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IV. Role of Boron in Plants.
BORON NUTRITION OF CROPS
Deficient tomato (Lycopersicon esculentum, Mill .) plants were found to translocate more sugar when 50 ppm of B was added with sucrose through a cut petiole
than when sucrose was applied alone. Subsequent studies by Dugger and Humphreys (1960) implied a direct involvement of B in the enzymatic reactions of
sucrose and starch synthesis. It has also been suggested that B deficiency possibly causes reduced synthesis of uridine diphosphate glucose (Birnbaum et al.,
Weiser et al. (1964) reported that B does not enhance sugar translocation in
plants, but it does enhance the foliar uptake of sucrose applied to the leaves.
They concluded that this phenomenon of enhanced foliar uptake of sucrose has
given rise in the past to the erroneous conclusion that B enhances sugar translocation.
Zapata (1973) found that sugar cane (Saccharurnoficinarum L.) plants receiving only traces of B suffered growth and quality losses without developing visual
B-deficiency symptoms. Lack of B lowered sucrose production in leaves and
significantly altered the rates of sugar transport in sugar cane storage tissues
(Zapata, 1973). In sugar beets the sucrose content of the storage roots started to
decrease at about the same point at which limiting B resulted in a drop in yield
(Vlamis and Ulrich, 1971).
The earliest morphological symptoms of B deficiency in mung bean
(Phaseolus uureus L.) appear to be a slowdown in root extension, followed by a
degeneration of meristematic tissue, possibly due to a repressive effect of B
deficiency on cell division (Jackson and Chapman, 1975). Results of Robertson
and Loughman (1974) indicated that it is unlikely that responses associated with
B deficiency are caused by interference with cell division, but they may be
related to the role of B in the metabolism, transport, or action of auxin-type
hormones in broad beans (Vicia fubu L.). Whittington (1959) found that
B-deficient field bean roots had enlarged apices and fewer cells than the normal
B-sufficient roots. Investigations of Kouchi and Kumazawa (1976) on tomato
root tips indicated that a lack of B distorted the shape and arrangement of cortical
cells and resulted in an abnormal accumulation of a “lipid-like substance.”
Also, there was an abnormal development of Golgi apparatus, which seemed to
be related to the irregular thickening of cell walls.
Cohen and Lepper (1977) established that cessation of root elongation of intact
squash (Cucurbitupepo L.) plants is an early result of B deficiency. They noted
that the ratio of cell length to cell width ranged from a low of 0.8 in B-sufficient
root meristems to a high of 3.0 in root meristems grown in a B-deficient nutrient
solution for 98 hours. It was concluded that a continuous supply of B is not
essential for cell elongation but is required for maintenance of meristematic
Bioassays showed that extracts of substances similar to indoleacetic acid
(IAA) taken from B-deficient roots were more inhibitory to the growth of bean-
UMESH C. GUFTA
root segments than were those from normal roots (Coke and Whittington, 1968).
The IAA treatment and B deficiency together restricted root growth more than
did either B deficiency or IAA treatment. Bohnsack and Albert (1977) demonstrated that B deficiency in squash resulted in increased IAA oxidase activity, but
root elongation was inhibited. Roots of plants subjected to 12 hours in a B-free
medium and then transferred to a medium containing B regained normal elongation rates and oxidase activity within 18 to 20 hours. They further suggested that
high levels of IAA under B-deficiency conditions may inhibit cell division and
lead to an induction of the IAA oxidase enzyme.
Deficiency of B results in browning of plant tissues, which is thought to be
related to the accumulation of polyphenolic compounds; it is also postulated that
B is involved in the synthesis of cell-wall components (Slack and Whittington,
1964). Lee and Aronoff (1967) suggested that B combines with
6-phosphogluconic acid to form an enzyme-inhibitor complex, which regulates
phenol synthesis, thereby preventing the typical necrosis and ultimate death of
B-deficient plants. Birnbaum et al. (1977) found that, when B was lacking in the
medium, cotton ovules accumulated brown substances.
Investigations of Jackson and Chapman (1975) with mung beans and broad
beans suggest that the earliest known response to the removal of B from a plant
culture medium is increased incorporation of precursor into the RNA of the root
tip region. These responses to B deficiency are markedly similar to some of the
effects of the application to plant tissue of such hormones as auxin, gibberellic
acid, and cytokinin.
Any element that is essential for the growth and development of plants must
have a direct or indirect influence on N metabolism, including synthesis of
proteins. Sherstnev and Kurilenok (1964) found that, in B-deficient sunflower
(Helianthus annuus L.) plants, the content of many amino acids increased significantly. Such higher content of amino acids was explained by an acceleration in
protein decomposition or by a deceleration in protein synthesis. It was suggested
that probably both processes occurred, but the deceleration in protein synthesis
was probably the dominant process. Studies by Kibalenko et al. (1973) showed
that during photosynthesis the rate of 14C02incorporation into free amino acids
was higher in sugar beet (Beta vulgaris L.) plants grown on a nutrient medium
containing B than in B-deficient plants. However, in leaves of sugar beet and pea
(Pisum sativum L.) plants, the level of free amino acids was higher and that of
protein was lower than in leaves of plants grown in a full nutrient medium. Thus,
B deficiency significantly inhibited protein synthesis.
Boron deficiency in tomatoes has been found to be induced by N bases, such
as 6-azauracil and 2-thiouracil, whereas uracil antagonized the effect of these
bases and prevented the appearance of B-deficiency symptoms (Albert, 1968).
Likewise, studies on cotton indicated that B-deficiency-like symptoms were
BORON NUTRlTION OF CROPS
induced by 6-azauracil and 6-azauridine in ovules growing in B-sufficient media
(Birnbaum et al., 1977). Other nucleoside-base analogs either reduced or had no
effect on overall growth, but did not cause typical B-deficient callus growth of
cotton ovules. Orotic acid and uracil countered the effects of 6-azauracil.
Similarities between symptoms of B deficiency and 6-azauracil injury, and the
ability of uracil to suppress both, suggest that B-deficiency symptoms are related
to reduced activity in the pyrimidine biosynthetic pathway and are not related to a
reduction in nucleic acid synthesis.
In field experiments on peat and mineral soils, ergot infestation of barley
(Hordeum vulgare L.) was decreased and grain yields increased by B fertilization (Simojoki, 1969). It was suggested that B deficiency caused structural
changes in the plant, resulting in greater susceptibility.
Pollard et al. (1977) reported that a deficiency of B in corn and broad beans
reduced the capacity for absorption of phosphate. They also found that the
B-deficient roots of corn had a reduced ATPase activity, which could be rapidly
restored by the addition of H3BO3 an hour before extraction of the enzyme. The
evidence strongly supports the view that B plays an essential role in the regulation of the functions of higher plant membranes and that the ATPase is a possible
component of transport process. The possible mechanisms whereby this control
is exercised include direct interaction of B with polyhydroxy components of the
membrane and the elevation of endogenous levels of auxins. Gupta and MacLeod
(1977) noted that B deficiency produced yellow and violet discoloration in
rutabaga leaf tissue. This discoloration may have been due in part to the reduced
uptake of P caused by a lack of B, as suggested by Pollard et al. (1977) for corn
and broad beans.
V. Factors Affecting Boron Requirement and Uptake in Plants
A. SOIL :pH,
CALCIUM, AND MAGNESIUM
Soil pH is one of the most important factors affecting the availability of B in
soil and plants. Generally, B becomes less available to plants with increasing soil
pH. Several workers have observed negative correlations between B uptake by
plants and soil pH (Bennett and Mathias, 1973; Bartlett and Picarelli, 1973;
Gupta, 1972b; Wolf, 1940). However, this relationship is not consistent, and
deviations from this effect occur, owing to factors such as crop species (Gupta,
1972a, 1977). Studies by Peterson and Newman (1976) and Gupta and MacLeod
(1977) have shown that a negative relationship between soil pH and plant B
occurs when soil pH levels are greater than 6.3-6.5. The availability of B to
plants decreases sharply at higher pH levels, but the relationship between soil pH
UMESH C. GUPTA
and plant B at soil pH values below 6.5 does not show a definite trend. Gupta
(1968) did not find any relationship between hws B and pH on 108 soil samples
from Eastern Canada ranging in pH from 4.5 to 6.5.
Eck and Campbell (1962) found that liming decreased B uptake when soil B
reserves were high. They attributed this effect to a high Ca content. Robertson el
al. (1975) reported that no close relationship between available soil B and soil pH
was found in Michigan soils. However, they reported that soil test levels of B in a
calcareous soil decreased rapidly after B application.
Tanaka (1967) reported that B uptake by radish (Raphanus sativus L.) was
reduced when the Ca content of the medium was increased. Beauchamp and
Hussain (1974), in their studies on rutabaga, found that increased Ca concentration in tissue generally increased the incidence of brown-heart. Wolf (1940)
found that Mg had a greater effect on B reduction in plants than did Ca, Na, or K ,
but the differences between Ca and Mg effects were small. However, in the
previous work no distinction was made between the effects of soil pH and levels
of Ca and/or Mg on B uptake.
Gupta and MacLeod (1977) conducted experiments to distinguish between the
effects of soil pH and sources of Ca and Mg on the availability of B to plants.
They found that, in the absence of added B, rutabaga roots and tops from Ca and
Mg carbonate treatments had more severe brown-heart condition than did roots
from the Ca and Mg sulfate treatments. The B concentrations in leaf tissue of
rutabaga from treatments with no B were lower at higher soil pH values where Ca
and/or Mg were applied as carbonates than they were at lower soil pH where
sulfate was used as a source of Ca and/or Mg (Table 111). In the presence of added
B this trend was not clear, but the levels were well above the deficiency limit.
The lower B concentrations in no-B treatments with carbonates than in those with
sulfates appear to be related to soil pH differences. It was also noted that the
effect of applications of lime on B uptake was not related to the availability of Ca
and/or Mg, since equivalent amounts of Ca and/or Mg were applied as sulfates,
compared with those added as carbonates; furthermore, Ca and Mg concentrations in the plant tissue were similar (Gupta and MacLeod, 1977). Barber
(1971) reported a reduced uptake of B by soybeans [Glycine max.(L.) Merr.] as
the soil pH increased. However, the author pointed out that the pH effect might
be important on some soils and have little effect on others.
Gupta and Cutcliffe (1972) noted an interaction between soil pH and hws B on
the severity of brown-heart in rutabaga. The degree of brown-heart was found to
be more severe at high soil pH than at low pH. However, at high rates of B, soil
pH as high as 6.8 had no effect. Decreased uptake of B with increased soil pH
has been reported for alfalfa, soybeans, and barley by Wear and Patterson
(1962), Barber (1971), and Gupta (1972b), respectively.
The data by Gupta and MacLeod (1977) showed no differences in B uptake
whether the plants were fed with Ca and/or Mg as long as the corresponding