Plants reach deep below surface soil to take water up into their shoots and leaves. Through a process called hydraulic lift, plants also leak water into the bone-dry surface soil to release nutrients and stir microbial activity critical to the plants’ survival. Microbes recycle the nutrient building blocks plants need to grow. ~ American biologist John Stark
Drought – prolonged water deprivation – initiates a cascade of acclimations. Initially, leaf area allocation and activity change.
First, leaf expansion halts. Smaller leaves, with less surface area, lose less water.
If plants become stressed after a substantial leaf area has developed, the older leaves are sacrificed first. Younger leaves remain and may even become more active.
One common response to drought is to slather more wax on the leaves, to reduce transpiration. This produces modest results, as cuticular transpiration accounts for only 5–10 % of total water loss.
Some desert plants take leaf area adjustment to an extreme: they lose all their leaves during a drought. Plants get most of their moisture through their roots.
There is a balancing act in the relations between root and shoot systems. A shoot grows until its water supply is limited. A root grows until its supply of photosynthetic product is limited.
This balance shifts during drought to root extension, to dig deeper for moisture. In well-watered soils, root systems tend to be shallow. Plants send their roots deeper as drought takes hold.
That said, different plant families have different strategies to deal with drought. Grasses suppress root growth as drought takes hold. This response allows the plant to slow water extraction from the soil, treating the residual moisture as a reserve. This root austerity is a temporary measure. When the soil moisture level rises, root growth resumes.
As soil dries, its water potential and matric potential turn negative. So, plants adjust their internal chemistry to absorb more water.
Water potential is the tendency of water to move from one area to another due to osmosis, gravity, pressure, or matrix effects, such as surface tension. Matric potential summarizes the adhesive intermolecular forces that water has for solid particles – in other words, water’s cling to things.
Plants can continue to absorb water only if their water potential remains less than the water source, and the matric potential stays favorable. So, they adjust: accumulating solutes and lowering the osmotic potential of cell sap.
Adjustments in internal solutes (increases in sugars, organic acids, and ions (particularly potassium)) are small but helpful. Shifting water around within cells improves water potential. The 2 actions dovetail to keep cells functioning.
Having experienced drought conditions, plants learn which responses are most effective, thereby letting them better deal with, and more quickly recover from, later dehydration events.