In another article (Rev Up Your Plants), we talked about how water was the true driving force in plant growth. This time, we’ll describe how watering works, that is, how travels from the potting medium into the plant and makes the journey upward into the leaves, overcoming the forces of gravity, and how your cultural conditions and watering habits can affect that.
The force at work is called transpirational pull, which can be visualized by imagining sipping a drink through a straw. Evaporation of water through stomata (tiny openings in leaves) creates the suction, pulling water continuously upward. The hotter, sunnier, and less humid the environment, the faster the evaporation and the faster the transpirational pull. Problems begin when the rate of transpiration exceeds the ability of the roots to absorb water and the plant wilts due to loss of turgor, or water pressure within their cells. Such loss of turgor is more difficult to see in most orchids, due to the presence of unique rigid structures throughout the plant that prevent the collapsing of the tissues, as happens with most other types of plants.
In epiphytic orchids, the velamen covering the roots instantly absorbs whatever water it comes in contact with. The water then enters the root, crosses the outer epidermis and moves toward the xylem (water-conducting tissue) by moving in the spaces between the cells or actually through them. Once the water reaches the xylem, which is a series of open tubes within the stele (that “string” that’s left when a root dies and rots), it moves quickly upward, continuing through xylem branches and into the tiny veins in the leaves, and ultimately into the cells themselves through osmosis.
In order to keep the whole thing working, it is important that water keeps moving in an unbroken flow. That is helped by the fact that water molecules bind together, or are cohesive. Those bonds are strong enough to keep the molecules stuck together even as they travel up to great heights (think about trees!). However, if the transpirational pull becomes greater than the water supply absorbed by the roots, cohesion is broken and an air bubble, or embolism, forms. Unless the bonds can be reestablished relatively quickly (sometimes water will enter the xylem from surrounding cells and fill the gap and force the air to dissolve), the flow of water is permanently interrupted. Water can be diverted around the embolism by moving laterally into other xylem tubes, but if too many embolisms are present the part of the plant above them will die. Fortunately, that is less common in orchids, as the rate of water transfer within the plant is much slower than that of many annuals and perennials, and the transpiration rate has been reduced through some evolutionary metabolic modifications
So how can your culture affect that? The obvious one is a lack of-, or too infrequent watering. If you tend to water infrequently, or insufficiently, the roots cannot take up as much water as they can when watered frequently. Even if the individual particles in the potting medium stay wet, that does not mean they can easily transfer it to the roots. Such a practice tends to “strain” the cohesivity of the water already within the plant, slowing the whole growth process. Likewise, growing in too dry of an environment accelerates the transpirational flow, which can outstrip the roots’ ability to replace the water, leading to desiccation and wilting, usually seen first in flowers.
Probably the least obvious cultural issue is excessive humidity. Yes, orchids have evolved, in many cases, to live in high-humidity environments, but if the relative humidity is constantly very high, the rate of evaporation of water from the leaf stomata is stifled, slowing the transpiration process, which ultimately slows the growth of your plants.
Summing this up, for optimal growth of your plants, water should be applied frequently and copiously to the airy root system, and the relative humidity should be kept moderately high, but not saturated.
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