The critical chilling temperature of cold-sensitive lima bean (Phaseolus lunatus L.) seedlings was determined to be approx. 8°C. The full-length cDNAs of PlSAD, PlFAD2, and PlFAD3 encoding three fatty acid desaturases were isolated from lima bean leaves. The PlSAD open reading frame (ORF) had 1,317 bp and a single intron of 601 bp, encoding a polypeptide of 438 amino acids that showing 86% homology with the plastidial stearoyl-acyl carrier protein desaturase in soybean. The PlFAD2 ORF contained 1,059 bp and was uninterrupted, encoding a polypeptide of 352 amino acids having 85% identity with the microsomal omega-6 desaturase GmFAD2-3 in soybean. PlFAD3 ORF had 1,116 bp and seven introns, encoding a polypeptide of 371 amino acids showing 91% homology with the microsomal omega-3 desaturase FAD3 in cowpea. PlSAD and PlFAD2 were expressed highly in leaves but poorly in roots and stems, while PlFAD3 was expressed in three tissues. All three genes were significantly induced in leaves by drought. PlSAD and PlFAD3 in leaves were down-regulated by high temperature and salinity, whereas PlFAD2 was up-regulated by the two stresses. The expression patterns of the three genes in lima bean leaves under suboptimal temperature, 18°C, were different from those under 8°C. The down-regulation of PlSAD transcript at chilling temperature might be an important factor contributing to chilling susceptibility for lima bean plants.

Fingered citron (Citrus medica L. var. sarcodactylis Swingle), a precious fruit ornamental plant, is sensitive to low temperature. Cold tolerance, evaluated by semi-lethal temperature, was lower in wild-type 'Qingpi' than in its mutant 'Aihua' trees obtained by c-radiation. The full-length cDNAs of two genes encoding fatty acid desaturases involved in unsaturated fatty acid biosynthesis were isolated from the fingered citron leaves. The Cms-FAD2 open reading frame (ORF) had 1,152 bp and was uninterrupted, encoding a polypeptide of 384 amino acids that showing 82% homology with the microsomal x-6 desaturase CiFAD2 in Davidia involucrate. The CmsFAD8 ORF contained 1,373 bp and 7 introns, encoding a polypeptide of 458 amino acids showing 76% homology with the plastidial x-3 desaturase BpFAD8 in Betula pendula. CmsFAD2 was expressed highly in leaves but low in roots and flowers, while CmsFAD8 was obviously expressed in three tissues. Compared with control group (28°C), the expression of CmsFAD2 and CmsFAD8 in leaves of two genotypes was significantly induced at 6°C. The increase of CmsFAD2 and CmsFAD8 was earlier and larger in coldtolerant 'Aihua' than in cold-sensitive 'Qingpi'. The linolenic acid content increased significantly in leaves of mutant 'Aihua' plants exposed to low temperature of 6°C. The results showed that a positive relationship between CmsFAD expression and genotype tolerance to cold may exist.