Relationship of chlorophyll, seed moisture and ABA levels in the maturing Brassica napus seed and effect of a mild freezing stress

Abstract

Chlorophyll (Chl) retention by mature seed of canola as the result of an early frost or other environmental factors (the “green seed problem”) causes serious economic losses. The relationship of seed degreening to rate of moisture loss by seed and silique and the role of ABA in this process were investigated as a function of developmental age. During the normal predesiccation stage (28–45 days after pollination), seed of Brassica napus (cv. Westar) loses Chl rapidly but seed moisture slowly. After a mild freezing stress, there is a rapid loss of moisture from silique walls, followed by accelerated loss of seed moisture. Chl degradation ceases at 35–45% seed moisture. ABA levels in silique walls of frozen plants (determined by enzyme‐linked immunosorbant assay) increased after freezing, apparently in response to moisture loss. In contrast, ABA levels in the seed increased dramatically 1 day after freezing, then decreased to control levels. The influence of the rate of seed moisture loss on Chl degradation was investigated by fast and slow drying of isolated seed under controlled humidity conditions. Seed dried rapidly at 22% RH retained most of its Chl, whereas seed dried slowly at 86% RH lost Chl as fast or faster than seed on control (unfrozen) plants. In all treatments, Chl loss stopped at about 40% seed moisture.

Non-lethal freezing effects on seed degreening in Brassica napus

“…1). Because it had been suggested (Green et al 1998) that disruption of tissue and interruption of the vascular connection to the seed coat might account for the loss of water in the silique, the vascular tissue of 32 DAF siliques was examined in detail (Fig. 4).…”

“…Free ABA, which accumulated after 21 DAF to levels above those in wildtype seeds, was unable to alter embryo development at this stage to allow normal seed maturation to take place. The change in reported ABA levels, in 27-30 DAF canola, following a non-lethal freezing stress, with an immediate increase in the levels in silique wall and seed, dropped to its original level after 1 day and prior to the seed undergoing rapid moisture loss (Green et al 1998).…”

“…ABI3, together with FUSCA3 (FUS3), LEAFY COTYLEDON1 (LEC1) and LEC2, encode key transcription factors that regulate seed maturation, including seed storage protein and lipid accumulation in Arabidopsis (Gazzarrini et al 2004;Kagaya et al 2005;Santos Mendoza et al 2005). ABA is integral to the regulation of these genes and therefore is an integral regulator of the seed maturation process, including seed chlorophyll degradation (Green et al 1998). ABA has been implicated in chlorophyll degradation (Green et al 1998).…”

“…ABA is integral to the regulation of these genes and therefore is an integral regulator of the seed maturation process, including seed chlorophyll degradation (Green et al 1998). ABA has been implicated in chlorophyll degradation (Green et al 1998). ABA applied to haploid Brassica embryos has been shown to promote their degreening (JohnsonFlanagan et al 1992), possibly through a down-regulation of expression of chlorophyll a/b light-harvesting proteins (Chang and Walling 1991).…”

“…While normal chlorophyll clearing of developing embryos occurs under slow moisture loss from both the seed and silique, a non-lethal freezing stress has been shown to induce a rapid loss of moisture from silique walls followed by increased loss of seed moisture, resulting in a seed moisture content that can no longer support chlorophyll degradation; the resulting green seed is viable and will germinate normally (Green et al 1998). The biochemical and physical changes required to bring about this rapid loss in silique and seed moisture are unknown.…”