The function of Lhca4, a gene encoding the photosystem I type IV chlorophyll a/b-binding protein complex in Arabidopsis, was investigated using antisense technology. Lhca4 protein was reduced in a number of mutant lines and abolished in one. The inhibition of protein was not correlated with the inhibition of mRNA. No depletion of Lhca1 was observed, but the low-temperature fluorescence emission spectrum was drastically altered in the mutants. The emission maximum was blue-shifted by 6 nm, showing that chlorophyll molecules bound to Lhca4 are responsible for most of the long-wavelength fluorescence emission. Some mutants also showed an unexplainable delay in flowering time and an increase in seed weight.