On the relationship between in vivo chlorophyll fluorescence spectrum and leaf temperature

The relationship between in vivo chlorophyll fluorescence spectrum and leaf temperature was evaluated on various plants differing in sensitivity to chilling no-freezing temperatures. Reducing the leaf temperature from 25 to 4°C the chlorophyll fluorescence over the whole 650-800 nm spectral range was seen to increase linearly in tomato, bean, pea and broad bean plants. However, the rate of fluorescence intensity increase per Celsius degree for the 730 nm band was about 2-times higher than that at 685 nm. Hence, the chlorophyll fluorescence ratio, F685/F730, was observed to be directly correlated to leaf temperature. In a second experiment, the F685/F730 ratio and the leaf temperature were monitored on plants in a climate chamber as the temperature was decreased from 23 to 4°C. The experiment was run simultaneously on the chilling-sensitive tomato, bean and maize and on the chilling-tolerant pea, broad bean and wheat plants. Two phases related to leaf temperature can be distinguished for all species: the first 5-hour period during which the leaf temperature decreased from 24 to 4°C, and a second period during which the leaf temperature slightly oscillated around 4°C. During the first phase, F685/F730 decreased markedly for the chilling-sensitive species and slightly for the chilling-resistant ones. In the following stage, the F685/F730 values for the chilling-resistant plants remained constant while those for the chilling-sensitive were still found to decrease. On the basis of our results, the use of the chlorophyll fluorescence ratio as indicator of plant chilling sensitivity can be envisaged.