Graphic by Ross
In this month’s blog, we’re sticking with the third wedge of the Aquaponics Cycle: the plants.
Water and nutrients have been absorbed by the plant roots and made their way into the bottom of the “Xylem,” an elevator that will deliver them to the cells of the stems, leaves, flowers and fruits of our plants. How does that work?
The Xylem elevator is solar-powered, in a round-about way, so let’s start at the sun’s surface. The surface of sun is so hot, and its gravity produces so much pressure, that two atoms merge together into a new element. This process, called fusion, also releases light in the form of photons. Imagine three photons of different wavelengths: a red one, a blue one, and a green one. They get released by the sun during fusion, and speed across space. They pass by Mercury, Venus, and skirt by the Moon. They pass through Earth’s atmosphere and come careening towards the surface of our planet. They pass through the glass of a greenhouse roof, and just before they hit the ground, they are intercepted by the leaf of a tomato plant.
The green photon bounces right off. Reflected. That’s why the leaf looks green to us. But the red and blue photons are absorbed by chlorophyll pigments. When one of these photons hits a pigment molecule, it pushes an electron out of the pigment. Light energy is converted into electrical energy. This electron flows into an electron transport chain. The reactions in this chain split a molecule of water into hydrogen and oxygen (how plants produce oxygen), and the electron’s energy is used to form two molecules, ADP and NADPH. The cell can now use these molecules as energy sources for other processes. Electrical energy is stored as chemical energy. This is the first step of photosynthesis.
The second step of photosynthesis is the Calvin Cycle. This cycle uses the chemical energy in the ADP and NADPH to convert carbon dioxide into sugars, like glucose, which can be sent throughout the plant to fuel the operation and growth of the plant cells. To get carbon dioxide into the cell to create sugars, small pores on the plant leaves, called stomata, have to open to let carbon dioxide in. While the carbon dioxide is coming in, water vapour is released out of the plant through these same pores, a process called transpiration.
So what does this have to do with the Xylem elevator? Well, because water is a polar molecule with positively and negatively charged ends, it is chemically “sticky.” Water molecules are attracted/connected to each other like links on a chain. When a stomatal pore opens to let in carbon dioxide and a water molecule is released, that water molecule “pulls” on the water molecule behind it, which pulls on the one behind it, and so on, pulling the whole water column “chain” up the plant, stretching all the way down at the roots. In fact, the evaporation of water from the surface of the leaf helps create the pressure difference (suction) that pulls water into the roots in the first place. As the water is pulled up the Xylem and into transpiring tissues, the nutrients dissolved in the Xylem get pulled up the plant as well, into the tissues that need them. While most elevators work via pulleys, the Xylem elevator does it with suction.
From here, the nutrients can be split into two camps: mobile nutrients and immobile nutrients. Immobile nutrients (including Calcium, Iron, Zinc, Copper, Manganese, Boron, and Molybdenum) flow up through the Xylem, get integrated into new tissues, and can’t be moved around again after that, they’re stuck. So if the plant is suddenly unable to absorb enough of these nutrients to grow (e.g., there isn’t enough in the aquaponic system water), nutrient deficiency symptoms like yellowing leaves or stunted growth will show up on new leaves at the top of the plant. Mobile nutrients (including nitrogen, phosphorus, potassium, magnesium and sulfur) also flow up through the Xylem and get integrated into new tissues, but they can leave those tissues and move up into new growth if needed. So if the plant is suddenly unable to absorb enough mobile nutrients to grow, those nutrients will be “robbed” from old growth (e.g., the leaves at the bottom of the plant) and moved up to new growth, meaning the older/lower leaves will show deficiency symptoms first.
What can a plant do when faced with a nutrient deficiency? In my next blog, we’ll discuss how the plant changes itself, the environment around its roots, and barters with microbes to attract the nutrients it needs.
