Coconut (Cocos nucifera) in vitro ecology: Modifications of headspace and medium additives can optimize somatic embryogenesis

Of those explants tested, immature zygotic embryo tissues proved to be the best for initiating callus with potential for somatic embryogenesis. Slicing of this tissue and use of the central sections (near to and including the meristematic tissue) gave the best embryogenic response. Slices that were placed under illumination necrosed more rapidly and to a greater degree than those incubated in the dark. Explant slice necrosis could be prevented or severely retarded by the addition of activated charcoal into the medium. Washing the explants for short periods of time prior to culture was also found to improve callus production. Prolonged washing resulted in low rates of callus production. In an attempt to prevent ethylene accumulation in the culture vessel headspace, AVG, an ethylene biosynthesis inhibitor and STS, a chemical which reduces the physiological action of ethylene, were successfully used to promote somatic embryogenesis. Spermidine, putrescine and spermine, polyamines that are known to delay plant senescence and promote somatic embryogenesis in some plant species, enhanced the rate of somatic embryogenesis when they were introduced into the callus induction medium. The use of polyethylene glycol in combination with abscisic acid helped promote somatic embryo formation and maturation as well as the subsequent formation of plantlets. The use of all of these improvements together has created a new and improved protocol for coconut somatic embryogenesis. This new protocol puts significant emphasis on improving the in vitro ecology of the explant, callus and somatic embryogenic tissues.

Suctions, fracture energy, and plastic cracking of cement mortar and concrete

Plastic cracking of cement mortar and concrete is primarily attributable to desiccation by evaporation from unprotected surfaces. This causes high suctions (negative pressures) to develop in the pore water adjacent to these surfaces. Dissolved salts in the pore water can also contribute significantly to suctions. Quantitative expressions are available for all of the components of the total suction. The development of suctions over time is illustrated by the results of desiccation tests conducted on cement mortars, supplemented by data from the literature. It is shown that ambient conditions conducive to plastic cracking can arise almost anywhere, but that the extremely high suctions that develop in mature cement mortar and concrete do not imply that compression failures should occur A high value of fracture energy is derived from data from the desiccation tests that implies that plastic cracking is characterized by a significant zone of plastic straining or microcracking.