Agriculture Reference
In-Depth Information
Temperature and soil moisture effects on field germination and emergence
A simplified model based on hydrothermal time concepts (Eqn 4.10) has given
good predictions of onion emergence from field sowings (Finch-Savage and
Phelps, 1993) and indicates the optimum timing of a single irrigation after
sowing to maximize the percentage seedling emergence and minimize the
spread of emergence (Finch-Savage, 1990; Finch-Savage and Steckel, 1994). It
is assumed that germination, i.e. the emergence of the radicle from the seedcoat,
is the most moisture-sensitive stage in germination and emergence. For the
radical to appear, the water potential around the seed must exceed a critical
threshold,
b , which is about -1.1 MPa for onions (Finch-Savage and Phelps,
1993). Therefore, the water potential
b is a critical barrier restricting seedling
development until it is exceeded. The timing of periods when
in the soil exceeds
b determines the timing of seedling emergence. In most species radical
extension growth is less sensitive to moisture stress than radicle emergence from
the seed, and the radicle also enters moister soil as it grows downwards. As
explained previously, germination also depends on temperature and requires a
certain quantity of thermal time,
.
Under field conditions both temperature and soil water potential vary
unpredictably, but under good horticultural practice seeds are sown into moist
soil and initial imbibation is rapid. Once imbibed, a seed can progress towards
germination provided it remains above
b progress
towards germination occurs according to a simple thermal time model (Eqn
4.6). If
b . Therefore, if
is above
falls below
b , germination cannot occur until either rainfall or
irrigation increases
b again. The thermal time for germination differs
for different percentiles of the seed population: for example, the 10th, 50th and
90th percentiles required 55, 77 and 111°C day, respectively (above base
1.4°C) in the experiments of Finch-Savage and Phelps (1993). Hence, the
germination and consequently the emergence of seedlings is spread over time
and, if soil water potential falls below 1.1 MPa within this range of thermal
time, only the lower percentiles will emerge until soil water potential increases
again above 1.1 MPa. This results in the flushes of emerging seedlings that are
frequently observed in seedbeds that dry out and are then re-wetted and a step-
like graph of the percentage emergence versus time (see Fig. 4.13).
Timing a light irrigation (e.g. 12-15 mm of water) to occur when
sufficient thermal time has elapsed for the initially imbibed seeds to be ready to
germinate results in consistently higher percentage emergence and reduced
spread of emergence time than pre-sowing irrigation or no irrigation of the
seedbed. Irrigation at about 90°C day above a base of 1.4°C means that most
seeds will be advanced to the point where they can germinate when
above
exceeds
b (see Fig. 4.14a, b). Unnecessary irrigations are best avoided as they can
damage soil structure and thereby make conditions less favourable for
emergence, so it is useful to optimize the timing of irrigation using this model.
The same principles, with a very similar day-degree requirement for timing
irrigation, have been shown to apply to leek (Finch-Savage and Steckel, 1994).
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