Sequential Loss of Photosynthetic Functions during Leaf Desiccation as Monitored by Chlorophyll Fluorescence Transient

The effect of reduction in leaf water content on the complete chlorophyll a fluorescence tran-sient of Dolichos leaf was monitored using a kinetic fluorescence monitor. Specific and sequen-tial alterations in the profile of the fluorescence transient was observed as leaf water content was progressively decreased. Of all the characteristic, the P S J M ^ M J T (cf. Govindjee and Papageorgiou 1971) points of the fluorescence induction curve, the M2 peak which is linked to CO2 fixation potential of leaf, was extremely sensitive to desiccation. The extent of PS * phase of the transient diminished almost proportionally to leaf water content until the water content ap-proached about 30%. The effect of strong additional illumination administered during the fluorescence transient suggests that the quenching of fluorescence due to desiccation is mostly link-ed to the redox-level of QA; the stable electron acceptor of photosytem II, and not to the changes in zJpH formation or 'state changes'. Rewatering of desiccated leaves restored the fluorescence transient except in case of severe desiccation. The monitoring of fluorescence transient thus offers a diagnostic tool to assess the nature and the level of impairments due to loss in leaf water content. Key words: Chlorophyll a fluorescence — Dolichos lablab — Fluorescence transient — Lea