Monocarboxylic acids most readily stimulated 90% germination
of dehulled red rice (Table I). Isobutyric and isovaleric
acids were as active as the linear forms. Nonaromatic hydroxyacids
(glycolic and lactic acid) required one order of magnitude
higher concentrations for responses similar to their parent
acids (acetic and propionic acid). Esters elicited activity
at 30 to 130 mm and showed increasing activity with increased
carbon number. A similar relationship was observed for the
alcohols. Acetaldehyde and propionaldehyde were effective at
ca. 40 mM. However, several substances of interest broke
dormancy but did not elicit 90% germination: butanol, succinic
acid, and salicylic acid. Therefore, to facilitate structureactivity
comparisons, concentrations required to obtain 50%
germination were obtained from dose-response experiments
(data not shown) for these chemicals as well as others
(Table II).
The log K01w was plotted versus the log concentration of
each substance required for 50% germination (Fig. 1). A
significant correlation (P < 0.001) between this index of
lipophilicity and dormancy-breaking activity was obtained.
The dormancy-breaking activity within the monocarboxylic
acid group was significantly correlated to log K01 (P < 0.001).
A similar relationship was obtained for the alcohols (P < 0.05)
(Fig. 1, inset). The slopes and intercepts of these two lines
were significantly different from each other, as shown by a
test of homogeneity of regression coefficients (P < 0.05).
Correlations between relative hydrophobicity, using ethane
(Fig. 2) or methane (data not shown) as the parent compounds,
and the log concentration of each substance required
for 50% germination were not statistically significant. Fortyeight-hour
exposures to propane, butane (each ca. 1 mM), or
pure carbon monoxide promoted germination no more than
5% above control levels.
The dormancy-breaking activity of substances with smaller
size dimensions (cyanide, azide, nitrite, formic acid, andketones, which do not possess dissociable protons. In this
report, we show that relative dormancy-breaking activity of a
wide array of substances is primarily related to their lipophilicity
but is modulated by the nature ofthe functional group(s)
present or by molecular size.