Branches of Botany: Plants need help dealing with dry days of summer

The dog days of summer have arrived in the islands, and with them come increased temperatures and decreased rainfall.

The National Weather Service’s Climate Prediction Center has forecast that the amount of precipitation in ­July will be below normal for the isles. This means that the green oasis in your yard or the floral menagerie on the lanai may need some extra help.

Most people recognize that when plants drop leaves, it can be a sure sign of dehydration. However, how and why plants use water might not be as clearly understood.

Water plays an integral role in growth and photosynthesis (when plants turn sunlight into food). In the chloroplasts — the green structures inside plant cells where photosynthesis takes place — water is broken down to yield electrons and protons in the form of a hydrogen ion, producing oxygen as a byproduct. These will be used during the Calvin cycle when plants convert carbon dioxide to food/sugar.

Now that we know why plants need water, how do they absorb it from their environment?

Plants absorb most of their water through their roots. While this can happen along the root’s entire length, it is maximized in the fine roots at the growing tips. To increase the efficiency of water uptake, these fine roots can be covered in root hairs that expand their absorptive surface area and optimize the contact between roots and soil. Additionally, a symbiosis — an interaction between two different organisms living in close physical association — between mycorrhizal fungi in the soil and plants can increase the root’s surface area manifold (where the roots split off a main stem).

Though the exact mechanism is not known, roots generally grow toward water. And they can travel impressive lengths to do so.

One study found that the roots of the Shepherd’s tree (Boscia albitrunca) can reach a depth of 223 feet and the longest lateral roots can grow around 164 feet from the side of a plant!

Once water makes its way into the root, it must cross several layers of cells and be transported across a water-impermeable layer to the xylem. This is a network of tissues that moves water throughout the interior of the plant.

There is no equivalent to a heart or pump in plants that aids the movement of water. So how does it get to the top of large tree species?

It might surprise you to learn this process is driven by evaporation. Tiny pores in the leaves called stomata open to take in carbon dioxide. While this occurs, it also allows for evaporation to happen.

Since water is a polar molecule — where one end is slightly positive and the other slightly negative — it bonds with itself. As water molecules evaporate and exit the plant’s leaves, they send the other molecules they are bonded to all the way down to the water in the roots. This process does have its limits; a study led by Dr. George Koch determined that the maximum height a tree can grow is between 400 to 426 feet.

Habitat also affects this process. In arid environments, it is not advantageous for plants to open their stomata during the day. To overcome water loss, certain lineages of plants have evolved a specific form of photosynthesis where they open their stomata at night to reduce water loss. Other plant groups, mainly grasses, separate the cells where photosynthesis and sugar production occur, which allows for less water loss.

Summer watering tips

1. Water during the early morning or evening to reduce water loss.

2. Replace plants that need a lot of water with native species from dry or coastal areas.

3. Investigate drip irrigation systems to reduce excess watering.

Check out the city Board of Water Supply’s website for more information.

Jesse Adams is a botanist at the Harold L. Lyon Arboretum, where he works to catalogue, propagate and conserve the plant diversity found there.