The growth of Arabidopsis plants in chilling conditions could be related to their levels of salicylic acid (SA). Plants with the SA hydroxylase NahG transgene grew at similar rates to Col-0 wild types at 23°C, and growth of both genotypes was slowed by transfer to 5°C. However, at 5°C, NahG plants displayed relative growth rates about one-third greater than Col-0, so that by 2 months NahG plants were typically 2.7-fold larger. This resulted primarily from greater cell expansion in NahG rosette leaves. Specific leaf areas and leaf area ratios remained similar in both genotypes. Net assimilation rates were similar in both genotypes at 23°C, but higher in NahG at 5°C. Chlorophyll fluorescence measurements revealed no PSII photodamage in chilled leaves of either genotype. Col-0 shoots at 5°C accumulated SA, particularly in glucosylated form. SA in NahG shoots showed similar tendencies at 5°C, but at greatly depleted levels. Catechol was not detected as a metabolite of the NahG transgene product. We also examined growth and SA levels in SA signaling and metabolism mutants at 5°C. The partially SAinsensitive npr1 mutant displayed growth intermediate between NahG and Col-0, while the SA-deficient eds5 mutant behaved like NahG. In contrast, the cpr1 mutant at 5°C accumulated very high levels of SA and its growth was much more inhibited than wild type. At both temperatures, cpr1 was the only SA-responsive genotype in which oxidative damage (measured as thiobarbituric acid-reactive substances) was significantly different from wild type.Salicylic acid (SA) has received much attention due to its association with economically important plant responses to disease and other stresses. Detailed evidence implicates SA in PR gene expression, systemic acquired resistance, and the hypersensitive response (Kunkel and Brooks, 2002). SA also seems to be involved in responses to abiotic stresses, such as ozone (Sharma et al., 1996;Rao and Davis, 1999;Koch et al., 2000), salt and osmotic stress (Borsani et al., 2001;Molina et al., 2002;Shim et al., 2003), UV-B (Surplus et al., 1998), drought (Senaratna et al., 2000Nemeth et al., 2002), paraquat (Kim et al., 2003, and heat (Dat et al., 1998a(Dat et al., , 1998b(Dat et al., , 2000Lopez-Delgado et al., 1998a;Senaratna et al., 2000;Larkindale and Knight, 2002; Clarke et al., 2004). Stress-influenced developmental transitions, including flowering (Hatayama and Takeno, 2003;Martinez et al., 2004), tuberization (Lopez-Delgado and Scott, 1997), and senescence (Morris et al., 2000), may also involve SA.Cold is one of the most important limitations to crop productivity and species distribution. Freezing (subzero) or chilling (low positive) temperatures can cause injury or reduced growth depending on the cold tolerance of the species (Schneider et al., 1995;Pearce, 1999;Humphreys et al., 2003). Recent studies describe potentially valuable effects of salicylate treatment on cold tolerance in maize, rice, and wheat (Janda et al., 1999;Szalai et al., 2000;Kang and Saltveit, 2002;Tasgin et al., 200...