Can screening criteria for drought resistance in Nicotiana tabacum L. be derived from the polyphasic rise of the chlorophyll a fluorescence transient (OJIP)?

Four tobacco cultivars (Nicotiana tabacum L.), known to differ with respect to their drought resistance, were subjected to a slowly intensifying drought stress (control level: ψPD=ca.−0.5MPa; and severe stress level: ψPD=ca.−2.5MPa), and rewatered under controlled environmental conditions in programmable growth rooms. Drought stress-induced changes in the fast-phase chlorophyll fluorescence induction kinetics during the stress and recovery periods were monitored with a shutterless fluorescence measuring system with a time resolution of 10μs (Plant Efficiency Analyser by Hansatech Instr., UK). Although chlorophyll fluorescence transients are excellent probes for measuring the effects of stress on the PSII photo-chemistry, in this article we evaluate their practical applicability in screening for drought resistance. During the entire drought stress and recovery period, the fluorescence transients followed a regular pattern of 0-J-I-P--T, with two intermediate inflections J (ca. 2ms) and I (ca. 20ms) appearing between the Fo and Fm levels. From the fluorescence data presented, it is clear that the single fluorescence parameter which differed the most in its response to drought stress between the drought-resistant (GS46 and ELSOMA) and drought-sensitive (TL33 and CDL28) cultivars was the Fo value normalized to Fo (t=o), which in confirmation of their drought resistance, increased dramatically in the former but not in the latter cultivars. In the interpretation of the results, however, care is expressed not to attempt to correlate changes observed in individual transient components with possible physiological causes, since these individual fluorescence parameters are influenced by an array of interacting physico-chemical and metabolic factors. The use and importance of normalized values, ratios and indices, in terms of the practical application of chlorophyll fluorescence measurements in drought-stress research is discussed