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Desiccation Sensitivity of Recalcitrant and Orthodox Seeds in Relation to the Stage of Seed Development and Germination
Thesis Abstract:
Response to desiccation is dependent on the seed's physiological status. Therefore, it could be hypothesized that orthodox and recalcitrant seeds will exhibit parallelism in their response to desiccation if viewed from the entire stage of seed development and germination.
The study aimed to determine the parallelism between soybean (Glycine max L. Merr) orthodox and cacao (Theobromo cacao L.) recalcitrant seeds· in relation to desiccation from seed development to germination.
The studies conducted were: I) dessication of soybean seed at different stages of seed development before physiological maturity; 2) desiccation of physiologically mature soybean seed at different stages of seed germination (phases rand H, radicle protrusion, and phase III); 3) seed development and maturity of cacao; 4) desiccation of cacao seed at different stages of pre- and post-physiological maturity; and 5) initiation of gennination of cacao seed after physiological maturity.
Results confirmed the parallelism of soybean and cacao seeds in response to desiccation. Both seeds were found to be dessication-sensitive at immature stage 25 days after anthesis (DAA) for soybean and 4.5 months after anthesis (MAF) for cacao. Soybean seeds became desiccation-tolerant at 30-35 DAA (slow drying) and 40 DAA (fast drying). r n cacao, this stage corresponded to 5 MAF irrespective of drying rate. Another desiccation-sensitive stage was observed at 18 hours after imbibition (radicle protrusion stage) and 5.5 MAF of soybean and cacao, respectively.
The tolerance of cacao seed to drying (a very short time, 5-5.5 MAF) was further limited by a relatively high critical moisture content (30%). Drying to 20 percent moisture content resulted in cacao with germination and vigor index of only I 0.67 and 1.33 percent, respectively.
On the other hand, soybean seed could be dried to a very low moisture content without reducing viability and vigor index. The physiological maturity stage was observed at 40 DAA and 5 MAF for soybean and cacao, respectively. It is suggested that these are the ideal times to harvest soybean and cacao.
Germinating soybean seed was sensitive to desiccation after 18 hours of imbibition. At 12 hours imbibition, drying increased germination without affecting the vigor index. This indicated that dehydration to invigorate soybean seeds should not be done beyond 12 hours of imbibition. Fast drying at 18 hours after imbibition, however, could minimize the decrease in germination compared to slow drying.
Initiation of germination was not observed on intact cacao pod. Thus, desiccation sensitivity could not be related to germination initiation.
Further study on germination initiation and related metabolism of other recalcitrant seeds through histological and biochemical studies was suggested to understand their mechanism of desiccation sensitivity.