DROUGHT AND CHLOROPHYLL FLUORESCENCE IN FIELD-GROWN POTATO (SOLANUM-TUBEROSUM)

Light interception, stomatal conductance and chlorophyll fluorescence were measured in potato (Solanum tuberosum L.) grown either irrigated, or droughted from the time of plant emergence. Compared with the irri gated treatment, drought reduced both light interception and stomatal conductance. In both treatments, the yields of variable fluorescence i n the dark- and light-adapted states (F(v)/F(m) and F(v)'/F(m)', respe ctively) were negatively correlated with photosynthetic photon flux de nsity (PPFD) and mirrored daytime changes in PPFD. Photochemical quenc hing was positively correlated with PPFD, but the dominant effect of F (v)'/F(m)' resulted in a decrease in the quantum yield of photosystem II (PSII) electron transport with increasing PPFD. Drought had no sign ificant effect on the functioning of PSII and the balance between phot ochemical and non-photochemical quenching was unaffected. Non-photoche mical quenching was not increased by drought and the quantum yield of PSII electron transport was unaffected. It is concluded that, in leave s of droughted plants, excess energy, resultant of stomatal limitation of photosynthesis, was dissipated by photochemical quenching such as increased photorespiration.