SummaryTo elucidate the molecular basis of loss of self-incompatibility in Lycopersicon, S-RNases and HTproteins were analysed in seven self-compatible (SC) and three self-incompatible (SI) taxa. No or low stylar RNase activity was a common feature in most SC taxa examined, in contrast to the uniformly high levels of activity found in all SI species. The S-RNase gene is most likely deleted in the four red-fruited SC taxa (L. esculentum, L. esculentum var. cerasiforme, L. pimpinellifolium and L. cheesmanii) because S-RNase genes could not be ampli®ed from genomic DNA. S-RNase genes could, however, be ampli®ed from the genomes of the three green-fruited SC taxa examined. L. chmielewskii and L. hirsutum f. glabratum show a decreased accumulation of transcripts, possibly re¯ecting changes in the 5¢¯anking regions of the S-RNase genes. The remaining green-fruited SC species, L. parvi¯orum, has a functional S-RNase gene in its genome that is expressed at high levels in the style, suggesting a genetic factor responsible for the low S-RNase activity. Together these results argue for several independent mutations in the S-RNase gene over the course of Lycopersicon diversi®cation, and that loss of S-RNase function is unlikely to the primary cause of the loss of self-incompatibility. We also examined the HT-B genes that play a role in self-incompatibility. HT-B transcripts were markedly reduced in the styles of all the SC taxa examined. A scenario is described where a mutation causing reduced transcription of HT-B in an ancestral SI species was central to the loss of self-incompatibility in Lycopersicon.

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Aluminum is usually regarded as the determining factor for plant growth in acid soils and nutrient deficiencies are often additional growth‐limiting factors in tropical acid soils. Taking into account the potential interactions between Al toxicity and nutrient deficiencies, the present study investigated sorghum (Sorghum bicolor Moench [L.]) and maize (Zea mays L.) cultivar differences for: (1) Al tolerance (relative growth in a one‐fifth strength nutrient solution [low‐nutrient medium, ionic strength: 4.5 mmol L−1] with Al and without Al), (2) low‐nutrient tolerance (relative growth in a low‐nutrient medium compared with growth in a full‐strength nutrient solution), (3) combined tolerance (relative growth in a low‐nutrient medium containing Al compared with a full‐strength medium lacking Al). The goal of the present study was to identify the predominant growth‐limiting factor using a solution culture medium that simulates the nutrient status of tropical acid soils. Differential Al tolerance among 15 cultivars of sorghum and 10 cultivars of maize in short‐term assays (2.5 or 20 µmol L−1 AlCl3 in 0.2 mmol L−1 CaCl2 at pH 5.0 or 4.9, respectively, for 24 h) was positively correlated with Al tolerance in long‐term cultures (11.1 or 42.6 µmol L−1 soluble Al in the low‐nutrient medium at pH 4.5 or 4.3, respectively, for 29 days). However, the level of Al tolerance in the short‐term assays was not correlated with the combined tolerance, suggesting that a short‐term screening technique may not be practically useful for estimating cultivar adaptation to a combination of stress factors in tropical acid soils. In sorghum, a less Al‐tolerant plant species, higher Al tolerance was associated with less Al absorption by the roots and greater K translocation into the shoots. In maize, a more Al‐tolerant plant species, there was no correlation between the accumulation or transport of elements and Al tolerance. Standardized partial regression coefficients suggested that low‐nutrient tolerance contributed more to combined tolerance than Al tolerance under most conditions (except for Al‐sensitive sorghum at 42.6 µmol L−1 AlCl3). A greater combined tolerance was associated with a higher K shoot concentration in sorghum and a higher Ca shoot level in maize. Plant nutritional characteristics linked to low‐nutrient tolerance should be evaluated as an important strategy for plant production in tropical acid soils, both for Al‐tolerant plant species and for Al‐sensitive plant species under low‐Al conditions.

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