Page 49 - BCI Bonsai & Stone Appreciation Q1 2025

of an oxygen atom and two hydrogen atoms bonded
through a covalent bond in a non-linear structure that
forms an angle, creating positive and negative poles
that allow interaction with other polar molecules.
This is why water is considered the universal solvent,
as most substances can dissolve in water, allowing the
movement of molecules within and between cells. Due
to its polar properties, water greatly influences the
structure and stability of molecules such as proteins and
sugars, as well as macro and microelements. Likewise,
cell expansion and the physicochemical integrity of the
cell wall depend on water. For many of our species, a
loss in water pressure is shown as the leaves wilt, which
is the first indication that there is no water uptake by
the roots.
The upper limit of water storage is often referred to
as the field capacity, while the lower limit is referred
to as the permanent wilting point. After a rainfall or
irrigation event that saturates the soil, there is a rapid
downward movement (drainage) of a portion of the
soil water due to the force of gravity and capillarity.
During the drainage process, soil moisture decreases
continuously. The rate of drainage is related to the
hydraulic conductivity of the soil. The rapid drainage
becomes negligible after some time, and at that point,
the soil moisture is referred to as field capacity. The
permanent wilting point is the soil moisture content at
which the plant can no longer absorb water from the soil,
causing the plant to wilt and die if additional water is not
provided. However, most plants will be under significant
water stress before this point, and they will likely suffer
a significant reduction in yield long before reaching the
wilting point. The total amount of water available for
plant uptake is the available plant water, often expressed
as a percentage by volume (volume of water/volume of
soil sample). The available water-holding capacity is
determined by multiplying the water available for the
plant (equivalent to the difference between an upper
and lower water limit) by the depth of the root zone
where water extraction occurs. Decreased water content
at the permanent wilting point negatively affects plant
health and yield. At this point, our trees begin to lose
leaves to prevent complete dehydration, followed by
branch dieback. In conifers, the wilting point is difficult
to define, which is reflected in the loss of many junipers
each year due to improper watering. Maintaining the
substrate at a high field capacity over an extended
period may result in oxygen loss, allowing conditions
for fungi and bacteria to invade weakened roots (as
in everything in life, there are several exceptions, like
Buttonwoods, Campeche, Bucida, Bald cypress, etc.).
Phytohormone Abscisic Acid (ABA)
To survive, plants require rapid adaptation and
the capacity to detect environmental signals. For
this purpose, one of the main hormones plants use
in response to different types of stress is abscisic acid
(ABA). To date, three central components make up
the ABA signaling mechanism in higher plants. The
first is the family of soluble ABA receptors referred to
as PYR/PYL/RCAR (pyrabactin resistance/pyrabactin
resistance-like/regulatory component of the ABA
receptor). These are proteins that can bind to the
hormone and produce a set of instructions for the
plant to respond. The second component is the group
A protein phosphatase 2C (PP2C-A); this protein
has the ability to modify other proteins by removing
phosphate groups that were added, a process called
dephosphorylation. The last component is SNF1-related
protein kinase 2 (SnRK2), which has the opposite role
of adding phosphate to selected proteins, a process
known as phosphorylation. In flowering trees, SnRK2
is the main positive regulator of ABA signaling, being
activated by phosphorylation. In the absence of ABA,
PP2C-A blocks the kinase activity of SnRK2 through
dephosphorylation. In the presence of ABA, PYR/
PYL/RCAR bound to ABA binds to PP2C-A, freeing
SnRK2 from being inhibited by PP2C-A. When SnRK2
is activated, it controls certain genetic sequences via a
transcription factor (AREB/ABF) that binds to ABA-
responsive elements for gene expression.
Macromolecules Involved
in a Water Deficit
When the water deficit develops slowly, there are
changes in developmental processes that have several
effects on growth. One of primary importance is the
specific limitation of leaf expansion. Although leaf
area is important because photosynthesis depends on
January/February/March 2025 | BCI | 47