Changes in Respiration and Cyanide Sensitivity of the Barley Floret during Development and Maturation

The 6-week period of development and maturation of the barley (Hordeum vulgare L.) floret from anthesis to harvest is characterized by two phases: an early phase of rapid increase in respiration rate and dry weight, and a late phase during which respiration decreased rapidly whereas dry weight remained unchanged. Consumption of O(2) by the embryo changed little during the entire developmental period, whereas O(2) uptake by the endosperm and the lemma and palea decreased significantly during the late phase.Maximal inhibition of O(2) uptake by cyanide in whole florets and floret parts coincided with maximal respiration and growth rates. Differences in sensitivity of embryo and endosperm to cyanide, as the floret matured, appeared to be related to developmental differences between these two tissues. Electron micrographs of embryo and endosperm cells are presented to illustrate some of their ultrastructural features, both at early and late stages of development. Increases in O(2) uptake and cyanide sensitivity of the caryopsis after the lemma and palea were removed suggested that the lemma, palea, and pericarp contributed partly to the increased resistance of the mature floret to cyanide by acting as barriers to gas exchange, thus restricting the movement of both cyanide and oxygen into the caryopsis.