Changes in germination performance and hydrolytic enzyme activity in wheat seeds (Triticum aestivum L.) caused by ageing and pre-sowing treatments : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Seed Technology at Massey University, Palmerston North, New Zealand
Three pre-sowing hydration-dehydration treatments were evaluated for their
capacity to protect or repair wheat seeds stored under two different sets
of artificial ageing conditions (accelerated ageing at 1 0 0 % RH, 40°C or controlled
deterioration at 1 5 % SMC, 3 5 ° C). Although similar losses in germination
capacity and decreases in radicle emergence rates occurred under both ageing
conditions, differences with respect to the physiology of ageing were highlighted
by changes in seedling growth and seed leakage. For example, increases
in seed leakage observed during storage at 1 5 % SMC were not found at
Longer hydration treatments (either 24 h at l 5°C in water or 20 h at 20°C
in -0.37 MPa PEG solution, followed by drying) improved the vigour of unaged
seeds, but treated material deteriorated rapidly in storage compared to untreated
controls. In contrast short hydration treatments (2 h at 25 ° C followed
by drying) offered some protection of germinability during subsequent storage
but did not affect the vigour of unaged seeds. When seeds were treated after
storage, longer hydration periods were effective in producing substantial
invigoration of viable deteriorated seeds (measured by evaluating T50 or
seedling growth) compared to little or no improvement by short hydration
treatments. These results support earlier suggestions from work on tomato
seeds that losses in seed vigour and viability are not necessarily a continuum
of the same deteriorative sequence.
The mechanisms of protection of germinability by short hydration treatments
were not clear. Small decreases in T50's of unaged or aged seeds as a result
of these treatments were due to leakage of germination inhibitory substances.
However, the rapid germination of unaged and improved responses from aged
seeds caused by longer hydration treatments suggested advances in germination
processes and repair activity under these conditions. This aspect was pursued
in further detail by studying changes in the hydrolytic metabolism of wheat
seeds using the 20 h PEG treatment.
Although the starchy endosperm of treated seeds showed some indications
of protein degradation, there were no changes in proteolytic activity (determined
as 'Azocoll' hydrolysing activity at pH 6.8) as a result of ageing or
pre-sowing treatment after storage. However, there were some indications
of loss of control over proteolytic activity in seeds subjected to treatment
before storage. Severe damage to m embrane permeability in these seeds
appears to be a post-mortem event as this was only found in samples showing
drastic losses in seed germinability.
Pre-sowing treatment caused a buildup of germinative a-amylase activity
in unaged but not in aged seeds, although both showed similar radicle emergence
rates. Quick resumption of a-amylase production during subsequent imbibition
by treated seeds, irrespective of ageing, suggests that components involved
in de novo enzyme synthesis are tolerant to desiccation in wheat seeds. Increased
a-amylase activity in treated seeds or its m aintenance during subsequent
storage, surprisingly did not cause damage to stored starch. There was no
relationship between increased a-amylase activity and early radicle emergence.
The ageing-induced delay in germinative a-amylase production appeared
to be due to delayed gibberellin synthesis by the aged embryo. Pre-sowing
treatment of seeds after storage effectively decreased the lag period for
enzyme production in deteriorated seeds. Ageing effects on aleurone were
characterised by investigating changes in the responsiveness of embryoless
half seeds to gibberellic acid with respect to a-amylase production in vitro.
Ageing of seeds caused a significant reduction in aleurone enzyme production.
These changes were at least in part, reversed by pre-sowing treatment of
Abbreviations: h = hours; PEG = polyethylene glycol; RH = relative humdity;
SMC = seed moisture content; T50 = time to 50% radicle