DELAYED LIGHT-EMISSION AND FLUORESCENCE RESPONSES OF PLANTS TO CHILLING

Delayed light emission (DLE) of chlorophyll has the same excitation an d emission spectra as chlorophyll fluorescence and was formerly called delayed fluorescence. DLE has a much longer time response than true c hlorophyll fluorescence and is detectable for times ranging from milli seconds to many minutes. DLE is induced by back reactions of the photo synthetic pathway and therefore requires functional chloroplasts. It i s detectable only in the dark following light excitation, yields very low energy, and decays very rapidly. DLE repetitively excited over tim e, which we term refreshed DLE (RDLE), shows a shoulder and broad peak in our measurements, indicating participation of at least two energy pools. DLE is altered by physiological stresses that affect chloroplas ts or photosynthesis, and as illustration, plant species known to be v ery susceptible or very tolerant to chilling were exposed to chilling temperatures for varying times. RDLE at 0.3 s (the initial shoulder on our curves) rose in response to chilling damage in the susceptible sp ecies. The major RDLE peak was greatly inhibited in the susceptible sp ecies and showed only small changes in the tolerant species. Fluoresce nce measurements made on the chilling-tolerant species indicated simil ar responses and similar coefficients of determination were derived. T hese results indicate that measurement of precisely timed delayed ligh t emission or of refreshed delayed light emission at a less precisely controlled time can be used to detect chilling stress.