California Avocado Society Yearbook
62 (1978): 118-125.
C.
K. Labanauskas, L H. Stolzy,
and G. A. Zentmyer
ROOTSTOCK,
SOIL OXYGEN, AND SOIL MOISTURE EFFECTS
ON GROWTH AND CONCENTRATION OF NUTRIENTS
IN AVOCADO PLANTS
Effects of the most common rootstocks of apple (Malus domestica), citrus (Citrus
spp.) and pear (Pyrus
communis) trees on nutrient uptake and
translocation have been extensively investigated (2, 4, 15, 17). Data presented
here clearly indicate the influence of rootstocks on nutrient uptake and
translocation to the plant tops. A considerable body of literature has been
published (7, 9, 12) on the influence of soil oxygen supply to the roots of
various plants on the nutrient concentrations in plant material. Low soil
oxygen in the rootzone resulted in significant
reduction in total nutrient uptake by the plant. Low soil oxygen resulted in an
accumulation of Na and Cl in citrus stems (7). Low
oxygen supply to the roots of avocado plants significantly decreased
concentrations of N, P, K, Ca, Mg, and B, increased Na and Fe in the tops (9),
and significantly decreased K, Mg, Na, and Cl in the
roots. Total N, P, K, Ca, Mg, Na, and Cl per plant
decreased with low soil oxygen supply.
Field experiments with bearing avocado trees showed
that excess soil moisture due to frequent irrigation decreased concentrations
of Fe and Zn in the leaves (6). A greenhouse avocado seedling experiment showed
that combined leaves and stems contained significantly lower concentrations of
N, P, K, and B, and higher concentrations of Na, Mn
and Fe when the water table was maintained at half the soil column for the
experimental period of 35 days (9).
Na concentration in the roots under the same
conditions was significantly lower than in the roots of control plants. Labanauskas et al. (5) studied moisture effects on the
nutrient uptake in citrus. They found that total N, Cl,
Na, Zn, Cu, and Fe per plant were significantly influenced by differential
irrigation treatments. Lower concentrations of Ca, Mg, and Fe were found in the
leaves of citrus seedlings grown in "wet soil" than in analogous
leaves from trees grown in "dry soils."
Density of roots governs soil exploitation and is
affected by varietal differences, soil oxygen supply,
and soil moisture level, hence the study of these factors on nutrient uptake
and translocation in avocado plants was combined in 1 experiment. This paper
reports the influence of 2 rootstocks, 2 soil oxygen levels, and 2 soil
moisture levels on the concentration of nutrients in avocado leaves, stems, and
roots, and total uptake of 11 macro- and micronutrients.
Materials and Methods
Hass avocado scion on two rootstocks, Duke and Topa Topa, were chosen to
evaluate the effects of soil oxygen and moisture on the nutrient concentrations
in the avocado leaves, stems, and roots, and total nutrient content per plant.
Duke has been considered sensitive and Topa Topa tolerant to high soil moisture and low soil oxygen as
related to growth and nutrient uptake (16).
Seedlings were grown in a soil medium consisting of
5 parts top soil, 3 parts silt, and 2 parts peat for 10 months, then grafted
with Hass scions. Nine months after grafting, plants were transplanted into 20
cm diameter and 50 cm high acrylic cylinders filled with Fallbrook sandy loam.
The soil in each container was packed to a bulk density of 1.42, and were
tightly sealed with plastic lids in order to alter the atmosphere above the
soil surface. The lids were provided with intake and exhaust ports through
which air or a gas mixture could be circulated over the surface to control the
oxygen supply to the soil system as previously described by Stolzy
et al. (13, 14). Openings for tensiometers were
provided in the lids.
The oxygen levels were maintained above the soil
surface in the containers: air—21% O2 and a mixture of 2.5% O2
plus 97.5% N. The soil O2 treatments will be referred to, hereafter,
as high and low, respectively.
Two levels of soil moisture were maintained in the
containers by using tensiometers inserted in the soil
at 15 cm and 40 cm depths. Half of the containers were watered with
0.l-strength Hoagland's solution when the tensiometer reading at the 15 cm depth indicated a soil water
potential of 15 centibars, and the other half were watered
with 0.l-strength Hoagland's solution when the
potential reached 55 centibars. These 2 moisture
levels will be referred to, hereafter, as high and low soil moisture
treatments, respectively. Each treatment was factorially
replicated 4 times. Experimental treatments in 2 consecutive years were
identical except for calendar period and ran for 90 days. The trees for both
experiments were raised identically starting from the seeds. The first
experiment was initiated in October, 1974, and terminated in December, 1974.
The second experiment was initiated in June, 1975, and terminated in September,
1975.
Greenhouse night temperatures were maintained at
about 22°—and day temperatures at 35 ° C during the hotter part of the day over
the course of the experimental periods. Soil root temperature was maintained at
25° ± 2°C in constant temperature tanks. Plants were harvested at the end of
the 90-day period. Each plant was divided into leaves, stems, and roots,
hand-washed in tap water containing 0.1% detergent (Joy), rinsed in demineralized water, and dried in a forced-draft oven at
60° C for 48 hr. Methods of sample preparation for nutrient analysis were as
described by Labanauskas and Bitters (4). The data
obtained from chemical analysis of tissues were subjected to statistical
analysis (1).
Results and Discussion
Rootstock Effects
Rootstocks had no significant effect on dry weight of the leaves, stems, or roots. Dry weight of the plants was not affected significantly by the 2 rootstocks, but the concentrations of N, P, and Cu were higher and Mn lower in the leaves of Hass on Duke rootstock (Table 1).
Concentrations of the other macro- and
micro-nutrients in the leaves were not affected significantly by the 2
rootstocks. Dry weight of stems was not affected measurably by the rootstock
differences; however, the concentrations of Mn, Cu,
and Fe in the scion stems on Duke rootstock were significantly higher.
Concentrations of the other macro- and micro-nutrients found in the stems
were not significantly influenced by the 2 rootstocks. Mn
and Fe concentrations were significantly higher and Mg lower in Duke roots.
Concentrations of other nutrients in the roots did not differ. Total amounts of
N and Fe taken up by the scion grown on Duke rootstock were significantly
higher. Thus, the 2 rootstocks produced little change in plant growth, nutrient
uptake, and nutrient translocation under the same environmental conditions.
Aeration Effects
The low soil oxygen reduced dry weight of the whole plant as compared to
plants supplied with high soil oxygen. Dry weights of the leaves and stems were
not affected significantly (Table 1). There was a significant reduction in dry
weight of roots of plants supplied with low soil oxygen.
Low soil oxygen treatment significantly reduced leaf concentrations of N, P,
K, Ca, Mg, Zn, Mn, and Cu, while Fe was increased (Table 1).
Concentrations of Na and Cl in leaves were not
affected by the soil oxygen treatments. Low soil oxygen significantly reduced
the concentrations of K, Mn, and Cu in the stems as
compared with plants supplied with high soil oxygen (Table 1). Dry weight of
stems was not affected by the soil oxygen treatments, but the concentrations of
Na and Cl in the stems were substantially higher
where the soil oxygen supply was low.
Decreased soil oxygen significantly reduced concentration of N, K, and Mg,
and increased Na, Cl, and Zn in the roots as compared
with high level oxygen. Total amounts of all nutrients studied were
significantly lower in plants grown under low soil oxygen, irrespective of
nutrient concentrations found in plant tissues.
The low oxygen treatment had little effect on the dry weight of leaves and
stems, but it lowered root dry weight very significantly—to produce high
significance in total plant weight. Dry weights of leaves and stems were not
reduced by the soil oxygen treatments, as reported earlier by Valorasetal (16). This confirmed earlier findings that
vegetative growth of some plants was relatively insensitive to low soil oxygen
(11). The findings presented in this paper agree closely with those reported on
citrus (7) in which low oxygen supply to the roots of citrus seedlings
significantly reduced dry weight of the roots, but showed no measurable effect
on the dry weights of the leaves and stems.
These data on effects of oxygen supply to the roots on nutrient
concentrations in plant tops are in fairly close agreement with previous
reports (9, 11). Those experiments showed that low soil oxygen increased Cl and Na concentrations in the stems, but not in the
leaves as had been previously assumed. Increased Na and Cl
concentrations in stems of avocado plants grown under low soil oxygen supply
were in effect cumulative, not related to dry weight reduction, and associated
with root injury. Concentrations of most of the nutrients determined were lower
in plant roots under low soil oxygen supply, with the exception of Na and Cl, which supported similar findings pertaining to Na and Cl in avocado (8, 9). Na and Cl
concentrations in stems were shown to have increased in the present and
previous experiments, whereas total amounts per plant were reduced under a low
soil oxygen supply. This indicates that poor soil aeration may lead to Na and Cl toxicity problems, particularly in plants such as
avocado which are extremely sensitive to Cl, and
which may occur although the soils are not particularly high in Cl.
The uptake of such ions as P and K by roots differs from Na accumulation
with respect to anaerobiosis. Both P and K
accumulations are immediately suppressed by anaerobic conditions and return to
normal only under aerobic conditions. Legget and Stolzy (10) found that Na is in part an exception to this
generalization—that uptake of Na by roots occurs under both aerobic and
anaerobic conditions. Previously observed accumulation of Na in plants under
anaerobic conditions over a long period of time was often considered a possible
passive entry due to damage of the plant's root system (3, 11).
The increase or decrease in the other macro- and micro-nutrient concentrations
in plants grown under differential soil oxygen supply may be related to dry
weight of the plant reduced. The total uptake of 11 nutrient elements decreased
in this experiment with decreasing dry weight when the oxygen supply to roots
was low.
Moisture Effects
Low soil moisture significantly reduced dry weight of leaves, stems, and the
total dry weight of the plants. Concentrations of Na, Mg and Cu in the leaves
of plants grown under low soil moisture were significantly higher. Low soil
moisture increased Ca concentration in the stems. Other macro- and
micro-nutrients in the stems were not influenced by moisture level in the soil.
The concentration of Mn was significantly lower in
the roots of plants grown under low soil moisture. Total N, P, K, Ca, Mg, Zn,
and Mn per plant were significantly lower in plants
grown under low soil moisture, irrespective of nutrient concentrations in the
leaves, stems, or roots. Interactions among rootstocks, oxygen, and moisture
treatments were not significant.
The effects of differential irrigation treatments on dry weight of plant
tissues produced, on nutrient concentrations in plant tissues, and total
amounts of nutrients taken up by avocado plants were in close agreement with
earlier reports (5, 6, 9). Higher concentrations of N, Mg, and Cu were found in
the leaves of plants grown in drier than in wetter soil. Concentrations of
nutrients found in the leaves, stems, or roots did not correspond to the total
amounts of nutrients taken up by the avocado plants which were significantly
lower in the plants grown under the low soil moisture regime. This was due to
lower amounts of dry weight produced by plants grown on dry soils than on wet.
Similar results from previous work with avocado have been reported earlier (5,
6, 9).
Experimental Effects
Experimental materials and methods were identical in both of these
experiments, but there were significant differences obtained in the dry weight
of avocado plants, attributable mainly to season (Table 1). Concentrations and
total nutrients per plant were significantly different each year. These
differences were attributed to differences in dry weight of plant material
produced in the different seasons.
Assessment of plant nutrient status cannot be made solely on the basis of
elemental concentrations because it is affected by soil oxygen, soil moisture,
rootstocks, and translocation in the plant. Consideration of dry weight
production, total nutrient uptake, and distribution within the plant are
essential to a proper description of plant nutrition status.
Summary
The effects of 2 avocado (Persea americana Mill.) rootstocks, 2 soil
oxygen levels, and 2 soil moisture levels on nutrient uptake and translocation
showed that seedling Duke and Topa Topa rootstocks produced little change in the growth of
Hass scion, nutrient concentrations in the leaves, stems, and roots, or the
total amount of nutrients absorbed per plant. Total amounts of 11 nutrients
studied were significantly lower, irrespective of concentrations in the various
plant tissues, in plants grown in the 2 % soil oxygen than in plants supplied
with 21 % soil oxygen. Low soil moisture reduced dry weights of leaves, stems,
and total dry weight of plants. Total amounts of N, P, K, Ca, Mg, Zn, and Mn per plant, irrespective of nutrient concentrations in
the leaves, stems, and roots, were significantly lower in plants grown under
low soil moisture.
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