Agriculture Reference
In-Depth Information
The bulbing ratio can be measured non-destructively and can therefore be
determined repeatedly on the same plant. If treatment comparisons are made
which avoid the complications discussed above - for example, for comparing
cultivars growing under the same conditions at a similar plant density - it is a
useful technique for assessing bulbing. The precision of determining the start
of bulbing from such measurements has been improved by graphing the
cumulative sum of (bulbing ratio minus 1.2) against sample date - a technique
termed 'cusums', derived from quality control statistical methods. Upturns
from a line following the
x
-axis clearly indicate the start of bulbing (Lancaster
et al.
, 1996).
The partitioning of dry matter during bulbing
Prior to bulbing, while the leaf canopy is developing, about 74% of the shoot
dry matter growth is partitioned into leaf blades and the rest into stem bases
and leaf sheaths (de Visser, 1994a; Tei
et al.
, 1996). After bulb scales have
initiated, previously differentiated leaf blades are still expanding, so there is a
transition period while the partitioning of shoot dry matter growth to leaf
blades decreases from 74 to 0%, and all new growth goes to the storage scales.
This partitioning is well described by a Gompertz equation (see Fig. 4.27).
The rapidity of the transition in partitioning will vary with the development
rate, which is determined by temperature (de Visser, 1994a; Daymond
et al.
,
1997).
Fig. 4.27.
The fraction of dry matter production partitioned to leaf blades in an
April-sown onion crop growing at Wellesbourne, UK, showing the decline to zero
following bulb initiation. The fitted line is a Gompertz curve (from Tei
et al
., 1996.
Courtesy of
Annals of Botany
).
