ABA Transport and Distribution in Relation to Its Function in Plants - Abscisic Acid: Metabolism, Transport and Signaling - page 60

4.1 ABA Content in Relation to ABA Biosynthesis
and Catabolism
ABA is ubiquitous in plant cells. The distribution and abundance of ABA change
greatly during development and under different environmental conditions. Under
non-stressed conditions, the ABA level in leaves is generally much higher than
in whole roots and stems. For example, the ABA content in the leaves of Zea
may (maize) plants is several fold higher than that of the roots and stems (200-
700 ng g − 1 DW vs. below 100 ng g − 1 DW, respectively) (Bahrun et al. 2002 ; Pekic
et al. 1995 ). Although the level of ABA varies in different leaves of an individual
plant, it does not differ much in different regions of the root. However, the level of
ABA in brace roots is several folds higher than that in the upper and lower roots
(Pekic et al. 1995 ). Furthermore, Pilet and Rivier ( 1981 ) found that the distribution
of ABA was asymmetrical in the elongation zone of horizontal maize roots, with
higher levels in the lower half and lower levels in the upper half of such roots. In
wheat ( Triticum aestivum ) plants, the ABA concentration in whole roots is about
45 pmol g-1 fresh weight (FW), whereas it is 98 pmol g − 1 FW in the apical parts
of roots (Vysotskaya et al. 2008 ). Rinne et al. ( 1994 ) reported that the ABA content
in the dormant buds of mountain birch ( Betzda pubescens ) is over 3,000 ng g − 1
FW, which is more than 15 times higher than that in maize leaves. Furthermore, the
ABA concentration in the terminal buds of short shoots is about four times higher
than in the basal buds. In the floral organs of navel orange ( Citrus sinensis Osbeck
cv. Washington), the highest ABA levels were observed in the stigma/style shortly
after anthesis (i.e., 11.5 or 3,036 ng g − 1 FW), and these levels were several times
higher than those measured in leaves (Harris and Dugoer 1986 ).
Relatively high ABA concentrations have been reported in fruits. For example,
in ripening grape berries ( Vitis vinifera cv. Doradillo) and strawberry ( Fragaria
ananassa ), the ABA content may reach 212 ng g − 1 FW Coombe and Hale (1973)
and 150 ng g − 1 FW (Jia et al. 2011 ), respectively. Much higher ABA concentra-
tions were found in some other fruit species. For example, in bilberry ( Vaccinium
myrtillus) fruits, the ABA content may reach over 30,000 ng g − 1 DW, which is
more than 70 times higher than that in leaves (Karppinen et al. 2013 ). Furthermore,
the ABA concentration in mangosteens ( Garcinia mangostana L. ) might be as high
as 6,600 ng g − 1 DW in some developmental stages (Kondo et al. 2002 ).
The ABA concentration in specific tissues is generally thought to be determined
by the rate of ABA biosynthesis. However, the rate of ABA catabolism may also
have an effect. The absolute rate of ABA catabolism was found to be proportional
to the level of ABA, i.e., the higher the level of ABA, the faster the catabolism
of this phytohormone (Jia and Zhang 1997 ). The rate of ABA catabolism has
been demonstrated to be quite high. In maize leaves, for example, the half-life of
ABA is less than an hour, and this implies that over 90 % of the ABA in a spe-
cific tissue would vanish as a result of ABA catabolism if ABA biosynthesis was
to cease completely. Given the rapid rate of ABA catabolism, stable levels of ABA
in specific tissues are due to a dynamic equilibrium between ABA biosynthesis