Monthly Botanical Term: Glycophytes and holophytes

Salt, or more specifically Na^+, Cl^-, Mg^2+ and So4^2- is a big deal when it comes to soil conditions. These salt ions have the potential to hold on to water that would otherwise be more available to plants. Worldwide about 6% of all the worlds land area is affected by salinity issues (Munns and Tester, 2008). Some plants, like the glycophytes are very sensitive to saline soils and don’t tend to fare too well in those areas. Other plants, the holophytes are salt tolerant plants that have some truly amazing ways to combat the salt in the soils.

First lets talk about the Glycophytes, Most of the plants we are familiar with are not salt tolerant. What does that really mean though? Tolerance is basically defined as a highly complicated conglomeration of all plant activity and the diversity of successful forms and physiologies confounds generalizations (Cheeseman, 2013). Basically, glycophytes are sensitive to salt, it messes with all of the plants processes. The main ways that plants are effected are as follows;

  1. High salinity leads to low water potential. Plants with too much salt in their soil will appear as if they are not being watered enough and to the untrained eye looks identical to water stress and droughts.
  2. Na^+ and Cl^- are usually quite toxic to plants
  3. NaCl can lead to ion imbalances and leads to nutrient deficiency because the salt will be absorbed at a similar rate as as potassium (Flowers and Colmer, 2008).

Holophytes are the salty survivors. They’ve come up with many different ways to avoid too much damage from salty conditions that we’ll talk about here shortly. First I wanted to talk about how some areas get too salty in the first place. The most common way is also the most natural. As rocks and minerals break down due to weathering on the earths surface it is over time carried by rivers or rain or winds into wetlands where the water evaporates but the salt and other metals do not. Over time the salt accumulates and eventually it will completely change the ecosystem around it. Agriculturally we human beings can make some mistakes sometimes. One of the big ones though is growing crops in arid climates because the water evaporates more quickly more water must be applied to compensate. Just like before, the metals and the salts do not evaporate so our agricultural croplands get saltier by the year and its a real race to figure out how to stop it. (Munns and Tester 2008, Munns and Gilliham 2015). With that covered lets get into some of the cool ways plants avoid the salt.

When it comes to flowers and fruits, most halophytes have altered their flowering times, collecting the essential resources needed at times of year where the salt would be more tolerable, for instance, if it were during a wet season when there is more water available to the plant. They will also utilize re-translocation of photosynthate to other parts of the plant. The leaves will partition salt into sheaths or petioles instead of the mesophyll and the epidermis. Like the flowers and fruits leaves can also undergo re-translocation of the salt but the leave may also excrete salt through the leaf. This can have a benefit of added protection against the sun, and giving the leaf an unpleasant taste for would be herbivores. The stems of some halophytes may actually store salt, as well as control its long distance transport from the roots to the leaves. Finally the roots may exclude as much as 95% of the salt the plants absorb. Further salt exudates come from the roots ability to pull salt out of the Xylem vacuoles. Finally, halophyte roots often have unique archetectual changes in shape and structure that make it harder for salt to get absorbed in the first place.

On a final note, to make it as confusing as possible, there is no single plant we can point out as a definitive halophyte or glycophyte. Like many things in nature the amount of salinity a plant can take is on a spectrum and may even differ from individual to individual. There is no line that is drawn where a holophyte is separated from a glycophyte definitively and often they grow together in the same environments. Imagine “holophyte” mangrove tress that grow along the salty coasts that at the same time have “glycophyte” epiphytes growing in their branches. The Epiphytes are in this case considered glycophytes comparatively, but in truth they partake in many of the strategies that the mangrove tree does as well such as salt exudation through the petiole of the leaf, but when compared to damn near any plant in the tropical rain forests of south America both would undoubtedly be called halophytes.

Isn’t botany fun??

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Author: garesgarden

ISA Certified Arborist, Amateur Botanist, and future Agricultural Engineer.

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