seed physiology - dessication / mature drying

- seed looses h2o
- seed looses connection from parent plant
- metabolism stops

orthodox (traditional) vs recalcitrant seeds

orthodox eg. date palm
- looses 95% water
- store at dry and low temp.

recalcitrant eg. avocado, mango, lychee
- tropical seeds
- can't loose too much h2o
- doesn't go through maturation drying
- cannot be stored for long periods

coffee is intermediate, can survive seed dessication but dies in dry and low temp.

major mechanisms in seed dessication tolerance
1. carbohydrates - prevents large scale phase transition from liquid crystalline to dry gel by inserting sugar (e.g. raffinose) OH groups to phospholipid polar head groups
2. lipid-soluble antioxidants eg. glutathione, ascorbic acid, prevents peroxidation damage to lipid membranes due to free radicals and free fatty acid accumulation by scavenging for them
3. LEA (Late Embryogenesis Abundant) and LEAII - dehydrins (denaturation resistant proteins)
- 5 groups
- 1. random coil - able to conform to structures they contact, many OH groups to solvate cystolic structures and prevent their crystallization
- 2. amphipathic alpha-helix - hydrophobic (interacts with lipid membrane) and hydrophilic side (h bonds with polar groups of macromolecules) preventing coagulation
- 3. eleven amino acid motif - helix forming bundles (dimer) - hydrophobic (binds membranes) and hydrophilic sides (binds extra ions resulted from drying), > 1.5mM ion scavenging ability
counteracts increasing ionic strength buildup in cytosols
4. repair proteins:
- molecular chaperones - helps in protein folding
- ubiquitin - degrades damaged proteins
5. reserves help maintain mechanical strength to the whole cell