PHYSIOLOGICAL ASPECTS OF THE EFFECT OF CONTINUOUS SOIL AERATION ON PLANT ...

17 Pages·2006·1.61 MB·English

PHYSIOLOGICAL ASPECTS OF THE EFFECT OF CONTINUOUS
SOIL AERATION ON PLANT GROWTH'
W. F. LOEHWING
Introduction
The experiments here reported deal with the effects of continuous
aeration of sand and soil cultures. They are the outgrowth of nutritional
studies which suggested that the beneficial effect of certain fertilizers was
ascribable to the production of larger root systems and increased absorption
of nutrients. Since it was known that soil aeration would increase the bulk
of root systems, experiments were undertaken to determine whether it was
thereby possible to duplicate the results of certain fertilizer treatments and
thus disclose that such nutrients acted indirectly by enlargement of the root
system. The results of the aeration experiments are reported separately
because they reveal significant effects of soil air on plant metabolism. Data
from nutritional studies will appear elsewhere.
CANNON and CLEMENTS have reviewed the earlier literature on plant
responses to soil aeration, and they have extended our knowledge of that
subject by a comprehensive set of experiments of their own (19, 24). The
existing data indicate that the growth of most roots depends upon free soil
oxygen (12, 39, 45) although some roots can develop anaerobically (16).
Anaerobic roots are characteristically devoid of root hairs (16, 18, 41) and
consequently absorptive processes differ from those of typical roots (26, 28).
Even those roots with low soil oxygen requirements, however, are readily
injured by moderately high concentrations of soil carbon dioxide (14, 15,
27, 37). Relatively high oxygen tensions are needed to offset otherwise
toxic carbon dioxide concentrations about roots (17).
Improper composition of soil air manifests itself in reduced, slow-grow-
ing root systems, inadequate absorption, short-lived, discolored foliage and
delay or failure of reproductive processes (1, 11, 19, 30, 34). The symp-
tomatic complex arising from impaired gas exchange of roots reflects a
general reduction in rate and magnitude of normal absorptive and growth
processes. The great bulk of existing evidence thus indicates that roots
are sensitive to variations in soil air. It also suggests that experimental
manipulation of soil atmosphere provides an effective means of studying
the role of root systems and their effect on the metabolism of the plant as
a whole. It must be noted, however, that the preponderance of existing
data deals chiefly with minimal oxygen requirements, carbon dioxide toler-
ance, and a general description of gross anatomical changes induced as
'An investigation aided by a research grant from the National Chapter of the
Society of Sigma Xi.
567
568 PLANT PHYSIOLOGY
critical concentrations of both gases are approached. The general char-
acter of the results of earlier workers nevertheless suggests that root ac-
tivity is influenced fully as much by soil air as by water and essential
mineral nutrients. The data given here indicate some of the metabolic dif-
ferences between aerated and unaerated plants.
Procedure
The experiment comprised a double series of pot cultures, one of which
contained coarse quartz sand and the other an ordinary fertile field loam.
Two-gallon, glazed earthenware jars were provided with a tubed 3"x 3"x 1'
porous basswood block to distribute the air supply. Pots of the sand series
were filled with 12 kg. of sand and each uniformly aerated with a con-
tinuous stream of air approximating 100 liters daily. The main air stream
was scrubbed by bubbling through two 10-liter bottles partly filled with
water. These bottles also served to regulate the pressure and to humidify
the air, thus maintaining the soil moisture of aerated pots very nearly
equal to that of controls. Use of dry air was found to cause injury largely
through its evaporational effect even at rather low rates of aeration. Hu-
midity and rate of flow were further regulated by interposing a smaller
water bottle between each pot and the main air supply (6). Sand cultures
were initially given 500 cc. of a neutralized Knop's nutrient solution which
was gradually brought up to a total of 1500 cc. at the end of the first month.
The soil solution varied between pH 7.4 and 7.0 during the course of the
experiment.
All pots were maintained at approximately constant weight by water-
ing as needed. A control series was similarly treated except that aeration
was omitted. A second series employing 10 kg. of fertile loam was also
aerated in the foregoing manner in order to determine the effect of differ-
ence in soil type. Dwarf sunflowers (Manus variety) were used in both
sand and loam cultures as well as inoculated Ito San soy beans in loam.
Initial analyses were made four weeks after germination while the plants
were still in the vegetative phase. The final analyses were made in seven
to nineweekswhentheplantswere in flowerbutbefore fruits had appeared.
Sand and soil were carefully washed out of the jars in order to harvest the
entire plant with minimum injury to the root system. Plants were sepa-
rated into tops and roots by cutting at the ground line. Roots were care-
fully freed of solid matter, rinsed in distilled water, blotted, and allowed
to air-dry for 30 minutes. Fresh weights were recorded and tissues were
then comminuted by hand or Nixtamal mill.
Material for carbohydrate and nitrogen analyses was preserved in 80
per cent. alcohol. Soluble carbohydrates were first hydrolyzed and the
precipitated copper estimated as glucose according to official methods (10).
569
LOEHWING: CONTINUOUS SOIL AERATION
The insoluble residue was refluxed with hydrochloric acid and further
analyzed as starch (10). Nitrogen of dry material and sap was deter-
mined by the Kjeldahl-Gunning method. Calcium was titrated as oxalate
against permanganate. Magnesium and phosphorus were estimated colori-
metrically as phosphates in a solution made up of ammonium molybdate,
sodium sulphite, and hydroquinon. Potassium was precipitated as chloro-
platinate and iron estimated colorimetrically in amyl alcohol as thiocyanate.
Tissue for sap analyses was placed in sealed vials, immediately frozen
at -600 C. in dry ice. Tissue was rapidly thawed prior to extraction.
Sap was expressed hydraulically and filtered through a double cloth filter
at 10,000 pounds' pressure. Conductivity, hydrion, buffer, and oxidation
measurements were made at 25° C. and these determinations as well as the
freezing-point depression were commenced immediately after extraction of
the sap (35).
Sap samples were immediately cleared, acid-hydrolyzed, and analyzed
for reducing components which are recorded as glucose. Oxidase activity
was determined colorimetrically with an indophenol reagent freshly pre-
pared for each determination as follows: 0.144 gm. alpha-naphthol was
dissolved in 10 cc. of 95 per cent. alcohol; then 0.209 gm. p-phenylene-
diamine-hydrochloride was added and the whole made up to 250 cc. with
water, and finally neutralized with anhydrous sodium carbonate. Exactly
1 cc. of sap was added to 50-cc. portions of the reagent and permitted to
stand for one hour in a 400-cc. beaker. Controls were similarly prepared
except that sap was omitted. After standing for one hour, 50 cc. of 95
per cent. alcohol were added to dissolve the indophenol; both control and
test sample were compared colorimetrically with a color standard contain-
ing 0.353 gm. indophenol per liter. Test samples generally showed forma-
tion of larger amounts of indophenol, and this quantity minus that formed
in the controls is expressed in the tables as grams of reagent oxidized per
liter of sap per hour.
Hydrion and buffer capacity were measured potentiometrically with a
calomel half-cell and hydrogen electrode. Two 4-cc. samples of fresh sap
were used for the acid and the alkali buffer curve respectively and the pH
recorded after successive additions of 0.5-cc. portions of N/40 acid and
alkali respectively. Osmotic pressures were computed from depression of
thefreezing-point determinedbythe Beckman method (35).
Data on sunflowers
The analytical data for sunflowers in the sand and loam series (tables
I, II) disclose a general increase in size and weight of roots as well as tops
in the aerated cultures. Seedling development is accelerated by aeration
and the precocity thus established is maintained well into the reproductive
570 PLANT PHYSIOLOGY
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phase. Although the controls are not invariably lower in percentage dry
weight of each constituent, they are generally lower in corresponding ab-
solute weights. Thus in the sand cultures (table I) the percentage ash in
unaerated tops and roots is higher but the total weight of ash per plant
is twice as great in young vegetative aerated plants. Differences in total
ash tend to diminish with increasing age Dut aerated plants retain an ap-
preciable excess even during senescence. Aerated plants absorb, transport,
and utilize mineral nutrients more rapidly and efficiently than the controls
if judged on the basis of rate of dry weight increase.
In connection with the essential inorganic materials, the small stature
of the control plants explains the initially higher percentage of calcium
despite the smaller total amount per plant in the sand cultures. There is
an obvious tendency of calcium to accumulate in the roots of the controls
in the maturation phases, both in sand and loam cultures. The absorption
and internal distribution of magnesium is striking, occurring earlier and
reaching the tops more rapidly in aerated young plants. This is evidenced
by the fact that young aerated tops are much higher than the controls in
magnesium while at the same time the unaerated roots run higher than
aerated roots in percentage and total magnesium, both in sand and soil
cultures (tables I, II). The original lag in magnesium intake by unaerated
controls in sand was more than compensated, however, by rapid absorption
during the seventh and eighth weeks. At the end of this time the total
magnesium content of the mature controls in sand exceeded that of the
aerated plants. The fact that this rapid intake of magnesium occurs late
in the life of the controls and that it is coincident with a definite lag in
potassium absorption may explain why the development of the sand con-
trols fails to overtake that of the corresponding aerated plants (C, table I).
Although less pronounced, similar results were observed in loam cultures.
Acceleration in rate of magnesium intake is thus concomitant with a reduc-
tion in rat-e of potassium absorption.
Thus the initial percentage of potash is high in both aerated and un-
aerated plants but falls with increasing maturity, owing in part to the
reason just given and also to the proportionately greater increase in dry
weight over potash intake. Although percentage of potash in roots fluctu-
ates, the aerated roots in sand are uniformly higher in absolute amounts
of this element. Percentages of phosphorus are rather low in all young
plants (39, 40) but increase significantly after the fifth week, especially in
roots. Aerated plants not only run uniformly higher in total phosphorus
but inaugurate reproduction earlier and maintain it longer than the
controls.
The organic metabolites, namely, organic nitrogen, total sugars, and
hydrolyzable polysaccharides, are uniformly higher in percentages but not
574 PLANT PHYSIOLOGY
in absolute amount in all unaerated sand plants. Aerated loam plants,
however, were higher in both percentage and total amounts of these con-
stituents. Although not given in the tables, supplemental analyses showed
a tendency for crude fiber and total amino nitrogen to be greater in aerated
plants, a condition suggestive of more rapid assimilation. Since high sugar
content coupled with abundant nitrate nitrogen generally characterizes im-
paired protein metabolism, this was probably also the case in the above
instance.
It will be noted, however, that the soluble constituents of the sap, as
indicated under dry weight composition thereof, tend to be higher in the
tops of older aerated plants in sand and loam. Judging from the data on
freezing-point depression by expressed fluids, their mineral constituents
are osmotically more effective than their organic solutes. The specific con-
ductivity of the sap of aerated and control plants, however, does not vary
as greatly as might be anticipated from the preceding data. Both the con-
ductivity and the buffer capacity of the sap were difficult to correlate with
the pH and the mineral content of the plants. Aerated sunflower tops in
sand appear better buffered than controls against acid, but as they reach
maturity these differences in acid buffering diminish greatly. Mature un-
aerated roots on the other hand possess a better buffer capacity against
alkali than aerated cultures of similar age. The greater free acidity of sap
from unaerated plants suggests a higher acid content than in aerated cul-
tures (21).
Total nitrogen dissolved in expressed sap is generally low but shows a
tendency to accumulate in unaerated roots of sunflowers grown in sand
(D, table I). This fact again implies not only slower translocation of
solutes but also impaired nitrogen metabolism in controls, especially in
older plants. On the whole, the sap of the plants showing the best de-
velopment was marked by a higher oxidation potential, shown by the
greater amount of indophenol produced. On the other hand, the top-root
ratio in terms of fresh weight of the best developed plants was consistently
lower in aerated plants than for the poorer controls of the same series (A,
tables I, II).
Data on soy beans
The soy bean loam cultures (table III) exhibit several points of con-
trast with sunflowers, owing to the difference in feeding habits of the two
species. Aerated soy beans displayed more abundant root nodule forma-
tion, and these plants were larger. In contrast with sunflowers, calcium
was more rapidly absorbed and translocated to the tops. Sunflowers and
soy beans were comparable, however, in the accumulation of calcium in the
roots of older unaerated plants.
LOEIWING: CONTINUOUS SOIL AERATION 575
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