4.3. Effect of water stress on seed quality
During both seasons seed vigor measured as percentage and rate
of seed germination or percentage field emergence was not affected
by water stress treatments (Tables 7B and 8B). This is in agreement
with the results of Ayars et al. (1991) in carrot who reported that
germination percentage was high irrespective of soil moisture.
Seedling vigor of seed lots obtained from treated main
umbels during both seasons showed somewhat variable results
under standard conditions (25 ◦C) and field conditions. Variations
between the seasons might be mainly due to the effect of different
temperatures during sowing and emergence and during
other growth and development stages of the crop. Under controlled
temperature, water stress on mother plants did not show
significant difference in the lengths of seedling shoots except
stress at seed maturation which resulted in significant reduction
in the first season (Table 7A). However, in the second season, no
reduction in shoot length was shown in Table 8A. This might be
due to improper moisture level with the towel papers used for
that seed lot which could delay the germination or retard the
growth of seedlings. Seedlings from stressed seed lots showed
increasing in root length in both seasons but the difference in
root lengths was very little, while seedling fresh and dry weights
were not affected, except the seedlings from plants stressed at
seed maturation. However, this latter treatment under field conditions
showed good result (Tables 7B and 8B). The result of
seedling vigor under field conditions was more or less similar to
that obtained in the laboratory with slight differences. Under all
conditions the water stressed mother plants at the stated growth
stages produced high quality seeds that sometimes exceeded
seeds or seedlings from control plants in some parameters of
seed and seedling vigor in the two seasons, even under artificial
deterioration of seed lots (accelerated aging test). Seeds from
all treatments exhibited no significant difference between their
high germination percentages. But, the seeds of control plants
gave the best rate of germination, which took a shorter time to
germinate (Table 8A).
Accelerated aging test is commonly used to obtain information
on seed vigor. It was also observed that under aging the germination
of all seed lots was not affected when compared to the standard
test. Similarly, some researchers reported that water stress during
seed development had no effect on seed germination (Fougereux
et al., 1997; Samarah and Alqudah, 2011). Moreover, Vieira et al.
(1992) found that drought stress imposed at the beginning of seed
development or later at the full stage of development had no effect
on seed quality, as estimated by seed germination and accelerated
tests. The results of the standard and accelerated aging tests predicted
the potential of seedling emergence of onion seeds under
water stress conditions. Seed quality can be considered as another
important trait affected by drought stress.