Roles of Soluble Sugars in Protecting Phytochrome- and Gibberellin A3-Mediated Germination Control in Skotodormant Lettuce Seeds

Abstract

Skotodormant seeds of Lactuca sativa Grand Rapids imbibed in darkness for 10 days (10-day DS) germinated poorly upon terminal treatment with red light (R) or gibberellin A 3 (GA 3 ). Soluble sugars in the imbibition solutions influenced the depth of skotodormancy. Ten-day DS seeds, imbibed in 50-500 mM sucrose or 100-500 mM glucose and given terminal GA 3 germinated completely and germinated about 80% when imbibed in 100 mM galactose, mannose, lactose, or maltose. In contrast, terminal R applied to 10-day DS seeds caused only 20-50% germination. If given R at day 0 and imbibed for 10 days in darkness in 500 mM sucrose or glucose, seeds washed free of exogenous glucose or sucrose then germinated about 50% in darkness in water. These seeds responded to terminal R or GA 3 with complete germination. When seeds were given FR at day 0, germination responses following terminal R or GA 3 were significantly lower when the duration of DS was increased from 7-10 day DS to 15 days. In 10-day DS seeds given initial FR and imbibed in either solutions of 50 or 100 mM sucrose and KNO 3 , either terminal R or GA 3 treatment gave complete or near complete germination. It is concluded that seed exposure to certain soluble sugars and/or nitrate during a 10-day DS protected certain substrates and thereby extended the sensitivity of the seeds to terminal R or GA 3 treatment. The study provides substantial evidence for nonhormonal factors associated with light and GA action in the control of seed skotodormancy.The exact relationship between the hormonal control of metabolic processes leading to germination and the phytochrome status within seeds is often complicated by interactions between light and other environmental factors such as temperature and nutritional status (Thomas 1992). Nitrate promotes the germination of a wide range of species, but its effect is best realized in combination with other factors such as temperature manipulations or light (see Hsiao and Quick 1996, McIntyre et al. 1996, and references cited therein). Hsiao and Quick (1996) showed that the presence of certain inorganic nitrogen salts at 25-50 mM in the dark storage (DS) imbibition solution influenced the depth of skotodormancy. The terminal red light (R) treatment was the most effective in seeds imbibed in NH 4 NO 3 for 10 days in darkness and washed before the terminal treatment. Terminal GA 3 caused complete germination in 10-day DS seeds imbibed in 25 mM NH 4 Cl or 50 mM NH 4 NO 3 , whereas complete germination followed either terminal R or GA 3 when these seeds were imbibed in higher concentrations of the four salts. We concluded that seed exposure to certain inorganic nitrogen salts, especially ammonium salts, during a 10-day DS could maintain the sensitivity of the seeds to terminal R or GA 3 treatment by reducing the depth of skotodormancy to levels at which the germination of the seeds still would be under the control of P fr and/or GA 3 .The objectives in this study were to examine whether seed exposure to soluble sugars during DS imbibition produce...