Pecan production is one of the most intensive
agricultural enterprises in the world. The
successful production of pecans revolves around a
complete, intensive management program. Not only
is the crop long term, May-November, but the tree
is a perennial plant. As such, poor management
decisions can have long term effects on the trees
and future production. However, the entire
management program is secondary to the soil on
which the orchard is planted and the water
management program. Tree and nut physiology can
only be explored once one has an understanding of
soil, water, air and root relationships in the soil.
Also remember that the physiological processes are
much more complex than this simplified version.
If the soil is poor, ie, shallow, poorly
drained, tight with poor aeration, the management
program, regardless of how good it is, cannot
overcome such soil limitations. Trees can be grown
on such soils but long term economical production
is not feasible.
Also, if ample water supplies are not
available, 8-10 gallons per minute per acre, long
term consistent production cannot be maintained.
As everyone well knows, water is critical to
obtain full sized nuts, as well as to fill out the
kernels. Water stress in mid-summer drastically
impairs quality and reduces next season's crop
potential too. Water is also important up until
the time of shuck split to help insure uniform
shuck opening. However, water is also important
in all the metabolic and physiological functions
which take place in the tree. It is both the
solvent and medium for all biochemical reaction.
The deficiency of water has its first effect
on growth, which is understandable and noticeable.
However, dry periods also drastically affect
nutrient absorption. Dry soil results in poor
nutrient mobility and hence can create a nutrient
deficiency. So the lack of water not only hurts
the plant directly, but indirectly as well. Water
is also super important in photosynthesis which
will be discussed in more detail later.
In order for the tree to be able to take up
the required nutrients and/or water, the soil
environment must be in proper balance. Normally,
when pecan growers look at soil, they only see or
feel the physical components of sand, silt and
clay, which go together to make up a sandy loam
or a sandy, clay loam, etc. However, the soil
environment consists of a solid, liquid and gaseous
phase (Figure 1).
Ideally, 40-50% of the soil volume would consist of
pore space which would mean the soil has a good
friable structure. Of this pore space, 25% should
be air/oxygen, 25% should be water and 50% should
be the physical soil particles. Such soil pore
space and characteristics are ideal for root growth
because they can store ample water and air as well
as allow penetration by roots. Naturally, the
greatest air/oxygen levels are in the top 6-12-18
inches of the soil. Hence this is the area of
greatest root concentration or density and the
area of greatest uptake. Remember, root metabolism
creates a demand for water and nutrients.
As alluded to earlier, all nutrients must be
dissolved in water in order for the tree to absorb
them. They can be transported to the roots by
either mass flow or diffusion. Mass flow occurs
with the movement of water to the root. The
amount of nutrients reaching the roots is dependent
on the water flow, water consumption of the plant,
and nutrient concentration of the water. Diffusion
occurs when an ion is transported from a higher to
a lower concentration. Plant roots absorbing
nutrients from the soil can thus create a sink to
which nutrients diffuse. Most plant nutrients are
supplied by diffusion.
All nutrients are absorbed as ions, NO3-, K+,
Ca2+, etc. I think many of us have the idea that
nutrients are somehow magically taken up like a
person drinking a soda through a straw. Such is
not the case and it is actually a very complicated
and complex process.
Generally speaking, nutrients are taken up by
one of two methods, either passive uptake or
active uptake. Nutrients in direct contact
(interception) with the roots contribute little to
uptake. Rather the bulk of macroelements, N, P, K,
etc. are transported to the root for absorption.
Hence, nutrient mobility is a major factor in
nutrient availability.
Passive uptake involves the movement of ions
down or along an electrochemical gradient. In
other words, movement from a high concentration to
a low concentration. Active uptake, on the other
hand, requires energy. It is involved when the
plant selectively accumulates ions against a
gradient. Usually absorption is not a problem if
the nutrients are at the proper levels in the soil
and ample air/oxygen and water is present in the
soil.
So soil, water and soil air relationships are
all vital and important to the growth of the plant.
However, technically speaking, what we really mean
is that they are critical for photosynthesis.
Photosynthesis is the conversion of light energy
into chemical energy. The tree is able to
accumulate very low concentrations of CO2 from the
atmosphere and convert it into sugars.
The pecan tree is a sun loving plant and does
not appear to have a light saturation point
(too much light). Rather poor light levels are
the reason for poor photosynthesis and food
manufacture. This is the reason, cloudy weather
in September and October can reduce nut quality.
Light is needed for food manufacture. When light
levels are optimum, low CO2 levels may reduce
photosynthesis, but this is usually not the case.
Proper water relationships are the keys to
driving photosynthesis when other conditions are
favorable. Water moves from a higher concentration
to a lower concentration; ie. from the soil to the
root, up the xylem via the transpirational stream,
into the stomatal pores and out into the
atmosphere, hence low humidity promotes greater
transpiration. The pecan tree, if given ample
room, exposes a considerable leaf surface area to
the atmosphere. This is necessary for the capture
of CO2. On the other hand, it means water loss by
transpiration is high. The pecan is unique in that
stomatal resistance does not increase (ie.
transpiration does not decrease) with reduced light
levels. This means the tree has decreased water
use efficiency, but that it is able to resume
photosynthesis more rapidly when favorable light
conditions again occur.
High temperatures and wind tend to promote
transpiration and hence possible photosynthesis.
The stomatal pores have to be open for
transpiration which allows CO2 to enter and hence
the longer the stomates are open, the more
potential CO2 which can be captured and used in
photosynthesis.
The nut development and physiology are
outlined in Figure 1-6. The
most critical aspect of this figure is the late
season "filling" of the kernel, mid August to
October. Hence, photosynthesis needs to be at its
peak to fill the nuts and store reserves for next
year's crop. It is critical that healthy,
functioning leaves be maintained to insure this
photosynthesis takes place. Anything which
impairs photosynthesis - water stress, low light
levels, cloudy conditions, will tend to reduce
quality. It is important to maintain leaves in a
green state to extend photosynthesis. It may be
that late season nitrogen applications will be
needed on heavily cropped trees to maintain the
dark green color needed for optimum photosynthesis.
It should be clear from this brief outline of
the physiological processes occurring in pecan
trees that leaves are of utmost importance in the
process. Hence, our objective is to grow as big
and as many leaves as possible and maintain them
until frost. Then, if our management program is
adequate, tree yields will take care of themselves.
For more information contact:
Larry Stein
Texas A&M Research Center
1619 Garner Field Road
Uvalde, Texas 78801
PHONE: (210)278-9151 Ext. 134
FAX: (210) 278-4008
E-MAIL: larrystein@tamu.edu