Being able to sow early to maximize the growing season and to escape drought stress has increased the importance of low-temperature tolerance in sunflower. Yet knowledge about the molecular basis of sunflower response to low temperature is still lacking. To address this issue, nylon microarrays containing >8000 putative unigenes were developed and used. Early-and late-flowering genotypes were sown at 15 8C and grown until the two-leaf stage when they were subjected to 7 8C until the four-leaf stage. The transcriptional profiles of low temperature-grown plants (15 8C and 7 8C) were compared with those grown under standard conditions (25 8C). Two-step ANOVA normalization and analysis models were used to identify the differentially expressed genes. A total of 108 cDNA clones having a P-value <10 23 were found to be differentially expressed between the low temperaturegrown plants (15 8C and 7 8C) and their corresponding two-leaf-and four-leaf-stage controls across the two genotypes. About 90% of these genes were downregulated. This includes genes potentially involved in the metabolism of carbohydrate and energy, protein synthesis, signal transduction, and transport function. Comparing gene expression profiles at 15 8C and 7 8C revealed that only four genes can be considered as differentially expressed, in both genotypes, suggesting that similar genetic programmes underlie the response of sunflower plants to these temperature regimes. The analysis also revealed that early-and late-flowering genotypes respond similarly to low-temperature tolerance as justified by the low number of genes showing a significant genotype3treatment interaction effect. It seems likely that the down-regulation and/or non-induction of genes having a critical role in lowtemperature tolerance may be responsible for the sensitivity of sunflower plants to low-temperature tolerance. The results reported provide an initial characterization of the transcriptome activity of sunflower, as a chilling-sensitive plant under suboptimal temperatures, and could be of importance to reveal the potential differences between chilling-sensitive and chilling-tolerant species.

Taken together, these results indicate that vegetative and reproductive tissues employ different transcriptome responses to these stress treatments. Careful examination of the putative functions of these genes revealed novel and specific responses. The potential roles of many of the differentially expressed genes in stress tolerance are mentioned and discussed.