A couple of summers ago, I grew a pot of cosmos from seed. These beautiful and healthy plants grew vertically to a great height, and the stems were an impressive circumference to an extent I had never seen. Sadly, they did not produce one blossom. However, I felt that harvesting firewood from the stems more than compensated for the lack of colorful blossoms one normally plants for. Besides, it gave me a great story to share with my friends; aside from enjoying the hyperbole, it sent me on a search for why this happened.
Having had an introduction to plant hormones during my Master Gardener training, I began my investigation there. Maybe you have been introduced to the five major hormones in the plant world: the usual suspects are auxin, gibberellin, cytokinin, ethylene and abscisic acid, which work separately and together to influence plant growth. Well, my cosmos had had a significant dose of something.
Right away, I affixed my Master Gardener ID badge to my shirt. Channeling my gardening guru, we quickly discarded ethylene, as it is a hormone that affects ripening and rotting. You’ve all heard the tale of the really ripe brown banana’s effect on the green bunch: Exposure to the ethylene gas ripens those green bananas quickly. This was not my cosmos’s issue.
So we’re down to four main hormones: auxin, gibberellin, cytokinin and abscisic acid. Now it gets more difficult.
Hormones get things done. They are usually found in very small concentrations. Auxin, discovered in the late 1920s, is responsible for stimulating growth (aha! We have a suspect here). If a plant does not naturally produce auxin itself, it will die. Auxin is involved in cell growth and cell expansion; it is found in the very tip of the stem as it is actively growing. Auxin moves downward in a plant from the stem to the root. Could this be the culprit? Auxin maintains apical dominance; it prevents lots of lateral buds and branches from growing. My guru looks at me and shrugs. Oh great, I’m on my own. There was no lateral growth to speak of on my cosmos stalks.
Let’s examine gibberellin, another suspect. It causes some effects similar to auxin, but it is a very different hormone. Research proves that it plays an important role in several developmental phases of a plant. It is notorious for causing the elongation of the distance between nodes on a stem. A place where a leaf attaches is called a node, for those of you unfamiliar with this term. This space between nodes is called internodes — go figure.
That leaves two hormones of interest: cytokinin and abscisic acid. Let’s examine cytokinin first. This hormone stimulated plant cells to divide when in culture with auxin. Now they’re ganging up. Two hormones are working together. This duplicity is involved in both cell division and in the making of new plant organs such as roots or shoots. In the cell cycle, they encourage cells to divide.
Unlike auxin, cytokinin works in the opposite direction. It is produced on the opposing end of the plant, the apical meristems at the very tip of the roots. It travels up the stem, catching a ride with water, not exerting any effort or energy on its own. This lazy hormone did not likely deprive me of my cosmos blossoms, as we are not considering new roots or shoots — just making gargantuan stems of a great height minus blooms.
Let’s take a look at that last possible contender. Abscisic acid has much to do with water. It serves as a chemical messenger to alert the rest of the plant that it is water stressed when there is a drought. It basically shuts down the valves that release water (guard cells) so the plant does not lose any more precious water. Now if ever there were such a hormone that served as a chemical messenger to prevent budding and subsequent flowering, it surely would be our guilty party. Abscisic acid has the water function only. Surely we can rule this one out.
At this time Hercule Poirot, world-renowned detective, sporting a walking stick and a potted plant, enters the garden with our five hormone suspects. His eyes canvass the plants quickly. Dropping his monocle, his gaze hesitates on auxin and a muscle twitches as he passes cytokinin. Much as I thought! In a challenging voice and a confrontational gesture, he halts his pace in front of me, turning abruptly to face my smug expression. Surely I was right, it had to be auxin!
“What did you feed your plants?” he asks accusingly. Shaking, I quickly remove my MG ID badge slipping it into my pocket. To be continued next month. …
Kim Woodford, WSU Master Gardener Class of 2016, gardens in Westport.