Understanding Plant Biology

We want our crops to grow with vigor and health, to produce lots of large tasty fruit with minimal disease and stress.

If we want to learn from the many informational articles available, and then put the best practices into effect on our own crops, we need to have a working knowledge of what’s happening inside the plant and better understand how each of the various recommendations we come across might affect other aspects of our crop.

Cells are the fundamental place to begin our exploration and the basic unit of life. Plant cells have their nucleus bound with a membrane (eukaryotic), and the DNA is enclosed within that nucleus.

The two most distinctive features of a plant cell are the cell wall outside this membrane and the organelles it contains. The wall is made of cellulose and its prime function is support and rigidity.

Cell Structure of a Plant

Plants are made of organs that are made from tissues. Tissues are simply groups of cells that work together to perform a specific job. Plant tissue, similar to human tissue in some ways, is built from specialized cells which in turn contain specific organelles that exist inside the cell itself.

Plant cell types are split into two groups: eukaryotic and prokaryotic cells. Prokaryotic cells do not have a nucleus or organelles. Bacteria are an example of a prokaryotic cell.

Eukaryotes

• Parenchyma cells are the most bountiful cell type in a plant. They have thin wall membranes as compared to others and make up the soft or herbal parts of plants. They are found on the insides of leaves, flowers and fruits. However, they are not part of the epidermis (outside skin) or veins.
• Collenchyma cells generally have thick primary walls composed of cellulose. These types of cells are narrowly elongated to provide structure, especially for areas of new growth. These cells are non-lignified, meaning they can stretch as the organ they comprise elongates.
• Sclerenchyma cells have thick, lignified, woody and stiff secondary walls and will quite often die when the plant is mature. They are mostly in the root system and remain after dying, providing woody or sturdy parts for the plant.

Cells make up tissues. There are three primary types of tissue in a plant:

• Dermal tissue is like a skin. Among other things, it protects the other plant organs from attack.
• Vascular tissue is responsible for transporting the nutrients from one part of the plant to another. These cells look very different from the other types.
• Ground tissue cells perform photosynthesis and store energy (as a sugar). Some also provide structural support.

Specialized tissues include:

• Meristem tissue, which may be at the tip, shoot or root of a plant. Meristematic cells give rise to the three fundamental cell types and comprise that tissue in many plants where growth takes place. These cells have the ability to divide and multiply. They have the same function in plants as do stem cells have in animals. When cloning your plants, it is vital to have meristem tissue present in the specimen. Apical meristem tissue will comprise the growing tip, or bud, on a branch.
• Mesophyll tissue is comprised of parenchyma cells. These type cells are mostly in the leaves and are loaded with chloroplasts. Within this group are palisade, located near the surface of the leaf, and spongy, located under the palisade group. Their primary role is photosynthesis.

Vascular or transporting tissues include:

• Xylem. Comprised of xylem cells, xylem tracheids, xylem tracheae, xylem fibers and xylem cells are also known as water-conducting cells. They are hard cells that bring water up to the leaves. They do not live past maturity but their cell walls remain to allow water to flow freely through the plant.
• Phloem. Comprised of conducting or sieve cells, companion cells and phloem parenchyma cells, these make up a system of cells that organizes itself to transport sugars produced in the leaves throughout the plant as needed. The cells in these tissues live past maturity.
• Cambium. Comprised of unspecialized meristem cells (a type of parenchyma cell), the cambium is a secondary vascular tissue and is key to successful grafting.

No matter what the cell or tissue type, food and oxygen is needed to survive and grow. Some break down food molecules to release the stored energy inside. This process is called cellular respiration. Oxygen is also used to process food through oxidation. Plants primarily oxidize sugar for food.

How do Plant Cells and Tissues Get Oxygen?

For the green tissues above ground, oxygen is a byproduct of the photosynthesis process. For root tissues which are in the dark and have no photosynthesis, this option is not possible. Root tissues must therefore take up oxygen from the soil in which they grow. The hair roots of the plant have a large surface to volume ratio and are semi-permeable. They can absorb oxygen from the pores in the soil.

Chloroplasts are key organelles within many plant cells for performing the function of photosynthesis. This function captures the energy from light provided. It is the chlorophyll within these chloroplasts that actually gives the ability to harvest energy.

Light energy is absorbed by the chlorophyll within a limited frequency range. The molecules within the chlorophyll will become excited, thus producing energy that is passed from one molecule to another. In the end, the electron in a molecule must be ejected from the chlorophyll in order for energy to be captured and used. The process of electron ejection takes place only within chlorophyll molecules, which are held in a protein complex known as a reaction center.

Understanding Cellular vs. Root Respiration

Mitochondria are the organelles within many plant cells that carry out the function of cellular respiration. Here, the cell converts the sugars or photosynthates back into energy for various life processes, including growth. This process is quite similar to oxidation that occurs when wood is burned.

So, the parallel image for these functions could be that photosynthesis creates, through cellular processes the glucose (or wood to burn), and then cellular respiration burns that wood to produce energy.

Because plant roots must breathe in oxygen to respire, the soil or other media they live in must provide this oxygen. In deep water culture, oxygen must be infused into the water for this purpose. In many other grow media, care must be taken not to overwater.

Soggy soil prevents oxygen from being present for the plant root system. In this case, the plant roots will soon no longer produce energy and fail. Water and nutrients will no longer be transported to the canopy above.

The plant wilts and eventually dies if not corrected. When soil no longer contains oxygen it is considered to be anaerobic and will eventually present a foul odor.

Often through this type of weakening, a plant becomes prey to diseases and insects. This is why when a gardener only treats the disease or controls insects it often is not long until that same plant once again is stressed and infested; this is mostly due to overwatering and soggy soil.

We know that we need to provide good radiant sunshine or synthetic lighting for our crop in order to obtain the amount of cellular photosynthesis our crops demand, but to ignore another part of this formula—thorough root respiration—only invites disappointment when it comes to maximum yields.

Our soil or grow media will have a positive effect on plant root respiration when it is adequately filled with oxygen.

Soil that is too wet and lacks sufficient oxygen will lead to plants with root systems that do not perform properly. In soil or other non-fluid grow media the goal is to fully saturate the medium during the watering cycle and then allow it to drain, evaporate and transpire through the plant long enough for healthy respiration to occur. Some plant species do much better in wet and soggy soil than others.

These types of plants are better capable of transporting oxygen produced by cellular respiration down to their roots. Where a plant can do this well, it can tolerate the lack of root respiration far better. Know this aspect of your crop, and do not keep the soil wet longer than that specie can handle.

Digging a little deeper into the science behind agriculture and horticulture will improve your understanding of various recommendations you receive.

When a recommendation does not seem to match the science, you will be tipped off to question it, and look deeper into the issue. Using a better understanding of plant biology to improve the vigor of your crops is a very satisfying experience.

Learn, take notes on each crop and be the best gardener you can be.

For more information, visit the reference webpage for this article. You will be able to read more about advanced plant biology terminology, and do additional research with several articles that have been referenced in this piece.