The natural variation in starch synthase IIa (SSIIa) of rice (Oryza sativa L.) was characterised using near-isogenic lines (NILs). SSIIa is a candidate for the alk gene regulating the alkali disintegration of rice grains, since both genes are genetically mapped at the same position on chromosome 6 and related to starch properties. In this study, we report that the alkali-susceptible cultivar Nipponbare lacked SSIIa activity in endosperm. However, the activity was detected with NILs having the alk allele of alkali-tolerant Kasalath. SSIIa protein was present even in Nipponbare endosperm, but it was not associated with starch granules at the milky stage of endosperm. Three single-nucleotide polymorphisms (SNPs) predicting amino acid substitutions existed between the cDNA sequences of SSIIa of Nipponbare and Kasalath were genotyped with 65 rice cultivars and four wild relatives of cultivated rice. The results obtained explain the potential importance of two of the amino acid residues for starch association of rice SSIIa. An analysis of the chain-length distribution of ␤-limit dextrin of amylopectin showed that without SSIIa activity, the relative number of A-chains (the short chains without branches) increased and that of B1-chains (the short chains with branches) decreased. This suggests that, given the SSIIa defect, short A-chains could not reach a sufficient length for branching enzymes to act on them to produce B1-chains.

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Oxygen isotope compositions of individual mineral grains in two spinel-bearing chondrules have been determined by secondary ion mass spectrometry. O isotope ratios of spinel grains in a plagioclase-rich chondrule were extremely 16 O-rich (δ 18 O ¾ δ 17 O ¾ 35‰) indicating close relationship to spinels in Ca-Al-rich inclusions (CAIs), whereas O isotope ratios of a spinel grain in barred-olivine chondrule were less enriched in 16 O (δ 18 O ¾ δ 17 O ¾ 10‰). The degree of 16 O-excesses among minerals within each chondrule was parallel to the crystallization sequences. These measurements suggest that some CAI-spinel grains were incorporated into the chondrule precursors, and that the O isotopic variation among minerals may result in incomplete exchange of O isotopes between 16 O-rich chondrule melt and 16 O-poor ambient nebular gas. The mesostasis having an O-isotopic composition near the terrestrial fractionation line may have been somewhat affected by the aqueous alteration in the parent body.

Reported differences in the relaxation of cell walls in enlarging stem tissues of soybean (Glycine max (9,11) similarly excised pea stems but they continued to grow for 0.5 to 1 h (9) and the walls required 3 to 5 h to relax when P was measured with a pressure probe (9, 11). The Y estimated by this method averaged 0.3 MPa below P, which was considerably lower than in soybean.Because of these discrepancies, our knowledge about the yield threshold remains uncertain in higher plants. It is possible that growth is regulated differently in different species or, because a psychrometer was used for one experiment (5) and a pressure probe for the others (9, 11), it is possible that some feature of the methods caused the differences.The magnitudes of the forces causing cells to enlarge indicate how growth rates are controlled and are necessary to obtain estimates of the extensibility of the cell walls. Without an understanding of Y, it is not possible to obtain this information. Because the reasons for variations in the previous measurements are not known, we investigated Y using both the pressure probe and the psychrometer. The experiments were conducted in a water-saturated atmosphere to ensure that transpiration did not complicate the measurements and all P determinations were affected only be growth. MATERIALS AND METHODS

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In this paper, we report the petrographic properties of visible tephra layers sampled from two drill cores (SG93 and SG06) from Lake Suigetsu, central Japan, because no fundamental petrographic data including the refractive indices of minerals and volcanic glass shards, the shapes of volcanic glasses, and mineral compositions of the Suigetsu tephra samples have been reported. The Suigetsu tephra layers can be broadly classified into those derived from calderas on Kyushu Island (e.g., Aso, Ata, and Aira) and those that derive from Daisen and Sambe volcanoes in the Chugoku district of southwest Japan. The tephra layers correlated with the Ulleung⊖Oki tephra from Ulleung Island on Japan Sea, the Sakate tephra which has a close relationship with the Sambe⊖Ukinuno tephra, and the scoria layer from an unknown source volcano were also found in the Suigetsu core samples. The Suigetsu tephra layer correlated with the San'in 1 tephra found in the drill core bored from Japan Sea can be also correlated with the Kuju-1 tephra which erupted from the Kuju caldera on Kyushu Island, based on the petrographic properties. The petrographic characteristics provide important constraints on the correlation and identification of tephras, which cannot be obtained by only the major element compositions of volcanic glass shards. However, the correlation of some tephra layers remains ambiguous. Therefore, additional geochemical data from volcanic glass shards including the concentrations of various trace elements are also necessary for a more rigorous identification and correlation of the Suigetsu tephra.

The major and trace element concentrations of volcanic glass shards from visible tephra layers in the SG93 and SG06 cores from Lake Suigetsu, central Japan, were determined by femtosecond laser ablation–inductively coupled plasma–mass spectrometry. The glass‐shard analyses, together with the petrographic properties of the tephra samples, allow the Suigetsu tephra layers to be broadly classified into tephras derived from calderas on Kyushu Island, and from Daisen and Sambe volcanoes in the Chugoku district of southwest Japan. The layers correlated with tephras from Kuju caldera and Daisen volcano, and with the younger Sambe tephras, have adakitic elemental features. A Suigetsu tephra sample correlated with the Sambe−Kisuki tephra based on petrographic properties has an elemental pattern similar to that of the Toya tephra from Hokkaido Island, northeast Japan. This match implies that tephras from northeast Japan, as well as Kyushu–Chugoku tephras, are possible correlatives of the Suigetsu tephra layers. Both petrographic properties and major–trace element data of volcanic glass shards are essential for robust tephra correlations, and hierarchical cluster analysis proved additionally useful in statistically evaluating relationships among the tephras.

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