Spinach (Spinacia oleracea L.) seed germination can be inhibited by high temperatures. An understanding of thermoinhibition in spinach is critical in predicting germination and emergence events. The purpose of this study was 3-fold: 1) to determine seed germination percentage and rate of spinach genotypes—`Cascade', `ACX 5044', `Fall Green', and `ARK 88-354'—exposed to constant and alternating temperatures; 2) to determine the nature and extent of inhibition imposed by the pericarp; and 3) to investigate leachate and oligosaccharide involvement in thermoinhibition. Germination inhibition began at >20 °C constant temperature and was totally suppressed at 35 °C. Alternating temperatures at 30/15 °C (12-hour day/12-hour night) resulted in greater germination than a constant 30 °C. The genotype sensitivity to supraoptimal temperatures was in the order of `ARK 88-354' ≤ `Fall Green' < `ACX 5044' < `Cascade', but the highly thermoinhibited `Cascade' seeds retained the ability to germinate when shifted to lower incubation temperatures. The pericarp inhibited germination, since seeds deprived of the pericarp had ≈90% germination at 30 °C. `ACX 5044' and `Cascade' had higher ABA content in the pericarp than `ARK 88-354' and `Fall Green'. Before imbibition at 30 °C, raffinose levels in each genotype were in the order of `ARK 88-354' > `Fall Green' > `Cascade'. After 48 hours of imbibition, sucrose and glucose levels were highest and raffinose levels were lowest in `ARK 88-354' and `Fall Green' seeds, while `Cascade' seeds remained less active metabolically. These data suggest that the pericarp apparently acts as a physical barrier as well as a source of inhibitors during thermoinhibition.
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