Pericarp, leachate, and carbohydrate involvement in thermoinhibition of germinating spinach seeds

Spinach (Spinacia oleracea L.) seed germination can be inhibited by high te mperatures. 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 gen otypes-'Casscade, 'ACX 5044', 'Fall Green', and 'ARK 88-354'-exposed to con stant and alternating temperatures; 2) to determine the nature and extent o f inhibition imposed by the pericarp; and 3) to investigate leachate and ol igosaccharide involvement in thermoinhibition. Germination inhibition began at >20 degrees C constant temperature and was totally suppressed at 35 deg rees C. Alternating temperatures at 30/15 degrees C (12-hour day/12-hour ni ght) resulted in greater germination than a constant 30 degrees C. The geno type sensitivity to supraoptimal temperatures was in the order of 'ARK 88-3 54' less than or equal to 'Fall Green' < 'ACX 5044' < 'Cascade', but the hi ghly thermoinhibited 'Cascade' seeds retained the ability to germinate when shifted to lower incubation temperatures. The pericarp inhibited germinati on, since seeds deprived of the pericarp had approximate to 90% germination at 30 degrees C. 'ACX 5044' and 'Cascade' had higher ABA content in the pe ricarp than 'ARK 88-354' and 'Fall Green'. Before imbibition at 30 degrees C, raffinose levels in each genotype were in the order of 'ARK 88-354' > 'F all Green' > 'Cascade'. After 48 hours of imbibition, sucrose and glucose l evels were highest and raffinose levels were lowest in 'ARK 88-354' and 'Fa ll Green' seeds, while 'Cascade' seeds remained less active metabolically. These data suggest that the pericarp apparently acts as thermoinhibition.