Agriculture Reference
In-Depth Information
emerged from the top of the pseudostem, the number of fully expanded leaves (on
which the ligule at the base of the leaf lamina can be seen) and the number of
senescent leaves show straight line increases when plotted against accumulated
day-degrees above 0°C (see Fig. 4.48).
For cv. 'Autumn Mammoth', Fig. 4.48 shows that leaves were initiated at a
constant rate of one per 92°C days (base temperature 0°C) and that leaves
appeared at one per 135°C days. Leaves took longer to appear than to initiate,
not because leaf initials accumulated at the shoot apex, as with cereal plants,
but because the pseudostem increased in length by a constant amount for
every leaf that emerged. Leaf extension growth occurs in the first 3-4 cm
above the apex and the rate of extension was almost the same for successive
leaves. Hence, although the rate of elongation of successive leaves was the same
(per degree-day), every leaf had to grow further than its predecessor to appear.
The process is summarized in simple mathematical terms by the equation:
DD per leaf appearance = DD per leaf initiation + Additional
pseudostem length per leaf/Rate of leaf elongation per DD
(Eqn 4.23)
Here, DD stands for day-degrees above 0°C (Hay and Kemp, 1992).
Fig. 4.48.
Leaf development in leek cv. 'Autumn Mammoth' grown from a mid-April
sowing in Scotland. Changes with accumulated thermal time (base 0°C) in the total
number of leaf structures initiated (s), the number of leaf tips visible above the leaf
sheath (t), the number of fully extended leaves with visible ligules (l), the number of
leaves that were at least 50% senescent (d) and the number of primordial leaves at
the shoot apex (p). The lines were fitted by linear regressions on data from 20 plants
per sample date (from Hay and Kemp, 1992. Courtesy of
Annals of Applied Biology
).
