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The last widely accepted classification of polyploidy recognized three major categories: autoploidy, segmental alloploidy, and true alloploidy. Criteria for recognizing these types were based largely on chromosome pairing in F1 hybrids, but until very recently there were no quantitative criteria to distinguish autoploids from segmental alloploids. This is theoretically possible, and Jackson and Casey (1982) and Jackson and Hauber (1982) have presented models and methods to quantitatively predict the frequencies of the different meiotic configurations in autotriploids through autooctoploids. The expected numbers of configurations then can be statistically tested for goodness of fit with observed data. Synthetic and natural autotetraploids have been shown to fit the models well. However, the exceptions that do not fit are of particular interest because they indicate the presence of Ph‐like (Pairing homoeologous) genes that cause fewer multivalents and more bivalents than expected in the autoploid models. The effect of Ph‐like genes at the nuclear level apparently involves placement of chromosome attachment sites on the nuclear membrane such that different genomes occupy slightly different sites. This would be far enough apart in true alloploids so that no pairing occurred in the original F1 hybrid. In true autoploids and normal diploids, homologues are equally close so that pairing obeys probability laws that make it possible to describe expected pairing events with appropriate models and equations. From the foregoing statements, one can deduce that the terms autoploid, segmental alloploid, and true alloploid need not and indeed should not be equated with results of crosses between any taxonomic levels. Rather, they represent conditions that can be attained by one or very few genes acting on nuclear membrane attachment sites. It is possible to deduce from hybrids at the diploid level the type of polyploid that can be obtained, and such data can have great agronomic value. A generally held dogma is that the inability of chromosomes to pair in F1 hybrids is due to considerable genetic divergence and perhaps numerous structural changes at the light microscope and/or ultrastructural level. Data from various sources show that this need not be true, and there is no reason to believe this is a factor in what has been called genome divergence in the classical sense. Genome divergence in terms of pairing ability can occur at the population level due to one or a few genes.

Methods are presented for determining the frequencies and numbers of various meiotic configurations expected in autopolyploids. This allows one to test polyploids of unknown origin for agreement with expected meiotic configurations. Rejection of the autoploid hypothesis may indicate the presence of Ph‐like genes or some type of alloploid. The models consider mean chiasma frequencies of 2, 3, and 4 per bivalent for triploids and tetraploids and 2 per bivalent for pentaploids, hexaploids, heptaploids, and octoploids. Literature data for a known autotriploid, autotetraploids, allotetraploids, and allopentaploids were tested against expectations of the models. There was generally good agreement between number of observed autoploid meiotic configuration and those expected in the models.

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The activities of key C4 enzymes in gel-filtered, whole-leaf extracts and the photosynthetic characteristics for reciprocal F, hybrids of Flaveria pringlei (C3) and F. brownii (C4-like species) were measured to determine whether any inherited C4-photosynthetic traits are responsible for their reduced CO2 compensation concentration values (AS Holaday, S Talkmitt, ME Doohan Plant Sci 41: 31-39). The activities of phosphoenolpyruvate carboxylase, pyruvate, orthophosphate dikinase, and NADP-malic enzyme (ME) for the reciprocal hybrids are only about 7 to 17% of those for F. brownii, but are three-to fivefold greater than the activities for F. pringlei. The low activities of these enzymes in the hybrids appear to be the result of a partial dominance of F. pringlei genes over certain F. brownii genes. However, no such dominance occurs with respect to the expression of genes for NADP-malate dehydrogenase, which is as active in the hybrids as in F. brownii. In contrast to the situation with the enzymes above, cytoplasmic factors appear to determine the inheritance of NAD-ME. The NAD-ME activity in each hybrid is comparable to that in the respective maternal parent. Pulse-chase 14CO2 incorporation analyses at ambient CO2 levels indicate that the hybrids initially assimilate 7 to 9% of the total assimilated CO2 into C4 acids as compared to 3.5% for F. pringlei. In the hybrids, the percentage of 14C in malate decreases from an average of 6.5 to 2.1% after a 60-second chase in 12C02/air. However, this apparent C4-cycle activity is too limited or inefficient to substantially alter CO2 exchange from that in F. pringlei, since the values of net photosynthesis and 02 inhibition of photosynthesis are similar for the hybrids and F. pringlei. Also, the ratio of the internal to the external CO2 concentration and the initial slopes of the plot of CO2 concentration versus net photosynthesis are essentially the same for the hybrids and F. pringlei. At 45 micromoles CO2 per mole and 0.21 mole 02 per mole, the hybrids assimilate nearly fivefold more CO2 into C4 acids than does F. pringlei. Some turnover of the malate pool occurs in the hybrids, but the labelling of the photorespiratory metabolites, glycine and serine, is the same in these plants as it is in F. pringlei. Thus, although limited C4-acid metabolism may operate in the hybrids, we conclude that it is not effective in altering 02 inhibition of CO2 assimilation. The ability of the hybrids to assimilate more CO2 via phosphoenolpyruvate carboxylase at low levels of CO2 than does F. pringlei may result in an increased rate of reassimilation of photorespiratory CO2 and CO2 compensation concentrations below that of their C3 parent. If the hybrids do possess a limited C4 cycle, it must operate intracellularly. They are not likely to have inherited an intercellular compartmentation of C4 enzymes, since F. brownii has incomplete compartmentation of key C3 and C4 enzymes.

Physiology and BiochemistrySinapate esters provide greater UV-B attenuation than f1avonoids in Arabidopsis thaliana (Brassicaceae) JOHN J. SHEAHAN 679