Increased Oxygen Bioavailability Improved Vigor and Germination of Aged Vegetable Seeds

Large and/or aged seeds are prone to hypoxic conditions during germination. Germination of selected vegetable seeds including corn (Zea mays L.), squash (Cucurbita pepo L.), and tomato (Solanum lycopersicum L.) was studied in water with different concentrations of hydrogen peroxide (H2O2) solution ranging from 0, 0.06% to 3.0% (v/v) or in aeroponics, all with 0.5 mM CaSO4. Imbibition, oxygen consumption, proton extrusion, and alcohol dehydrogenase (ADHase) activity of corn seeds were measured gravimetrically, electrochemically, and colorimetrically as appropriate. The results showed that 0.15% H2O2 provided the optimum oxygen concentration for seed germination. The germination percentage of aged corn seeds treated with H2O2 was significantly greater than those without H2O2 treatment. Corn embryo orientation in relation to a moist substrate also significantly impacted oxygen bioavailability to the embryo and hence ADHase activity. Corn seeds without H2O2 imbibed significantly more slowly than those with oxygen fortification by 0.15% H2O2. Increased oxygen bioavailability improved the metabolism of the seeds, which extruded 5-fold more protons from the embryos. Each treated embryo consumed twice the amount of oxygen as compared with the untreated one and likewise for treated and untreated endosperms. Increased oxygen bioavailability may be used to improve production of the tested crops. The results from this research imply that consideration should be given to including oxygen fortification in seed coatings for aged seeds and for large seeds regardless of age. The artificial provision of bioavailable oxygen might be effective in rescuing the germplasm in aged seeds in plant breeding and in crop production. A high seed germination percentage is a prerequisite for successful commercial vegetable production. Seed vigor, appropriate temperature, and oxygen and water bioavailability are key factors in high percentages of seed germination (Bewley and Black, 1994). Seed vigor and bioavailable oxygen are closely associated. Particularly, aged or large seeds are especially prone to lack of oxygen bioavailability (Geigenberger et al., 2000). However, the sufficiency of oxygen bioavailability is not easily secured in soil or water. For example, water saturated with air contains 250 mM oxygen (Lide, 1998), which is too little to supply sufficient bioavailable oxygen to support the germination of relatively large or aged seeds. In fact, seed hypoxia caused by flooding is a worldwide problem (Jackson et al., 2009) and a detrimental factor in plant growth and development of vegetable (Edelstein et al., 1995). The rainy periods in the spring often introduce much uncertainty into growers’ decision-making process pertaining to corn and vegetable planting. Because a corn seed has a volume six to 10 times larger than that of wheat or rice, corn seed is more likely to suffer from oxygen deficiency than the latter two species as measured (data not shown). Specific surface area is a material property of solids, which measures the total surface area per unit of bulk volume. It is defined by surface area divided by the volume (units of m/m). Specific surface area of corn seeds is only 0.1% to 0.5% that of the other two types of seeds based on our calculations. Additionally, waterlogging (i.e., oxygen bioavailability deficiency) sometimes causes up to 50% loss of crop yield (Dennis et al., 2000). Such yield losses have usually resulted from failure both of seed germination and plant growth and development. Oxygen bioavailability deficiency can be caused by different factors including waterlogging or aging and may be responsible for failures of seed germination. Oxygen bioavailability is, therefore, a significant factor in determining seed vigor and seed germination (Bewley and Black, 1994). Early aging or degeneration of seeds may also result in losses of seed vigor and vegetable yield as a result of unfavorable cultivation or storage conditions (Hong, 2001). Heavy rainfall after sowing often causes oxygen deficiency in the seedbed sufficiently severe to hamper emergence (Håkansson et al., 2012). Aged seeds are characterized by poor germination and slow post-germination growth, and they have low plasma membrane H-ATPase activity. This low enzyme activity may partially explain poor germination and low post-germination root elongation and growth of seedlings developed from aged seeds (Sveinsdottir et al., 2009). The essence of slow and/or low germination percentage is the retardation of biochemical and physiological reactions in aged seeds. Energetically, such retardation is related to a deficiency of active chemical energy, adenosine triphosphate (ATP), because aged seeds may have reduced content of nonsymbiotic hemoglobins. These hemoglobins act in plants to maintain the energy status of cells in hypoxic conditions such as during flooding, and they accomplish this by increasing substrate-level phosphorylation. Nonsymbiotic hemoglobins likely sequester oxygen in hypoxic environments, providing a source of oxygen to oxidize NADH to produce ATP for cell growth and development (Sowa et al., 1998). ATP deficiency may be common both in aged and flooded seeds. Therefore, new technologies are needed to coat large seeds like corn or other vegetable seeds with oxygen fertilizers to improve seed germination and crop success. Such technologies may be also advantageous in accelerating crop breeding for rescuing rare plant germplasm from aged or improperly stored seeds. However, there are no published accounts of an adequate understanding of the relationship between Received for publication 4 Sept. 2012. Accepted for publication 15 Oct. 2012. This project was financially supported by the U.S. Department of Agriculture, Grant No. NRICGP