?
How plants branch is more than just fascinating science. More branches means more stems, leaves, and fruit, and ultimately bigger crop yields. Understanding how plants control branching and how environmental factors influence this decision, will help increase crop production – particularly important now reducing fertilizer use is a priority.
So if you want to see the result of all this competition, keep an eye on our sunflower bed in the Understanding Plants area. The sunflower plants at either end should grow skywards on single stems and those in the middle section that have been decapitated, should form branches. We are growing dwarf cultivars at the front of the bed, sloping up to the taller cultivars, with the giants towering at the back. They will surely give the science of branching the impact it deserves.
Alison Murray, Interpretation Associate, Education Department, Cambridge University Botanic Garden