Practice question

Formation of artificial seed beads depends on ion exchange crosslinking reaction between sodium alginate and divalent cation source typically calcium nitrate Ca(NO3)2 or calcium chloride CaCl2. Sodium alginate is water soluble sodium salt of alginic acid; when droplets containing somatic embryos fall into calcium nitrate bath 50-100 millimolar, rapid exchange occurs sodium ions diffuse out calcium ions diffuse in binding cooperatively to guluronic acid residues of adjacent polymer chains forming junction zones described as egg box structure. Within seconds liquid alginate droplet transforms into firm insoluble calcium alginate hydrogel encapsulating embryo maintaining spherical shape. Calcium nitrate preferred over chloride in some protocols because nitrate supplies additional nitrogen nutrition for embryo growth during storage and germination while chloride can be toxic at high concentration. Hardening time 20-30 minutes determines mechanical strength, too short yields fragile beads prone to rupture during handling, too long yields hard beads impeding embryo emergence and oxygen diffusion. After hardening beads washed with sterilized water to remove excess calcium preventing inhibition of conversion and stored moistly until sowing in nursery or field.