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TEMAT: The plant root

The plant root 8 lata 5 miesiąc temu #1

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The plant root
Plant roots have two major functions:
• Physically anchor the plant to the growing medium
• Act as an avenue through which water and ions enter into the plant
for redistribution to all parts of the plant
Although the first role given here is important, it is the second role that
deserves our attention in this discussion. The book edited by Carson (1974)
provides detailed information on plant roots and their many important
functions, and the book chapter by Wignarajah (1995) discusses the cur-
rent concepts on nutrient element uptake.
Root architecture is determined by plant species and the physical
environment surrounding the roots. Plant roots grow outward and down-
ward. However, in soil, it has been observed that feeder roots grow up,
not down. This is why plants, particularly trees, do poorly when the soil
surface is compacted or physically disturbed. In soil, any root restriction
can have a significant impact on plant growth and development due to the
reduction in soil–root contact. Root pruning, whether done purposely (to
bonsai plants) or as the result of natural phenomena (due to the presence of
plow or clay pans), will also affect plant growth and development in soil.
In most hydroponic growing systems, roots may extend into a much
greater volume of growing area or medium than would occur in soil.
Root size, measured in terms of length and extent of branching as
well as color, is a characteristic that is affected by the nature of the rooting
environment. Normally, vigorous plant growth is associated with long,
white, and highly branched roots. It is uncertain whether vigorous top
growth is a result of vigorous root growth or vice versa.
Tops tend to grow at the expense of roots, with root growth slowing
during fruit set. Shoot-to-root ratios are frequently used to describe the
relationship that exists between them, with ratios ranging from as low
as 0.5 to a high of 15.0. Root growth is dependent on the supply of carbo-
hydrates from the tops and, in turn, the top is dependent on the root for
water and the required essential elements. The loss or restriction of roots
can significantly affect top growth. Therefore, it is believed that the goal
should be to provide and maintain those conditions that promote good,
healthy root development, neither excessive nor restrictive.
The physical characteristics of the root itself play a major role in ele-
mental uptake. The rooting medium and the elements in the medium will
determine to a considerable degree root appearance. For example, root
hairs will be almost absent on roots exposed to a high concentration (100
mg/L, ppm) of NO 3– . High P in the rooting medium will also reduce root
hair development, whereas changing concentrations of the major cations,
K + , Ca 2+ , and Mg 2+ , will have little effect on root hair development. Root
hairs markedly increase the surface available for ion absorption and also
increase the surface contact between roots and the water film around par-
ticles in a soilless medium; therefore, their presence can have a marked
effect on water and ion uptake. Normally, hydroponic plant roots do not
have root hairs.
The question that arises is, “What constitutes healthy functioning
roots for the hydroponic growing system?” The size and extent of root
development are not as critical as in soil. It has been demonstrated that
one functioning root is sufficient to provide all the essential elements
required by the plant, with size and extensiveness of the roots being pri-
marily important for water uptake. Therefore, in most hydroponic sys-
tems, root growth and extension are probably far greater than needed,
which may actually have a detrimental effect on plant growth and perfor-
mance. It should be remembered that root growth and function require
a continuous supply of carbohydrates, which are generated by photosyn-
thesis. Therefore, an ever expanding and actively functioning root sys-
tem will take carbohydrates away from vegetative expansion and fruit
growth. Therefore, some degree of root growth control may be essential
for extensive plant growth and high fruit yields.
A large and extensive root system may not be the best for most
hydroponic growing systems. Rather than the large root mass, active,
efficiently functioning roots are needed, since the nutrient solution con-
tinuously bathes most of the root system, thereby requiring less surface
area for absorption to take place. One of the major problems with the NFT
(Nutrient Film Technique) tomato hydroponic system,
for example, is the large root mass that develops in the rooting channel,
which eventually restricts O 2 (Antkowiak 1993) and nutrient solution pen-
etration; the end result is a problem called “root death.” Similar exten-
sive root growth occurs with other types of growing systems, particularly
with flood-and-drain systems, where roots frequently grow into the pip-
ing that delivers and drains the growing bed of nutrient solution, restrict-
ing even flow.
Similar extensive root growth is obtained with most hydroponic sys-
tems with roots frequently filling bags and blocks of media; in addition,
sometimes roots grow through the openings in the outer walls of bags
and media containers. The question is, “Does a large root mass translate
into high plant performance?” The answer is probably no if there is more
root surface for absorption than needed. In addition, roots require a con-
tinuous supply of carbohydrates, which can be better used to expand top
growth and contribute to fruit yield. A large root mass also requires sub-
stantial quantities of O 2 to remain fully functional.
Unfortunately, the question as to root size has yet to be addressed
adequately. It should also be remembered that roots require a continuous
supply of O 2 to remain healthy and functioning. Roots will not survive
in anaerobic conditions. Hydroponically speaking, a large, ever expand-
ing root system probably does not necessarily translate into greater top
growth and yield and, in fact, may actually have some detrimental effect.
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