2002 Volume 71 Issue 1 Pages 101-106
Pulse amplitude modulation (PAM) chlorophyll (Chl) fluorescence yield measurement was evaluated for estimating the high-temperature stress tolerance of vegetable crops. For this purpose, twenty two kinds of vegetable crops comprising of 17 taxonomic species were grown in controlled environments at the following three temperature regimes : (1) the air temperature during the 14-hr photo period was maintained at 20°C ; (2) it was raised stepwise at 5°C hr-1 from 20°C to 35°C, and then immediately decreased stepwise to 20°C, and (3) it was changed similarly to (2) except that the duration at 35°C was prolonged to 3 hr. At night the ambient air was cooled to 10°C in all three pretreatments. Following the pretreatments for 48 days, photosystem (PS) II Chl. fluorescence yields of leaf sections were determined by PAM before and during exposure of leaf sections to 45°C for up to 5 hr. Based on the heat sensitivity of PS II fluorescence yields, the vegetable crops tested were grouped into the following three classes : (A) crops with heat-tolerant PS II, (B) those with PS II sensitive to heat but capable of high-temperature acclimation, and (C) those with PS II sensitive to heat and incapable of high-temperature acclimation. These groupings coincide well with those based on the field performance of the crops at high temperatures. It is thus concluded that the Chl fluorescence yield measurement is a simple and reliable method for estimating the high-temperature stress tolerance of vegetable crops.
