[1] The onset conditions for ice nucleation on H 2 SO 4 coated, (NH 4 ) 2 SO 4 coated, and uncoated kaolinite particles at temperatures ranging from 233 to 246 K were studied. We define the onset conditions as the relative humidity and temperature at which the first ice nucleation event was observed. Uncoated particles were excellent ice nuclei; the onset relative humidity with respect to ice (RH i ) was below 110% at all temperatures studied, consistent with previous measurements. H 2 SO 4 coatings, however, drastically altered the ice nucleating ability of kaolinite particles, increasing the RH i required for ice nucleation by approximately 30%, similar to the recent measurements by Möhler et al. [2008b]. (NH 4 ) 2 SO 4 coated particles were poor ice nuclei at 245 K, but effective ice nuclei at 236 K. The differences between H 2 SO 4 and (NH 4 ) 2 SO 4 coatings may be explained by the deliquescence and efflorescence properties of (NH 4 ) 2 SO 4 . These results support the idea that emissions of SO 2 and NH 3 may influence the ice nucleating properties of mineral dust particles. Citation: Eastwood, M. L., S. Cremel, M.

Higher plants produce seed through pollination, using specific interactions between pollen and pistil. Self-incompatibility (SI) is an important mechanism used in many species to prevent inbreeding, and is controlled by a multi-allelic S locus1,2. “Self” (incompatible) pollen is discriminated from “non-self” (compatible) pollen, by interaction of pollen and pistil S locus components, and is subsequently inhibited. In Papaver rhoeas, the pistil S locus product is a small protein that interacts with incompatible pollen, triggering a Ca2+-dependent signalling network, resulting in pollen inhibition and programmed cell death3-7. Here we have cloned three alleles of a highly polymorphic pollen-expressed gene, PrpS, from Papaver and provide evidence that this encodes the pollen S locus determinant. PrpS is a single copy gene linked to the pistil S gene, PrsS. Sequence analysis indicates that PrsS and PrpS are equally ancient and are likely to have co-evolved. PrpS encodes a novel ~20 kDa protein. Consistent with predictions that it is a transmembrane protein, PrpS is associated with the plasma membrane. We show that a predicted extracellular loop segment of PrpS interacts with PrsS and, using PrpS antisense oligonucleotides, we demonstrate that PrpS is involved in S-specific inhibition of incompatible pollen. Identification of PrpS represents a major advance in our understanding of the Papaver SI system. As a novel cell-cell recognition determinant it contributes to the available information concerning the origins and evolution of cell-cell recognition systems involved in discrimination between “self” and “non-self”, which also include histocompatibility systems in primitive chordates and vertebrates.

Ice nucleation on mineral dust particles is known to be an important process in the atmosphere. To accurately implement ice nucleation on mineral dust particles in atmospheric simulations, a suitable theory or scheme is desirable to describe laboratory freezing data in atmospheric models. In the following, we investigated ice nucleation by supermicron mineral dust particles [kaolinite and Arizona Test Dust (ATD)] in the immersion mode. The median freezing temperature for ATD was measured to be approximately -30 °C compared with approximately -36 °C for kaolinite. The freezing results were then used to test four different schemes previously used to describe ice nucleation in atmospheric models. In terms of ability to fit the data (quantified by calculating the reduced chi-squared values), the following order was found for ATD (from best to worst): active site, pdf-α, deterministic, single-α. For kaolinite, the following order was found (from best to worst): active site, deterministic, pdf-α, single-α. The variation in the predicted median freezing temperature per decade change in the cooling rate for each of the schemes was also compared with experimental results from other studies. The deterministic model predicts the median freezing temperature to be independent of cooling rate, while experimental results show a weak dependence on cooling rate. The single-α, pdf-α, and active site schemes all agree with the experimental results within roughly a factor of 2. On the basis of our results and previous results where different schemes were tested, the active site scheme is recommended for describing the freezing of ATD and kaolinite particles. We also used our ice nucleation results to determine the ice nucleation active site (INAS) density for the supermicron dust particles tested. Using the data, we show that the INAS densities of supermicron kaolinite and ATD particles studied here are smaller than the INAS densities of submicron kaolinite and ATD particles previously reported in the literature.