South African
Avocado Growers’ Association Yearbook 1987. 10:47-48.
Y BAR1, U KAFKAFI2 and
E LAHAV3
1Regional Experiment Station, Akko;
2 Dept of Soil Science, Faculty of Agriculture, Rehovot;
3Dept of Fruit Trees, Agric Res Org, Bet Dagan, Israel
SYNOPSIS
Three levels of nitrate (2, 8
and 16 mMol) and four levels of chloride (2, 4, 8 and 16 mMol), in a complete
factorial experiment, were given for two months to two avocado rootstocks,
Mexican (salt-sensitive) and West Indian (salt-tolerant). An increase in the
concentrations of nitrate in the irrigation solution reduced the Cl-
content in the leaves and enhanced the ability of the avocado plant to use
increasing concentrations of saline water. Good growth was obtained at 16 mMol
Cl- in the presence of equimolar N03 concentration. The
results of this study suggest that by raising the nitrate content in the
irrigation water, the tolerance of avocado plants to salinity is increased.
The avocado tree is one of the cultivated fruit trees most sensitive to
salinity (6). Chloride concentrations in the irrigation water considered as
tolerable for most crops, are detrimental to avocado. Accumulation of chloride
in the leaves induces leaf necrosis, which develops from the leaf tip to its
base. It results in reduced photosynthetic surface of the tree, earlier leaf
shedding and decreased yield.
Secondary damage induced by salinity also occurs in the avocado. Trees
grown under saline conditions are more sensitive to frost, mineral
deficiencies, water stress, etc. Such trees suffer more from sunburn, because
of their reduced foliage. The damage to the trees is also expressed in reduced
root growth and hence, reduced water uptake from the soil.
In many species, competition occurs between chloride and nitrate uptake.
Chloride uptake may be reduced by increased nitrate uptake (8). Sixty years
ago, Haas mentioned that low nitrate concentration in the soil was followed by
uptake of higher chloride quantities than those taken up when the nitrate
concentration was high (4). In several experiments with avocado, increased
nitrate nutrition resulted in decreased chloride toxicity (3,5,6). A field
trial conducted in Israel showed reduced chloride content in the leaves and
reduced leaf burn as a result of increased N fertilisation (2).
An experiment was conducted in Western Galilee, Israel, to investigate
the relationship between chloride and nitrate and their effects on two avocado
rootstocks. The results are still preliminary, but due to their high practical
importance, it was decided to report on them before the end of the experiment
and the drawing of final conclusions.
MATERIALS AND METHODS
Two rootstocks, West Indian (relatively salt-resistant) and Mexican
(salt-sensitive), were seeded in the autumn of 1985. The seedlings were
irrigated with tap water for six months, at which time they were transferred to
10 L containers filled with sand. During the following two months the seedlings
were irrigated with nutrient solutions containing four levels of chloride
(2,4,8,16 meq / L = 70,140,280,560 mg CI- / L) and three levels of
nitrate (2,8,16 meq / L = 28,112,224 mg N-N03 / L), for a total of
12 treatments. Each treatment included eight seedlings of each rootstock, with
one plant serving as a replication.
The nutrient solutions were prepared to maintain a constant ratio among
the cations K+:Ca2+:Mg2+ (6:10:2
respectively). All solutions were included, also chloride and nitrate salts of
K, Ca and Mg, also KH2PO4 (0,6 meq / L), MgSO4
(0,2 meq / L), CaSO4 (1,0 meq / L) and Coratin-mixed fertiliser of
micro-elements (50 mg / L). Chloride levels covered the whole range of saline
water used for avocado irrigation from 2 meq / L Cl-, which is
considered very good water, to 16 meq / L Cl-, which contains more
than the highest level recommended for avocado irrigation. The N-NO3
levels also cover the full range of nitrogen applied to avocado plantations.
RESULTS
The data presented here were obtained after two months of irrigation
with the nutrient solutions. Increased N-NO3 concentrations had a
significant effect in reducing damage under the highest CI-
concentration in the nutrient solution (16 meq /L). It was proved with the two
rootstocks, but the Cl- toxicity was more pronounced in the case of
the Mexican rootstock.
Chloride levels in the leaves were generally higher in the Mexican
leaves than in the West Indian leaves. The increased Cl-
concentration in the solution induced a higher Cl- content in the
leaves, but usually this rise was reduced by increased N-NO3
concentrations in the nutrient solution (Table 1).
The chloride concentration in the roots of the Mexican rootstock was
lower than that in the roots of the West Indian (Table 2). With the increased
Cl- concentration in the nutrient solution, the Cl- level
in the roots also increased; the increase was more moderate in the West Indian
than in the Mexican seedlings. Generally, Cl- concentration in the
roots was reduced with the increased N-NO3 levels in the solution;
the reduction was more pronounced in the Mexican rootstock.
|
TABLE 1 Effect of chloride and nitrate
concentrations in the nutrient solution on chloride content (% dw) in the
leaves of two avocado rootstocks |
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|
|
||||||
|
ROOTSTOCK |
N-NO3
cntr (meq / L) |
CHLORIDE CONCENTRATION(meq/1) |
||||
|
2 |
4 |
8 |
16 |
Average |
||
|
MEXICAN |
2 |
0.68 |
1.08 |
0.88 |
1.97 |
1.15 x |
|
|
8 |
0.56 |
0.41 |
0.70 |
1.51 |
0.80 y |
|
|
16 |
0.53 |
0.53 |
0.78 |
1.11 |
0.74 y |
|
|
Average |
0.59 c |
0.68 c |
0.79 b |
1.54 a |
0.90 |
|
WEST |
2 |
0.48 |
1.11 |
0.76 |
1.89 |
1.06 x |
|
INDIAN |
8 |
0.33 |
0.61 |
0.82 |
1.42 |
0.80 y |
|
|
16 |
0.33 |
0.37 |
0.78 |
1.02 |
0.63 y |
|
|
Average |
0.38 c |
0.70 b |
0.79 b |
1.45 a |
0.83 |
|
Averages followed
by a common letter do not differ significantly at P=0.05. |
||||||
|
TABLE 2 Effect of
chloride and nitrate concentrations in the nutrient solution on chloride
content (% dw) in the roots of two avocado rootstocks. |
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|
|
|
|
|
|
|
|
||||
|
ROOTSTOCK |
N-N03 cntr (meq / L) |
CHLORIDE CONCENTRATION (meq / L) |
||||||||
|
2 |
4 |
8 |
16 |
Average |
||||||
|
MEXICAN |
2 |
1.00 |
1.05 |
0.80 |
1.58 |
1.11 x |
||||
|
|
8 |
0.81 |
0.83 |
0.93 |
0.90 |
0.87 y |
||||
|
|
16 |
0.83 |
0.58 |
0.71 |
0.76 |
0.72 y |
||||
|
|
Average |
0.88 b |
0.82 b |
0.82 b |
1.08 a |
0.90 |
||||
|
WEST |
2 |
1.00 |
1.22 |
1.24 |
1.33 |
1.19 x |
||||
|
INDIAN |
8 |
1.03 |
1.07 |
1.30 |
1.57 |
1.24 x |
||||
|
|
16 |
0.93 |
0.92 |
0.94 |
1.09 |
0.97 y |
||||
|
|
Average |
0.99 c |
1.07 bc |
1.16 b |
1.33 a |
1.13 |
||||
|
Averages followed
by a common letter do not differ significantly at P = 0.05. |
||||||||||
The effect of nitrate on reduced chloride uptake was more pronounced in
the Mexican than the West Indian rootstock. An eightfold increase in the Cl-
concentration in the nutrient solution induced a 1,2 to 1,5-fold increase in
the Cl- concentration in the West Indian roots vs a 1,6-fold rise in
the Mexican roots. This was observed in the low N-N03-treated
seedlings, while in the high N-NO3 treatment, despite an eightfold
increase in the chloride concentration in the nutrient solution, Cl-
did not increase at all in the Mexican roots.
The reduced Cl- uptake resulting from the nitrate
application, as well as the reduced damage due to salinity in the avocado
seedlings, were also found in other species such as cotton, snap beans (1,11),
tomato (9,10) and barley (7).
The preliminary observations reported on are of significance in reducing
salinity damage to avocado plantations. Constant application of nitrates to
saline irrigation water might reduce Cl- damage to avocado trees,
especially those grafted on the salt-sensitive Mexican rootstocks.
1 Abdel Halim, MA, 1984. Physiological aspects of chlorate-nitrate
antagonism in higher plants. Proc VI Int
Coll for the Optimization of Plant Nutrition, Vol 1, 1-10. Ed Martin
Prevel, Montpellier, France.
2 Brom, M, Sne, M. Hausenberg, I, Har Gani, H & Ben-Ya'acov, A,
1984. Avocado irrigation under various levels of salinity and nitrogen. Tel
lsaak 1982/3 Report. Agric Res
Organisation, Bet Dagan, Israel. (Internal Report, in Hebrew.)
3 Embleton, TW, Garber, MG, Jones, WW & Richards, SJ, 1958. Effects
of irrigation treatments and rates of nitrogen fertilisation on young Hass
avocado trees. IV: Macro-nutrient content of leaves. Proc Am Soc Hort Sci
71, 310-314.
4 Haas, ARC, 1928. Relation of chlorine content to tipburn of avocado
leaves. Calif Avocado Soc Yrb, 57.
5 Haas, ARC, 1929. Composition of avocado seedlings in relation to
chlorosis and tip-burn. Bet Gaz, 87,
422-430.
6 Haas, ARC & Brusca, JN, 1955. Chloride toxicity in avocados. Calif Agric, 9(2), 12-14.
7 Hiatt, AS & Leggett, JE, 1971. Ionic interaction and antagonism in
plants. In: Carson, EN [Ed]. The Plant
Root and its Environment, 104-134. Charlottsville, Univ Press of Virginia.
8 Kafkafi, U, 1984. Plant nutrition under saline conditions. In:
Shainberg, I & Shalhevet J [Ed]. Soil Salinity under Irrigation. Ecological studies, No 51, 319-338.
Springer-Verlag, Berlin.
9 Kafkafi, U, Valoras, N & Letey, J, 1982. Chloride interaction with
nitrate and phosphate nutrition in tomato. J
PI Nutr, 5, 1369-1385.
10 Kirkby, EA & Knight, AH, 1977. Influence of the level of nitrate
nutrition on ion uptake and assimilation, organic acid accumulation and
cation-anion balance in whole tomato plants. PI Physiol, 60, 349-353.
11 Weigel, RC Jr, Schillinger, JA, McCaw, BA, Gauch, HG & Hsiao, E,
1973. Nutrient nitrate levels and the accumulation of chloride in leaves of
snap beans and roots of soybeans. Crop
Sci, 13, 411-412.