Abbreviations: PDA, potato dextrose agar; sh2, shrunken2; su, sugury1. This research was supported by the United States-Israel Binational Agricultural Research and Development (BARD) grant no. US-1709-89, the Illinois Agricultural Experiment
Abbreviations: CNS-711, `Crisp N' Sweet 711'; HSII, `How Sweet It Is'; sh2, shrunken-2; SMP, solid matrix priming. Univ. of Florida Agricultural Experimental Station Journal Series no. R-01320. We acknowledge useful discussions with J.D. Eastin
The shrunken-2 (sh2) mutant of maize (Zea mays L.) increases sucrose and reduces starch in developing endosperm. An associated trait is poor seed and seedling vigor in seeds containing the mutation. The specific effects of sh2 mutant endosperm on embryo and seedling vigor were determined by analyzing seeds that contained either concordant wild-type or nonconcordant combinations of mutant and wild-type embryo and endosperm genotypes. The nonconcordant seed types that contained a wild-type embryo in association with a sh2 mutant endosperm or a sh2 mutant embryo in association with a wild-type endosperm were generated using the TB-3La translocation chromosome in which a wild-type Sh2 gene is attached to the centromeric portion of a B chromosome. Under stress conditions (complex stress vigor test), the seeds with mutant endosperm had lower germination, seedling fresh and dry weight, and index of conductivity than seeds with wild-type endosperm. Mutant endosperm and embryos excised from mutant endosperm imbibed more water than wild-type endosperm or embryos excised from wild-type endosperm. Because of the high concentration of osmotic solutes in the mutant endosperm, a rapid water uptake may induce a membrane disorganization. Leachate conductivities of seeds with mutant endosperm were higher than seeds with wild-type endosperm. In addition, a higher sucrose content and a lower raffinose to sucrose ratio were measured in the wild-type embryos associated with mutant endosperms than in the normal embryos excised from concordant wild-type seeds. These results suggest that a high rate of water uptake caused by the elevated concentration of osmotic solutes in seeds with mutant endosperms may affect membrane integrity during imbibition. Alternatively, the lower raffinose to sucrose ratio present in the mutant endosperm class might affect stabilization of cell membranes during seed desiccation. Embryos cultured in media containing 10% starch or no carbohydrate produced smaller seedlings than embryos cultured in 5% or 10% sucrose. Wild-type embryos excised from mutant endosperms exhibited lower germination in 0% and 5% sucrose media than embryos from concordant seed, indicating that reduced water uptake rates associated with lower external osmotic potential (10% sucrose) can improve vigor of embryos associated with sh2 mutant endosperm. The reduced vigor of embryos and seedlings that develop in association with sh2 mutant endosperm can be traced to the physiological and biochemical effects of the elevated sucrose levels present during seed formation and imbibition.
Germination and stand establishment of shrunken-2 sweet corn is a major production-related problem, particularly during early spring in the upper Midwest U.S. Several potential physical and physiological factors have been identified as contributors to poor stand establishment. Poor pericarp integrity and high sucrose levels may lead to non-uniform moisturization and imbibitional injury. Studies were conducted to determine if the use of “controlled moisturization” preplant treatments reduced imbibitional injury and improved germination and seedling vigor. Both laboratory and field results will be discussed.
This study was conducted to determine if changes in the raffinose: sucrose ratio in embryos of shrunken-2 sweet corn (Zea mays L.) hybrids were related to differences in seed leachate conductivity between two hybrids harvested at four maturities and artificially dried to 0.10 g H2O/g fresh weight. The ratio of raffinose: sucrose differed for `Crisp N' Sweet 710' (CNS) and `How Sweet It Is' (HSII). The mass ratio of raffinose: sucrose in CNS was >0.3 in seed harvested between 0.44 to 0.64 g H2O/g fresh weight and increased as seed dried from the initial harvest moisture to 0.10 g H2O/g fresh weight. Raffinose: sucrose ratios of HSII were <0.3 at all harvests between 0.55 to 0.72 g H2O/g fresh weight, but changes during desiccation were not as pronounced. Leachate conductivity of whole seeds of CNS and HSII decreased as seeds were harvested at progressively lower moisture contents. We suggest that a higher raffinose: sucrose ratio may be indicative of increased seed vigor in shrunken-2 hybrids.
Mutant endosperm associated with shrunken-2 sweet corn possesses a high osmotic potential that increases the rate of imbibition. Membrane damage associated with the rapid influx of water during imbibition can play a role in the poor emergence and seedling vigor associated with sweet corn germination. Film-coating as a seed treatment has been used to improve germination and vigor in sweet corn. This improvement may be associated with alterations in the kinetics of imbibition. Two seed lots of shrunken-2 sweet corn, low-vigor `Even Sweeter' and high vigor `Sugar Bowl', were treated with a polymer film-coating and evaluated for differences in water uptake. Imbibition curves were established for nontreated and film-coated seeds. Seeds were weighed every hour for 6 hours and showed a significant difference between the two treatments in fresh weight for both cultivars. This pattern continues throughout the imbibition phase of germination and continues into the lag period. Bulk conductivity tests resulted in no significant mean difference between untreated and film treated seeds after 24 hours. Film treatment assumes characteristics of a hydrophilic polymer. Electrolyte leakage is not reduced and imbibition rate increases by 18% for both varieties of film-coated seeds.
Experiments were conducted to determine if the fungicide imazalil infused into shrunken-2 sweet corn (Zea mays L. var. rugosa Bonaf.) seed via acetone could protect against soil- and seedborne fungi enough to improve germination and vigor. `Florida Staysweet' and `Crisp-n-Sweet 710' seeds were infused for 0.25 hours with 1% or 2% (w/w) imazalil-acetone (LA) solutions, air-dried, and subjected to a modified laboratory seedling growth cold test using sterile soil or soil inoculated with Fusarium moniliforme Sheldon. Both IA concentrations significantly reduced the incidence of diseased seedlings in soil inoculated with F. moniliforme when compared to nontreated controls. Neither treatment significantly reduced the incidence of seedborne fungi. Chemical name used: 1-[2-(2,4-dichlorophenyl)-2-(2-propenyloxy)ethyl-H-imidazole (imazalil).
One possible influence film-coating may have on seeds is modifying water uptake and electrolyte leaking during imibibition. Film-coating is a seed treatment that can improve sweet corn germination, especially under cold soil conditions. Two shrunken-2 sweet corn varieties (`Even Sweeter' and `Sugar Bowl') were treated with a polymer film-coating and evaluated for water uptake patterns during imibibition. `Even Sweeter' is a low-vigor sweet corn, while `Sugar Bowl' is a high-vigor variety. Standard germination tests were performed according to AOSA rules and suggest film-coated seeds germinated at a slower rate than untreated seeds. After 4 days of imibibition, `Sugar Bowl' film-coated seeds had 5% germination, while untreated seeds had ≈20% germination. However, after 7 days, film-coated seeds had 94% germination with untreated seeds at 80% germination. Results were similar for `Even Sweeter'. Bulk electrical conductivity readings were taken over 24 h to determine the amount of electrolyte leakage during imibibition. Low-vigor `Even Sweeter' had 92% higher overall leakage than high-vigor `Sugar Bowl'. Additional conductivity readings were taken for both seed lots every 2 h for 12 h. Film-treated seeds leaked 15% less than untreated seeds for `Sugar Bowl'. However, `Even Sweeter' film-coated seeds actually leaked 17% more than the untreated seeds. In both cases, 70% of electrolyte leakage occurred within the first 12 h of imibibition. An imibibition curve was established for the two seed lots comparing untreated and film-coated seeds. During the first 6 h of water uptake, film-treated seeds weighed ≈50% more than the untreated seeds for both `Even Sweeter' and `Sugar Bowl'. Pathways for water uptake as influenced by film-coating shrunken-2 seeds will also be presented.
In a test to overcome poor seed germination and seedling vigor of sweet corn (Zea mays L.) seeds carrying the shrunken-2 (sh2) mutant endosperm, primed seeds of two sh2 sweet corn cultivars—Crisp N'Sweet 711 (CNS-711) and How Sweet It Is (HSII)—were redried at 15, 20, 30, or 40C and 25% relative humidity after solid matrix priming (SMP). The dehydration rate was significantly lower in `CNS-711' than `HSII' at all temperatures. In both cultivars, the drying temperature after SMP was critical for seed performance. Primed seeds with a higher dehydration rate (dried at 30 or 40C) had better seed vigor, greater field emergence and seedling vigor, lower leachate conductivity and imbibition rate, and a higher respiration rate and glutamic acid decarboxylase activity than primed seeds redried at the lower temperatures or control seeds. Increased incidence of pathogen growth was observed on seeds dried at 15 and 20C relative to those dried at 30 or 40C, probably as a consequence of greater leakage from the seeds at a lower redrying temperatures. Lack of tolerance to dehydration at 15 and 20C was another factor adversely affecting the seeds redried at low temperature. A more rapid dehydration rate at a higher temperature after priming sh2 sweet corn improved many of the physiological characteristics used to measure seed quality and the subsequent emergence and vigor of the seedlings under field conditions.
Senescence occurs at the cellular and tissue levels. It is under genetic and environmental control and factors affecting initiation and speed of development of senescence can be passed from parental to F1 plants. This study was conducted in the greenhouse and field to determine how senescence patterns in F1 plants of a shrunken2 sweet corn (Zea mays L.) hybrid compared to those of parental inbreds. Greenhouse grown plants were left intact and field grown hybrids and parental inbreds had one or both reproductive organs removed or were left intact. Senescence patterns in stalk internodes were similar in greenhouse and field grown F1 and inbred plants. Senescence patterns in shank internodes in greenhouse grown plants were different from those of field grown plants. Senescence ratings in stalks increased as developmental stage advanced. Expression of stalk senescence in internodes below the node bearing ears appears to be suppressed by hybrid vigor. In field tests, destruction of the tassel before expansion (decapitation) appears to suppress senescence in internodes above I7, with this effect somewhat dependent on plant developmental stage.