Agriculture Reference
In-Depth Information
for growth of pollen tubes. Decreases in crop propagative yield or seed/fruit quality
in low B soils can be due to diminished reproductive development early or late in
the flowering/fruiting cycle. It has often been seen that reproductive growth, mainly
flowering, fruit and seed set and seed yield, is particularly sensitive to B deficiency
compared to asexual growth (e.g. Woodbridge et al.
1971
; Dear and Lipsett
1987
;
Noppakoonwong et al.
1997
). Likewise, substantial yield decreases can arise
without expression of indications of deficiency during prior somatic growth. B
also plays an important role in synthesis and metabolism of nucleic acids (Hundt
et al.
1970
).
As described above, B is responsible for enhancing chlorophyll content and rate
of photosynthesis (
P
n
), as well as inducing dry matter production in plants, and
therefore may result both in enhancing flowering and also the transport of photo-
synthetic products to reproductive stages, ultimately leading to yield improvement
(Du Ying Qiong et al.
1999
).
Factors influencing the impact of a low external B supply on sexual reproduc-
tion in flowering plants are likely to include: the capacity of roots to obtain B
from soil (Hu and Brown
1997
); the mobility of B in the phloem (Brown and
Shelp
1997
); the relative sink size in floral parts for photosynthate; the capacity to
redistribute B from vegetative tissues to reproductive organs; the rate of transpi-
ration by floral organs; the functional necessity for B in reproductive tissues;
and the distribution and richness of B-binding compounds in the apoplastic
pathway between the vein endings and the most distal floral tissue. The B
requirement for flowering is indicated by the sensitivity of pollen development
to low B and the generally high concentrations of B that occur in reproductive
parts of the flower. Under conditions of low external B supply, levels in
the anthers and pistils do not decline to the low levels measured in leaves.
B concentrations are higher in the stamen than in the pistil. The physiological
roles for B in sexual reproduction have yet to be fully defined and there is a need
for experimentation in this area.
Many of the studies that have been undertaken do not give definitive informa-
tion as to whether plants were critically deficient in B at the time of flowering, or
the B status of floral tissues was not determined at the time of impairment in
cellular development/function, or cell structure and metabolism were examined
long after the primary effect of B took place. An example is the observation that B
deficiency results in male sterility, a condition that can be induced by deficiencies
in other nutrients (e.g. Mn, Cu) or unfavourable environmental conditions
(e.g. water deficit, high/low temperature). These observations reveal nothing
about the processes that are being affected by low B supply and do not enable
us to conclude whether the effect of low B supply on male sterility is a direct or
indirect event. As root function is greatly impaired in severely B-deficient plants
in containers and this can impact on whole plant physiology, the requirement of
pollen development for B should be studied under conditions of controlled
external supply.
