IMPROVING VIGOR IN SHRUNKEN-2 CORN SEEDLINGS

The shrunken-2 (sh2) mutant of maize (Zea mays L.) increases sucrose a nd reduces starch in developing endosperm, An associated trait is poor seed and seedling vigor in seeds containing the mutation, The specifi c effects of sh2 mutant endosperm on embryo and seedling vigor were de termined by analyzing seeds that contained either concordant wild-type or nonconcordant combinations of mutant and wild-type embryo and endo sperm genotypes, The nonconcordant seed types that contained a wild-ty pe embryo in association with a sh2 mutant endosperm or a sh2 mutant e mbryo in association with a wild-type endosperm were generated using t he TB-3La translocation chromosome in which a wild-type Sh2 gene is at tached to the centromeric portion of a B chromosome, Under stress cond itions (complex stress vigor test), the seeds with mutant endosperm ha d lower germination, seedling fresh and dry weight, and index of condu ctivity than seeds with wild-type endosperm, Mutant endosperm and embr yos excised from mutant endosperm imbibed more water than wild-type en dosperm or embryos excised from wild-type endosperm, Because of the hi gh concentration of osmotic solutes in the mutant endosperm, a rapid w ater uptake may induce a membrane disorganization. Leachate conductivi ties of seeds with mutant endosperm were higher than seeds with wild-t ype endosperm, In addition, a higher sucrose content and a lower raffi nose to sucrose ratio were measured in the wild-type embryos associate d with mutant endosperms than in the normal embryos excised from conco rdant wild-type seeds, These results suggest that a high rate of water uptake caused by the elevated concentration of osmotic solutes in see ds with mutant endosperms may affect membrane integrity during imbibit ion, Alternatively, the lower raffinose to sucrose ratio present in th e mutant endosperm class might affect stabilization of cell membranes during seed desiccation, Embryos cultured in media containing 10% star ch or no carbohydrate produced smaller seedlings than embryos cultured in 5% or 10% sucrose. Wild-type embryos excised from mutant endosperm s exhibited lower germination in 0% and 5% sucrose media than embryos from concordant seed, indicating that reduced water uptake rates assoc iated with lower external osmotic potential (10% sucrose) can improve vigor of embryos associated with sh2 mutant endosperm, The reduced vig or of embryos and seedlings that develop in association with sh2 mutan t endosperm can be traced to the physiological and biochemical effects of the elevated sucrose levels present during seed formation and imbi bition.