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Nitric oxide (NO) is one of the more recently discovered signalling molecules but has been shown already to participate in a wide range of plant processes. One example is the assimilation of N via nitrate reductase (NR), the subject of a study by Jin et al. (Zhejiang University, Hangzhou, China and La Trobe University, Bundoora, Australia, pp. 9–17). Tomato seedlings were grown hydroponically under low nitrate (0.5 mm) or high nitrate (5.0 mm) conditions. After 2 weeks' growth under these conditions, plants were treated with a NO donor (sodium nitroprusside, SNP, or diethylamine NONOate sodium, NONOate) or with a NO scavenger (cPTIO), or were untreated (controls). In the untreated plants, NR activity was approx. 3-fold higher under high-N than under low-N conditions. NO, supplied either via SNP or NONOate, increased NR activity in low-N plants but decreased it in high-N plants. The reverse was true for the NO scavenger, cPTIO. A specific study of the effects of SNP showed that it did not affect either NR gene transcription or the amount of NR protein. Its effects were thus mediated at the post-translational level. The authors also tested the direct effects of NO on enzyme extracts. Addition of NO stimulated NR activity in extracts from low-N plants while addition of the NO scavenger inhibited enzyme activity. In high-N plants low concentrations of NO stimulated NR activity but higher concentrations inhibited it, as did, once again, the NO scavenger. Interpretation of these results is made more complicated by the fact that NR activity itself produces NO. Indeed, the authors' in situ measurements of NO in low-N and high-N roots is consistent with this. Nevertheless, it is clear that there is an interaction between NO and N-supply in the observed effects on NR activity. Further, the effects of NO may be mediated by direct interaction with the protein, as has been shown previously for interactions with haemoglobin.
Seeds are so familiar to us that we often forget what remarkable structures they are: embryonic organisms, dried down to a very low water content and capable of surviving in that state for months or even many years. They are therefore excellent subjects for germplasm conservation. However, even under ideal storage conditions there is extensive variation in seed life span between species, as discussed by Probert et al. (Wakehurst Laboratory, Royal Botanic Gardens, Kew, pp. 57–69). They collected, from a very wide range of habitats, seeds of 195 species belonging to 71 different families. In order to make observations within a realistic time-frame, seeds were subjected to rapid ageing (45 °C/60 % RH, or 60 °C/60 % RH). Seeds were sampled regularly for germination tests; time taken for viability to fall to 50 % (p50) was determined. As under more ideal storage conditions, there was a very wide range in seed life span. At 45 °C/60 % RH, p50 ranged from 0·1 days to 771 days. There was no obvious relationship between life span and taxonomic position: most orders exhibited a wide range of longevities. Order Myrtales contained the longest-lived species, Calothamnus rupestris, and order Liliales contained the shortest-lived, Narthecium ossifragum. Seed life span was correlated significantly with the absence of endosperm (mean p50 of 20·3 with endosperm and 65·7 days without endosperm). The climate at the place of a seed's origin was shown to be important: life span was positively correlated with mean annual temperature and negatively correlated with total annual rainfall. This leads to the conclusion that seeds without endosperm from plants growing in hot, dry environments are likely to be long-lived. Early angiosperms had endospermic seeds with small embryos and probably lived in moist environments. The authors suggest that these early flowering plants would have had short-lived seeds and that seed longevity evolved either as an adaptation to climatic drying or in association with invasion of hot, dry environments.
The work of Henk van Dijk at Université Lille I (pp. 115–124) reminds us that there are some important projects that simply cannot be fitted into a typical pattern of short-term grant funding. In a very well-planned and carefully executed long-term study, he investigated flowering and seed production in relation to plant age in sea beet (Beta vulgaris ssp. maritima). Plants grown from seed collected at different locations in Western Europe showed marked differences in mean life span between populations. In addition to these natural populations, the author established a synthetic population, based on plants originating from 93 different wild populations. All plants were grown in a greenhouse under natural day length at ‘temperatures slightly warmer than external temperatures’. Date of flowering, seed set and root diameter were determined each year until plant death. Pooled populations from different regions exhibited mean life spans ranging from 2·2 years (inland France) to 7·1 years (NW Brittany); the synthetic population showed a mean of 5·7 years. There was a clear effect of age on the date of flowering which, right across the age range, was 1·31 days later per year. Later flowering was correlated with a decline in seed production, probably because the period for seed and fruit ripening was shorter. In the year before death, these trends were especially evident, with flowering occurring 3·3 days later than in plants that had at least one more year to live. Seed production also declined further, as did investment in root biomass. These data thus suggest two different mechanisms (or sets of mechanisms) operating during a plant's life span, one involving a slow, longer-term decline and one involving a more catastrophic decline towards the end of life. The reader is referred to the paper for further discussion; at this point I simply wish to acknowledge again the commitment and patience involved in carrying out this important and interesting investigation.
During its long history the earth has experienced many climatic changes. In some of the warmest periods, characterized by very high atmospheric CO2 concentrations, coniferous forests extended into the polar regions. How then did the trees respond to the elevated CO2 concentrations in regions where there is 24 h of daylight in high summer? Llorens et al., at Sheffield (pp. 179–188) have used ‘living fossil’ conifers in experiments aimed at answering this question. Sequoia sempervirens, Metasequoia glyptostroboides and Taxodium distichum are all members of genera represented in the Mesozoic Arctic forests and are thus appropriate experimental models. Plants were grown for 3 years under conditions that mimicked the Cretaceous Arctic climate, including the seasonal variations in daylight hours and temperature. CO2 concentrations were either elevated (800 µmol mol−1) or ambient (400 µmol mol−1). In general it is expected that elevated CO2 concentrations will lead to an increase in water-use efficiency (WUE). However, the authors suggest that this effect will be negated by the very long daylight hours of summer in which there is little or no time to ‘recover’ from day-time conditions. This hypothesis was apparently supported by the data from Sequoia and Metasequoia, in which improvements in leaf WUE were only observed early and late in the growing season. However, in Taxodium the effect persisted throughout the season. Further, when measured at whole-plant level and integrated over the whole growing season, WUE of all three species was stimulated by elevated CO2. Analysis of the components that make up WUE showed that, surprisingly, transpiration rates were little affected by elevated CO2, such that there was no significant effect on total plant water use. The major effect was on increased rates of photosynthetic CO2 uptake. This occurred despite a lower carboxylation efficiency (as indicated by isotope discrimination measurements), but was probably aided by a reduction in photorespiration at the elevated CO2 concentrations.