7322894041

22

were t| = time before glyphosate-based herbicide exposure and t₂= 48 hours after exposure

Then reported to % of growth compared to control treatment, i.e. without herbicide exposure:

[(p control - p sample) / (p control)] x 100    (2.2)

2.5.3 Photosynthetic capacity measurcments

Photosynthetic capacity was estimated using tlić Plant Efficiency Analyser (Handy PEA, Hansatech Instruments LTD, England), which measures chlorophyll a fluorescence of photosystem II (PSII) in d ;-adapted plant cells, following the O-J-l-P transients (Strasser, 1995). The first phase (O-J) represents the reduction of plastoquinone molecules (Qa to Q_A), occurring after the accept< e of electrons from pheophytin (Dwivedi and Bhardwaj,I995) under constant illumination. The P !evel is reached when all the plastoquinone molecules are reduced and I level is an intcrmcdiate level (Strasser, 1995).

After 48 hrs exposure to glyphosate, all samples were dark-adapted for a period of 15 minutes, prior to chloroph> a fluorescence measurement with PEA. Then, cach sample was homogenized and transferred to a 2 ml cuvette, for liquid-phase measurements, which consists in a 6 second i mination (3500 pmol photons m’² s'¹). The initial level of chlorophyll a fluorescence (F₀) is the fluorescence level at 50ps (F_o=F5_0m») when all plastoquinone molecules are open, i.e. in a non-reduced State. The maximal level (F_M) corresponds to the higher fluorescence value of the total kinetic and occur when all transporters are reduced orclosed (Maxwell and Johnson, 2000). These two values represent two diflerent energy States occurring in PSII. The following formula was uscd to calculate the photosynthetic yield:

(F_M-F₅o_|tf)/F_M    (2.3)

Photosynthetic parameters calculated from the O-J-l-P transients were measured following the energy fluxes model developed by Force et al. (2003) (table II), where O corresponds to F 5o_ms, J to F_2ms, 1 to F3o,,)5 and P to F_M. The PSII energy flux model is a simplified model of the