Changes in laser induced chlorophyll fluorescence signatures during regeneration of kacholam and colocasia plants from water stress

In vivo laser induced chlorophyll fluorescence (LICF) spectra were measured on a Jobin Yvon double monochromator with He-Ne laser (632.8 nm) excitation during the initiation and recovery phases of water stress in kacholam (Kaempferia galanga) and colocasia (Colocasia antiquorum) plants. The fluorescence intensity ratio (F685/F730) of the chlorophyll bands at 685 and 730 nm were determined during induction and recovery phases of water stress. The fluorescence induction kinetics (Kautsky effect) was also measured on dark adapted intact plant leaves at the chlorophyll bands for determining the vitality indices (Rfd values) and the stress adaptation index (Ap). It is seen that F685/F730 ratio for kacholam increased from 1.7 to 2.1 during 17 days of water stress; but regained to its initial value, within a few days of removal of stress. In the case of colocasia, the ratio increased from 2.0 to 3.7 in 12 days of treatment but recovered only to 3.5 on removal of stress. The colocasia plants could no longer sustain the photosynthetic functioning and its leaves dried up, subsequently. The Rfd-685, Rfd-730 and Ap values showed a decreasing trend with increasing water stress for kacholam and colocasia plants. Kacholam plants with Ap values of ca. 0.26 recovered fully to their normal conditions on re-watering. For colocasia, during the same water stress period, the mean Ap value decreased from 0.2 to 0.089; but could attain only values of 0.093 with 4 days of re-watering and further recovery was impossible as plants became dry with total impairment of the photosynthetic apparatus. Colocasia plants with lower Ap values suffered from stress earlier than kacholam plants that have higher Ap values. When the Ap values fall considerably below 0.1 during water stress, recovery becomes remote as the cytoplasm and chloroplast storma get dehydrated. The results demonstrate the usefulness of Ap and Rfd values in determining the potential photosynthetic activity of plants and its application in understanding the regeneration of photosynthetic functioning on removal of water deficit.