Yoshida - 1981 - Fundamentals of Rice Crop Science

154 FUNDAMENTALS OF RICE CROP SCIENCE
concentrations in the soil solution alone cannot explain why rice suffers from zinc
deficiency on Kala Shah Kaku soil.
3.10.3. Effects of bicarbonate and organic acid on zinc absorption
Bicarbonate and organic acid may accumulate to toxic levels in submerged soils
depending on soil pH, organic matter content, and soil temperature.
A concentration of 15 or 30 mM HCO 3
-
causes a reduction of about 70% in the
65
Zn content of the shoots, but only a reduction of about 5% in the roots (Fig.
3.20). These results suggest that the effect of HCO 3
-
on zinc transport to the shoots
is greater than its effect on absorption by the roots. HCO 3
-
has a similar effect on
zinc transport from root to shoot in wheat (Forno et al 1975a). In calcareous and
other alkaline soils, bicarbonate concentrations exceeding 10 mM are common
during 3-6 weeks after submergence. Thus, the inhibitory effect of bicarbonate on
zinc absorption is likely to be one of the major causes of zinc deficiency in alkaline
soils.
The effect of acetic acid on zinc absorption varies with pH. At pH 4.5, acetic
acid at a concentration of only 0.3 mM reduces the 65 Zn in the shoots by 70% and in
the roots by about 77%. At concentrations of 10 and 30 mM, plants wilt, and the
65
Zn activity in the shoot and root tissues is reduced by 94% and 97%, respectively
(Fig. 3.21). At pH 7.5, however, additions to the culture solution of acetic acid up
to 3 mM have no detectable effect on 65 Zn uptake; the effects are small even at a
concentration of 10 mM. However, at 30 mM, the 65 Zn activity in the roots is
reduced by 51%, while that in the shoots is reduced by 75%.
3.20. Effect of NaHCO 3 on the 65 Zn absorbed
by the rice variety IR8 at pH 7.3 (Forno et al
1975a).