Determining seed vigour in selected Brassica species
Determining seed vigour in selected Brassica species
Leeks, Conrad R. F.
Leeks, Conrad R. F.
Date
2006
Type
Thesis
Fields of Research
Abstract
Variables for the accelerated ageing (AA) test, methods for reducing fungal
contamination during the AA test, using the conductivity test as a vigour test, the
effect of seed size on seed vigour and the relationship between laboratory test results
and field perfonnance in selected Brassica spp were investigated. In the first
experiment, three seed lots of turnip rape hybrid (B. rapa x campestris), turnip (B.
campestris) and forage rape (B. napus); and seven seed lots of Asian rape (B. napus),
six seed lots of Asian kale (B. oleraceae var. alboglabra L.) and five seed lots of
choisum (B. rapa var. pekinensis) with germinations above 90% were aged at two
different temperatures (41 and 42°C ± 0.3°C) and three ageing times (24, 48 and 72 ±
15 minutes). The second experiment was divided into three sections. In the first, the
same seed lots and species were aged at one temperature (41°C) and time (72 h), but
either 40 ml of saturated salts; KCl (83%RH), NaCl (76%RH), NaBr (55%RH); or
distilled water (96%RH) were used as the ageing solutions. In the second, one turnip
rape hyprid seed lot was aged at three temperatures (41, 42 and 45°C) and two times
(72 and 96h), again using the three saturated salts and distilled water as ageing
solutions. In the third, three turnip rape hybrid seed lots and three Asian kale seed lots
were surface sterilised (1 % sodium hypochlorite) prior to ageing at one temperature
(41°C) and time (72 h). In the third experiment, the same species and seed lots used in
experiment one at their original seed moisture content (SMC) were tested for
conductivity after soaking in deionised water for 4, 8, 12, 16, 20 and 24 h. They were
then re-tested after the SMC had been adjusted to 8.5%. In the fourth experiment,
three seed lots of forage rape and three seed lots of Asian kale were graded into three
seed size categories; large (retained on a 2.0 mm screen), medium (retained on a 1.7
mm screen) and small (passed through a 1.7 mm screen). Graded seeds were then
tested for standard germination, AA (41°C/48 h) and conductivity (measured at 16 and
24 h). In the final experiment, the relationships between laboratory tests for the six
species (each consisting of three seed lots), field emergence from three sowings, and
cold room emergence were evaluated.
Both time and temperature influenced post-AA germination. Increasing the
ageing period from 48 to 72 hours at 41°C, and 24 to 48 hours at 42°C resulted in
decreased mean germination percentage for all species but not always clear separation
of seed lots. While there were sometimes few differences between ageing at 41°C and
42°C, the former is preferred because it is already the temperature used for other
species. For Asian rape, choisum and turnip, the previously recommended testing
conditions of 41°C/72 h provided good seed lot separation, but for Asian kale and
turnip rape hybrid, AA testing at 41°C/48 h provided better results. Seed moisture
content after ageing ranged from 29-37% depending on species. Fungal growth on
seeds during the ageing period appeared to reduce post-ageing germination in some
seed lots .
Substituting saturated salts for distilled water did not stress seed lots in the AA
test, due to the lowered RH%, the exception being seed lots 1210 and 1296. For
forage and Asian species, seed lot germination mostly remained above 90% when
aged for 72 h at lowered RH%. Increasing the ageing duration from 72 to 96 hours
resulted in some decreases in post-AA germination but no clear separation of seed
lots. Surface sterilising the seeds prior to the AA test resulted in a lower incidence of
contaminant fungi which was associated with a lower percentage of abnormal
seedlings.
The conductivity test was mostly able to identify vigour differences among
forage and Asian vegetable brassica seed lots. Differences in conductivity readings
were observed among seed lots in all species. Increasing the period of imbibition
resulted in increased conductivity from most seed lots but radicle emergence occurred
after 16-20 h of imbibition. Variation was observed in the time to reach 95%
maximum of the imbibition curve for most species. Conductivity readings at 16 h
would avoid possible influences of radicle emergence on results. Adjusting the SMC
to 8.5% resulted in reduced variation in conductivity among replicates of seed lots,
due to a reduction in imbibition damage.
Seed size had a significant effect on both post-AA germination and
conductivity results. In forage rape, large size seeds had higher post-AA germination
cf. medium cf. small size seeds. In Asian kale, large size seeds had higher post-AA
germination compared with small size seeds. For both forage rape and Asian kale,
large size seeds had lower conductivity readings cf. small size seeds.
The correlation analyses demonstrated significant relationships between AA
testing and field emergence parameters (percentage emergence, emergence index and
emergence rate). Significant relationships were also observed between conductivity
testing and these field emergence parameters. Based on the correlation analysis, AA
testing at 41°C/48 hand/or 42°C/48 h could be recommended to be used as an AA
test for turnip and Asian rape; and 41°C/48 hand/or 41°C/72 h for Asian kale and
choisum. Based on the correlation analysis, conductivity testing at 16 h can be used to
predict the field emergence potential of forage and Asian vegetable seed lots. Vigour
tests were consistently able to provide better indicators of field perfonnance than the
standard germination test, although these relationships did vary with the different field
sowings.