Agriculture Reference
In-Depth Information
Fig. 4.13.
The observed onion seedling emergence from a sandy loam soil at
Wellesbourne, UK from sowings made on 23 March (a), 5 April (b) and 17 April (c)
1984 and the emergence predicted by a model based on thermal time and a critical
minimum soil water potential for root emergence from seeds (lines). Water
potentials and temperature round the seeds were calculated using another model
developed for this site that uses daily mean temperature and rainfall as input data.
The fitted lines were constrained to the maximum emergence observed in the field
(from Finch-Savage and Phelps, 1993. Courtesy of
Journal of Experimental Botany
).
The growth of seedlings after germination but before emergence from soil
As with germination, the rate of elongation of the cotyledon and primary root
depends on temperature and water potential but, in soil, mechanical
impedance also affects elongation. Figure 4.15a shows the linear relation
between temperature above a base temperature of 1.4°C and the rate of
cotyledon elongation of onion before formation of the cotyledon hook (Wheeler
