A comparison of pre-soaking and pre-washing as methods of increasing beet (Beta vulgaris) and New Zealand spinach (Tetragonia tetragonioides) seed germination : a thesis presented in partial fulfilment of the requirements for the degree of Master of AgriScience in Seed Science and Technology at Massey University, Palmerston North, New Zealand
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Date
2011
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Massey University
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Abstract
Water soluble germination inhibitors located in the pericarp of Beta vulgaris fruits have
historically been reported to be one of the causes of seed dormancy in fruit of this species.
Dormancy can lead to low and non-uniform germination, which is a problem in the seed
production system. Dormancy-breaking treatments that have been applied to B. vulgaris fruit
include steeping, priming, rubbing the fruit coat or scarification, and washing and soaking the
fruits prior to sowing as a way of leaching out the inhibitors. Anecdotal evidence suggests
that soaking the fruits is as effective as the washing dormancy-breaking treatment
recommended by the International Seed Testing Association (ISTA).
In this study, the presence or absence of dormancy in 18 B. vulgaris (11 vegetable beet, 4
fodder beet, and 3 sugar beet) seed lots was assessed by conducting germination tests on
fruits in the absence of dormancy-breaking treatments. There was little or no dormancy in the
seed lots tested. Six seed lots had over 90% germination, another six seed lots had more than
80% germination. Four seed lots had germination ranging from 76 to 79% while two seed lots
had germination lower than 67%. These low germination percentages were due to empty
fruits rather than dormancy. This suggests that dormancy may no longer be a problem in
modern B. vulgaris cultivars.
Two B. vulgaris seed lots were identified as having some residual dormancy as indicated by a
high percentage of ungerminated viable fruits. These were Flores sugar beet and an
“unnamed” (Kings Seeds) sugar beet cultivar. These had 14% and 16% ungerminated viable
fruits respectively. Two other seed lots identified had less than 90% germination, Brigadier
and Kyros fodder beets, and had high percentages of ungerminated viable fruits (5%)
compared with other seed lots in the same germination range. Dormancy-breaking treatments
were conducted on the four seed lots. The dormancy-breaking treatments were i) fruits
washed in running water at 25 °C for two hours, and ii) fruits soaked in 25 °C water for two
hours, changing the water every 30 minutes. The aim of this experiment was to compare the
effectiveness of the two treatments in removing residual dormancy by leaching inhibitors
from the fruit coat. The effectiveness of these dormancy-breaking treatments was determined
by measuring the germination percentage, time to 50% germination and the uniformity of
germination. No significant differences were found between the germination of soaked or
washed fruits compared with the control i.e. 84%, 89%, and 89% for Flores sugar beet, 77%,
80%, and 71% for the Kings Seeds sugar beet, 75%, 80%, and 73% for Brigadier fodder beet,
and 93%, 91%, and 91% for Kyros fodder beet for control, soaking and washing treatments
respectively. This suggested that water soluble germination inhibitors are absent from the
fruit coats as the water treatments did not alleviate the residual dormancy. There were no
significant differences in the time to 50% germination (T50) of the seed lots following the
treatments, except in Flores sugar beet where the soaking slowed down germination (49.9,
72.26, and 71.17 hours for the control, soaking, and washing treatments respectively).
Tetragonia tetragonioides was the only other species in the ISTA Rules for which the two
hour washing treatment was recommended as a dormancy-breaking technique. The same
dormancy-breaking treatments were applied to fruits of a T. tetragonioides seed lot. There
were no significant differences in germination for the control, washing and soaking
treatments (72%, 63%, and 70% respectively). The high percentages of ungerminated viable
fruits of 26%, 32%, and 28% remaining for untreated, soaked, and washed fruits respectively
indicated high levels of dormancy and inability of the dormancy-breaking treatments to
alleviate this. A follow up experiment following an ISTA Rules change in 2011 that changed
the two hour washing treatment to soaking the fruits for 24 hours was conducted to assess the
effectiveness of the new 24 hour soaking recommendation in alleviating dormancy. The 24
hour soaking treatment gave a significantly lower germination (40%) and high percentage of
ungerminated fruits (60%) compared with the control, two hour soaking, and two hour
washing treatments. Cutting the endocarp of ungerminated fruits at the end of the 35 days
germination period alleviated the dormancy in all remaining fresh viable fruits. The study
also suggests that T. tetragonioides germination is inhibited by the presence of excess water
past the early stages of imbibition, but further work is required to confirm this.
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Keywords
Beets, Seeds, Germination, Tetragonia tetragonioides, Beta vulgaris, Pre-soaking, Pre-washing