Page 429 - Biosystems Engineering

406 Cha pte r F i f tee n

(e.g., on the proton pump mechanism, permeability of cell walls, and
26
metabolism of the ions taken up). Faust and Miller divided exoge-
nous BRs into ones that affect these processes through changes in the
metabolism of IAA (NAA, IBA, PB) and gibberellins (GA , GA , PB),
3 4+7
and the ones whose mode of action is complex and so far not speci-
fied definitively (daminozide).
BRs are believed to affect the calcium content of fruit directly,
through changes in the transport of the auxin IAA (indole acetic acid),
and indirectly, as a result of changes in fruit size, limiting competition
among shoots, or changes in root weight.
According to Faust and Miller, BRs, when acting directly, usu-
26
ally cause a drop in the calcium content of fruit. It is generally diffi-
cult to determine when BRs act indirectly and when their direct effect
begins. The problem is quite complicated, particularly in the case of
trees that are already bearing fruit. It is easier to explain it using
young plants that have not yet started bearing fruit because it is pos-
sible to do experiments on such plants under controlled conditions.
However, the results obtained for nonfruiting seedlings do not fully
reflect the mineral nutritional status of mature, fruit-bearing trees.

15.1.1 Auxins and the Mineral Status of Fruits
Calcium migrates upward within a plant. 2,4,29 The direct effect of BRs
on Ca uptake is related to the transport of IAA in the plant, and the
upward transport of calcium depends on the downward transport of
the auxin IAA. Auxin transport is modified by BRs, proved by Ban-
14
son and Stahly. These researchers, using TIBA (2,3,5-triiodobenzoic
acid), an auxin transport inhibitor 2 to 6 weeks after flowering, found
a marked decrease in the calcium content of apples. TIBA caused the
greatest reduction in calcium levels when it was applied 2 weeks
after flowering. Because the treatment was delayed, the preparation
had a progressively weaker effect on calcium transport. The TIBA-
treated apples, however, always contained less calcium than the con-
trol apples, even when apples of the same size were compared. Par-
ticularly large differences in calcium levels between the TIBA-treated
and TIBA-untreated apples were found 7 weeks after flowering. As a
result of spraying with TIBA, a large number of apples were affected
by a bitter pit. Other studies by Stahly and Benson 66,67 indicated that
TIBA, while reducing calcium levels, increased the levels of potas-
sium at the same time, but not until the second half of the summer.
This caused an increase in the potassium-to-calcium ratio, which
meant a reduction in the storability of apples. TIBA did not affect the
levels of such elements as Mg, P, or B in apples but did cause a higher
nitrogen content.
Because the reduction in auxin transport decreased the supply of
calcium to the fruit, it could be expected that increased levels of aux-
ins in a tree would cause greater accumulation of calcium in its fruits