Botany for Gardeners: Photosynthesis (part 2)

Food begins with photosynthesis
All food begins with photosynthesis

(If you missed last week’s post about how photosynthesis works, you might want to read it now. I’ll refer to it below.)

As gardeners, we all want to grow healthy plants. Knowing what they need is helpful, but knowing why they need it is even better. Today I’m going to go over what plants need in order to feed themselves—and us. That’s what photosynthesis is for.

As I mentioned last Thursday, there are specific elements that form the building blocks of plant life. Some are incorporated into the structure of the plant, others are needed during the process of photosynthesis, but all are essential at one point or another. Too little (or too much!) of any of these elements will kill your plants.

Nutrients from the soil
The first step in making sure your garden has the resources it needs is a soil test. Here along the Front Range of Colorado, most soils have plenty of phosphorus (P) and potassium (K) but lack nitrogen (N). Adding a balanced fertilizer (containing N, P, and K in relatively equal amounts) can quickly elevate P & K to toxic levels; it’s often better to stick with just N.

As you recall, there are other elements needed as well: magnesium, manganese, copper, calcium, sulfur, boron, iron, molybdenum, and zinc. Again, most soils have plenty of these. If a test shows anything lacking, seaweed meal is a great source of micronutrients. The old garden advice to add Epsom salts to your soil stems from its composition—sulfur and magnesium. If your soil is lacking, adding Epsom salts will help. Otherwise, you’re wasting your money.

In many cases, plants can’t absorb these nutrients in their elemental form. They must be joined with other atoms to become available to the roots. Soil humus plays an important part in the conversion of basic elements into these molecules. The optimum humus level for most garden plants is about 5%.

H2O and CO2
What about the other elements necessary for photosynthesis—hydrogen, oxygen, and carbon—available as water and carbon dioxide? We all know that plants need water, but surely there’s plenty of CO2 in the atmosphere!

Tomato leaf stoma, WikicommonsSurprisingly, the way plants obtain these two molecules is interrelated. CO2 enters the leaf through pores called stomata (shown in this electron microscope photo from WikiCommons). These pores are usually open, allowing a two-way exchange of gasses—including water vapor. However, if the plant is water-stressed it can close its stomata, minimizing water loss. The problem is that by closing its stomata, the plant also prevents CO2 from entering. Eventually photosynthesis stops. The plant goes dormant or, if the drought lasts long enough, it dies of starvation.

Of course, water is also used directly in the photosynthetic process, so it’s doubly important. However, too much water can rot plant roots. The right amount of water is so important that plants have all sorts of ways to manage it.

Atriplex canescens - Fourwing Saltbush @XG 9aug05 LAH 142Plants adapted to dry climates (like this Fourwinged Saltbush) may have smaller leaves that have waxy coatings on their upper surface, minimizing water loss. Some have leaves that curl downward, raising the humidity underneath where the stomata are. Cacti have gone farther—they lack leaves entirely, and photosynthesize through their enlarged green stems. And some, such as crabgrass, even have  a modified form of photosynthesis that allows them to open their stomata less often!

On the other hand, plants in wet areas have wide, thin leaves so they can evaporate a lot of water vapor. (The extra leaf area also helps them gather more light in what is typically a habitat filled with thick vegetation and cloudy days.) Water lilies have stomata on the tops of their leaves instead of underneath, so they can still “breathe” while floating on the water’s surface.

When choosing plants for our gardens, we need to be aware of these differing needs for water. Especially in a xeriscape, grouping plants according to watering zones allows variety while making the most of every drop.

Finally, remember that one of the end products of photosynthesis is sugar. The plants use this sugar in the process of respiration—they “burn” the sugar, combining it with oxygen, to produce energy for growth. Animals do this as well; it’s why we have to eat.

Respiration can occur whether or not the sun is shining. In areas with warm, humid summers, plants respire—and grow—all night. But here in Colorado, our nights are cool enough that this growth largely grinds to a halt, particularly among garden veggies. That’s why the “days to maturity” you read about in catalogs don’t apply to us. I usually double the number of days they give to determine if our growing season is long enough to harvest a crop.

Carrots_home_20091103_LAH_5355Lucky for us, well-grown plants produce more sugar than they need. In fact, the more sugars they are able to make, the better they’ll taste (as in these sweet carrots). By catering to their demands, we allow our plants to do their best at providing us with gorgeous flowers, healthy foliage, and delicious food.

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