Benzoxazolin-2(3H)-one (BOA) induced changes in leaf water relations, photosynthesis and carbon isotope...

The effects are reported here of Benzoxazolin-2(3H)-one (BOA), an allelopathic compound, on plant water relations, growth, components of chlorophyll fluorescence, and carbon isotope discrimination in lettuce (Lactuca sativa L.). Lettuce seedlings were grown in 1:1 Hoagland solution in perlite culture medium in environmentally controlled glasshouse. After 30 days, BOA was applied at concentration of 0.1, 0.5, 1.0 and 1.5 mM and distilled water (control). BOA, in the range (0.1-1.5 mM), decreased the shoot length, root length, leaf and root fresh weight. Within this concentration range, BOA significantly reduced relative water content while leaf osmotic potential remained unaltered. Stress response of lettuce was evaluated on the basis of six days of treatment with 1.5 mM BOA by analyzing several chlorophyll fluorescence parameters determined under dark-adapted and steady state conditions. There was no change in initial fluorescence (F₀) in response to BOA treatment while maximum chlorophyll fluorescence (F(m)) was significantly reduced. BOA treatment significantly reduced variable fluorescence (F(v)) on first, second, third, fourth, fifth and sixth day. Quantum efficiency of open PSII reaction centers (F(v)/F(m)) in the dark-adapted state was significantly reduced in response to BOA treatment. Quantum yield of photosystem II (ΦPSII) electron transport was significantly reduced because of decrease in the efficiency of excitation energy trapping of PSII reaction centers. Maximum fluorescence in light-adapted leaves (F'(m)) was significantly decreased but there was no change in initial fluorescence in light-adapted state (F'₀) in response to 1.5 mM BOA treatment. BOA application significantly reduced photochemical fluorescence quenching (qP) indicating that the balance between excitation rate and electron transfer rate has changed leading to a more reduced state of PSII reaction centers. Non photochemical quenching (NPQ) was also significantly reduced by BOA treatment on third, fourth and fifth day. BOA had dominant effect on C isotope ratios (δ¹³C) that was significantly less negative (-26.93) at 1.0 mM concentration as compared to control (-27.61). Carbon isotope discrimination (Δ¹³C) values were significantly less (19.45) as compared to control (20.17) at 1.0 mM. BOA also affect ratio of intercellular to air CO₂ concentration (ci/ca) that was significantly less (0.66) as compared to control (0.69) when treated with 1.0 mM BOA. Protein content of lettuce leaf tissue decreased under BOA treatment at 1.5 mM concentration as compared to control.