The Physiology of Flowering Plants - KHAM PHA MOI

GERMINATION AND THE RESUMPTION OF GROWTH 315This is known as vivipary (live birth) or cryptovivipary (hidden livebirth) if the seedling does not emerge from the fruit. Vivipary hasbeen extensively investigated in mangrove. An interesting study byFarnsworth and Farrant (1998) showed that ABA levels were consistentlylower in the embryos of viviparous mangrove species comparedwith that in related species which produce desiccation-tolerant seed,suggesting that reductions in ABA allowed vivipary, similar to thatobserved in mutants of maize.11.15 Germination and the resumption of growthGermination commences with the uptake of water by the dry seed(imbibition) and is complete once the growing embryo emergesfrom the seed coat. The initial uptake of water is rapid.Macromolecules and structures are rehydrated and assume theirfunctional forms; during this period, low-molecular-weight solutescan be lost from the seed, presumably because the lipids in thedehydrated membranes take a little time to pass from a leaky gelphase to the more impermeable hydrated semi-crystalline phase.Respiratory activity rises dramatically. As the seed coat may impairO 2 diffusion, germinating seeds often undergo anaerobic respirationand accumulate significant quantities of ethanol (Chapter 1) beforethe testa splits, but then the mitochondria are activated and mitochondrialreplication is also stimulated. Protein synthesis commences,initially directed by pre-stored mRNAs, followed bytranscription and translation of new mRNA as genes necessary forseedling development become active.To fuel the growth of the embryo the seed reserves are mobilized,i.e. converted from their insoluble forms to soluble, transportableand/or metabolizable derivatives. The best-studied system isundoubtedly the cereal endosperm. During germination, enzymessuch as amylase and maltase are produced which will break downthe starchy endosperm to glucose. These are produced in the aleuronelayer which surrounds the endosperm. Experiments with cerealsdemonstrated that the embryo produces GA, triggering this process.If the embryo is removed from the seed, no starch breakdown occurseven after prolonged periods of incubation. However, if the embryo isplaced on the surface of an agar plate in close proximity to the rest ofthe seed, GA diffuses across the gap and starch degradation results.Application of GA also stimulates starch hydrolysis.In the cotyledons of oil storing seeds, fatty acids are releasedfrom the oil bodies by lipoxygenases. The fatty acids enterthe glyoxysomes (small organelles) where successive rounds of b-oxidation generate acetyl CoA, which are in turn used to formsuccinate in the glyoxylate cycle. This organic acid enters themitochondria and hence the Krebs cycle. Oxaloacetate fromthe TCA cycle then acts as a substrate for sucrose synthesis