The Physiology of Garden Love

When the days begin to warm, the gardener bides her time. Once the soil is also warm and the frost promises to hold off until fall, a new growing season can begin. The gardener wasn’t idle while her soil was frozen and covered in snow. She pored over seed catalogs and diagrammed the most efficient use of space or most beautiful arrangement of ornamentals. When only a few weeks remain before soil fully thaws, the gardener peels open fresh packets of seeds and tucks them into potting soil where they’ll become seedlings before braving the outdoors.

Seeds have thick protective coats housing an embryo inside that will eventually sprout to become roots and foliage. That protective coat prevents water from entering or exiting the seed and also limits its interaction with gases in the air. For a seed to sprout, it must shed the seed coat.

Tomatoes, kale, eggplant, leeks, broccoli, basil. Tasty recipes emerge from the soil, unbending their first stem as though letting out a slow yawn. Slowly, leaves take on their unique shape, texture, and scent. The gardener has anticipated the scent of fresh basil since plucking the last withered herbs in October. A tomato’s velvet stem reminds her of the supple fruit it will yield in July.

Temperature is crucial in shedding a seed coat. It isn’t simply that seeds need to be warmed, many seeds require warmer temperatures during the day and cooler temperatures at night. Holding a seed at a constantly warm temperature reduces germination rates and can lead to poor plant growth. Once a seed senses the proper temperature alternation, usually 5-10ºC difference between day and night, it sheds the seed coat and begins to interact with water, gases, and nutrients around it.

As the gardener patiently waits for her orderly array of tiny green plants to become robust and ready for the world, she can enjoy the first flowers of spring. Little purple, yellow, and white crocuses dot the front yard, hinting at the colorful rejuvenation the earth is about to experience. The crocus flowers bask in the noon rays and then quietly close their petals in the evening, sleeping like the gardener, who dreams of transplanting.

Deciding when to emerge from a seed coat is one of many instances where temperature influences plant development. Some flowers, like tulips and crocuses, close in cooler temperatures and open in warmer temperatures. When ambient temperature increases, growth of cells on the inside of petals greatly increases, pushing the flower open. Conversely, temperature decreases spur cell growth on the outer petal surface, causing petals to fold together.

The gardener pampers the seedlings as they grow. Water drips from their tiny leaves after she carefully spray each tray until reaching adequate soil moisture. A full spectrum light provides all of their electromagnetic needs. She thins plants that crowd one another and treats herself to the first fresh flavors of the season by rolling extracted sprouts around on her tongue. Even after two weeks of growth, her palette identifies broccoli leaves without reading their label. The red cabbage is developing too, she recognizes its purple tinged leaves before the flavor reaches her mouth.

A plant’s color, or pigmentation, indicates the presence of different of molecules. Molecules absorb and reflect different wavelengths of light. When a molecule can’t absorb a wavelength visible to the human eye, it appears to be the color of the unabsorbed light. Chlorophyll is a molecule that helps plants turn sunlight into energy and because of that function, growing plants put it anywhere light can be captured. Chlorophyll is responsible for the vibrant green color of a growing plant because it absorbs blue and red light but reflects green. Another group of pigments, flavonoids, protect plants from ultraviolet rays and attract pollinators. Flavonoids are responsible for the red of a rose and the purple of a violet. In addition to reflecting red hues of light, flavonoids also reflect invisible ultraviolet rays. By reflecting ultraviolet rays, flavonoids prevent damage to the plant and also create attractive patterns to insects that recognize ultraviolet light. Just like people can see the purple design flavonoids create in cabbage’s cross section, insects see ultraviolet flavonoid designs.

Finally, the gardener decides the plants are adequately developed and outdoor temperatures are conducive. Gingerly removing the plants from their black plastic casing, the gardener prepares to deliver them to their final earthen address. Roots encircle the potting soil under each seedling, dashing around the limited rectangular resource, worried by impermeable plastic walls. The gardener untangles the roots and placates their desire for more nutrients and water by delivering them into loosened soil.

Cluster root image from Wikipedia

As roots extend through soil, they create a haven for microorganisms. Bacteria colonize the area around roots because they are attracted to sugars that root tips secrete as they grow. Some bacteria change soil nutrients into forms that plants can use, this function can be especially important for rendering nitrogen available to plants. In this way, roots and bacteria feed one another and are symbiotic. Sometimes bacteria mimic a hormone produced by plants to initiate more growth. If a bacteria colony releases a plant growth hormone, it can result in dense, localized root growth called a cluster root. Bacteria can even colonize the root itself, enlarging a root and causing it to form nodules.

Pleased with her progress, the gardener wipes her hands on her legs and rises from her knees. Seedlings line recently hoed rows, looking frail in their expanded environment. But at some point a gardener must trust the plants’ physiology. As the gardener retreats, already their chlorophyll is collecting sunlight and bacteria is befriending their roots.

Physiology knowledge referenced from Introduction to Plant Physiology by William G. Hopkins and Norman P. A. Hüner, 3rd Edition.


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