Metabolic heat and CO2 production rates during germination of melon (Cucumis melo L.) seeds measured by microcalorimetry

Metabolic heat and CO2 production rates were measured by microcalorimetry d uring germination of two melon (Cucumis melo L.) accessions, Noy Yizre'el ( a cold-sensitive cultivar) and Persia 202 (a cold-tolerant breeding line), differing in their ability to germinate at low temperature (14 degreesC). I n seeds that were imbibed at either 14 or 25 degreesC, both heat production rates (phi) and CO2 evolution rates (Rco(2)) were higher in Persia 202 com pared to Noy Yizre'el. At 14 degreesC, phi and Rco(2) of intact Noy Yizre'e l seeds remained close to zero and germination was inhibited, but metabolic activity increased with time in decoated seeds (testa removed) and most em bryos germinated. The presence of the testa had less effect on metabolic ac tivity or germination of Persia 202 seeds at 14 degreesC. The ratio of phi /Pco(2) (calorespirometric ratio) is an indication of the substrate being u tilized for respiration, with lower values (similar to 455 kJ mol(-1)) asso ciated with carbohydrate substrates and higher values (similar to 800 kJ mo l-1) associated with lipid substrates. Removal of the testae of Noy Yizre'e l seeds increased phi /Rco(2) at 14 degreesC, suggesting that improved oxyg en supply to the embryo promotes a shift from carbohydrate to lipid respira tory substrates associated with germination. Consistent with this interpret ation, when the hilum apertures of the seed coats were sealed with silicone , phi, Rco(2) and /Rco(2) of Noy Yizre'el seeds remained low and germinatio n was inhibited at 25 degreesC, while in Persia 202 seeds the same treatmen t had relatively little effect on phi and Rco(2), phi /Rco(2) approached th e value expected for lipid respiratory substrates (700 kJ mol(-1)), and ger mination occurred. This confirms earlier anatomical work indicating that th e testae of Persia 202 seeds were more porous than were those of Noy Yizre' el seeds, contributing to the greater tolerance of Persia 202 seeds to low temperatures. Abscisic acid (ABA) was also inhibitory to melon seed germina tion; imbibition of seeds at 25 degreesC in 250 muM ABA decreased phi and R co(2), and phi /Rco(2) remained lower than in control seeds in both genotyp es. This effect of ABA on metabolic activity could be a cause or consequenc e of inhibition of germination.