Entropy-driven phase transition in binary mixtures.

Based on the principle of entropy maximum, a transparent method to study the phase separation is proposed. The excluded volume effects of binary mixtures of hard spheres with two different diameters are analyzed and the role of entropy is emphasized. As a result of the entropy variation caused by the packing of large spheres, there is a critical volume fraction to denote the phase boundary. It is shown that the variation of free volume fraction is influenced by the ratio alpha=d(L)/d(S) of large to small sphere diameters and the ratio x=eta(L)/(eta(L)+eta(S)) of large-sphere volume fraction to the total volume fraction of large- and small-spheres. We introduce a modification factor beta to describe the overlap degree of two large spheres excluded volumes when they pack together. The critical volume fractions for large-sphere packing with different values of alpha and x are calculated, and the corresponding phase boundaries are determined. Our results are in quite good agreement with previous experimental measurements.

High temperature-induced thermotolerance in pollen tubes of tradescantia and heat-shock proteins.

Growing pollen tubes of Tradescantia paludosa are protected from inhibition of growth at 41 degrees C by a prior exposure to gradually increasing temperatures. Heat shock proteins (hsps) are not synthesized by pollen tubes as determined by labeling with [(35)S]methionine and two-dimensional gel electrophoresis, during either a heat shock at 41 degrees C or a gradual temperature increase to 41 degrees C. A comparison after two-dimensional electrophoresis of silver-stained spots and radioactive spots after autoradiography of an extract of ungerminated pollen mixed with a trace amount of [(35)S]methionine-labeled vegetative tissue heat shocked at 41 degrees C to act as a hsps marker, indicates that the majority, if not all, of the major hsps are not present in the pollen grain at anthesis. The type of thermotolerance seen with pollen tubes can thus be achieved without the presence or the new synthesis of the hsps.

Messenger RNAs in corn pollen and protein synthesis during germination and pollen tube growth.

Mature ungerminated pollen grains of Zea mays L. contain presynthesized messenger RNAs. This has been demonstrated by the isolation of poly(A)RNA and its translation in the wheat germ and reticulocyte cell free systems into polypeptides many of which are similar to those synthesized in germinating pollen. Each corn pollen grain contains between 352-705 pg of total RNA and 8.9-17.8 pg of poly(A)RNA. During germination of corn pollen at least 260 different polypeptides are synthesized as determined by labeling and 2-dimensional gel electrophoresis. These results are discussed with reference to other plants and the number of different genes expressed during pollen development.