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

Abstract

Metabolic heat and CO 2 production rates were measured by microcalorimetry during 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°C). In seeds that were imbibed at either 14 or 25°C, both heat production rates (φ) and CO 2 evolution rates (R CO 2) WERE HIGHER IN PERSIA 202 COMPARED TO NOY YIZRE’EL. AT 14°C, Φ AND R CO 2 of intact Noy Yizre’el seeds remained close to zero and germination was inhibited, but metabolic activity increased with time in decoated seeds (testa removed) and most embryos germinated. The presence of the testa had less effect on metabolic activity or germination of Persia 202 seeds at 14°C. The ratio of φ/R CO 2(calorespirometric ratio) is an indication of the substrate being utilized for respiration, with lower values (∼455 kJ mol–1) associated with carbohydrate substrates and higher values (∼800 kJ mol–1) associated with lipid substrates. Removal of the testae of Noy Yizre’el seeds increased φ/R CO 2 at 14°C, suggesting that improved oxygen supply to the embryo promotes a shift from carbohydrate to lipid respiratory substrates associated with germination. Consistent with this interpretation, when the hilum apertures of the seed coats were sealed with silicone, φ, R CO 2 and φ/R CO 2 of Noy Yizre’el seeds remained low and germination was inhibited at 25°C, while in Persia 202 seeds the same treatment had relatively little effect on φ and R CO 2, φ/R CO 2 approached the value expected for lipid respiratory substrates (700 kJ mol–1), and germination occurred. This confirms earlier anatomical work indicating that the 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 germination; imbibition of seeds at 25°C in 250 μM ABA decreased φ and R CO 2, and φ/R CO 2 remained lower than in control seeds in both genotypes. This effect of ABA on metabolic activity could be a cause or consequence of inhibition of germination.