Agriculture Reference
In-Depth Information
Fig. 4.16.
The effect of soil water potential on the rate (a) and the percentage (b) of
emergence of onion seeds at 15°C. The rate was measured as the reciprocal of the
time to reach 50% of the final number emerging. The differences in soil water
potential were caused by differences in osmotic potential rather than matric
potential (redrawn from Hegarty, 1976).
The results of these experiments were summarized by a quantitative model
expressed by Equations 4.14 and 4.15:
Rate of elongation in thermal time =
dL/d(t - t
g
) = b(1 -
/
L
)(1 - (q / q
L
)
n
)
(Eqn 4.14, see Table 4.4 for
[A(t - t
g
)(1 -
/
L
)(1 - (q / q
L
)
n
) - L]
explanation of symbols).
The maximum shoot length attainable decreases if the shoot is slowed or
prevented from growth by mechanical impedance (see Fig. 4.20). This is likely
to be due to the depletion of seed reserves and the rate of depletion to depend on
temperature, and therefore a function of thermal time is appropriate to model
the effect. To account for this, the term A(t - t
g
) in Eqn 4.13 decreases as
thermal time elapses, according to Eqn 4.15:
A(t - t
g
) = C / (1 + [(t - t
g
) / d)]
m
)
(Eqn 4.15)
where
d = fitted constant, value 356°C day (base 2.1°C)
C = fitted constant, value 83.16 mm
m = dimensionless scaling parameter, value 1.735
A, t and t
g
are as defined for Eqn 4.14.
