Change In Photosynthetic Rate Of Lucerne Residue And Its Mechanism Responding To Cutting Under Different Water Treatments

The regrowth after cutting(defoliating) is essential to the utilization of forage and grassland.After forage cutting,the residue has to change its original function to adapt new situation.Some clue showed that photosynthetic rate(Pn) of the aboveground part would increase after cutting and would even keep higher for a longer period.That might be resulted from the change of light illumination and so on. However,Pn change shortly after cutting was still not clear and the mechanisms under cutting stimulus awaited further exploitation.In this study,Lucerne variety(Medicago sativa cv.Xinjiangdaye) was chosen as the experimental material.One group of materials was kept 70%field water capacity (FWC) and another was 35%FWC.On a clear day,plant Pn and stomatal conductance (gs) were measured every two hours using LI-6400 after forage cutting.The content of leaf chlorophyll a and b,malondialchehyche(MDA),solubale sugar(SS),proline and related antioxidases were also measured 5,10 and 30 h after cutting to investigate the mechanisms of Pn change after forage cutting.Higher Pn was observed in the residue after cutting,especially 5 h after cutting. Greater gs was also measured under cutting treatment.It suggested that gs elevation promises enough CO₂ supply for Pn increase after cutting.Leaf chlorophyll a and b contents and some fluorescence parameters decreased after cutting while a/b ratio increased,indicating that non-stomatal control also play roles in the regulation of Pn after cutting.Furthermore,after cutting leaf MDA content decreased,which showed less damage to cell membrane.Though leaf SS content decreased,leaf proline content increased,promising the maintanence of cell osmotic potential.This maintanence benefits the normal or even enhanced cell function,i.e.the functions of guard cell and the chloroplast within cells.The activities of SOD(superoxidase),POD(peroxidase) and CAT(catalase) were greater under cutting than without cutting 5 h after cutting. The greater activity of antioxidases is advantageous for cells to eliminate the reactive oxygen species,protecting them from membrane injury.In conclusion,greater gs promised enough CO₂ supply,thus resulting in higher Pn.The enhanced antioxidases would efficiently clean out free radicals resulting from cutting stimulus and more proline would help to maintain cell osmotic potential,thus maintain cell normal function,including guard cell/stomatal function.Additionally, more water supplies(i.e.70%FWC) could keep plant more sensitive and efficient in response to cutting stimulus.CAT was found very significant responding to cutting, suggesting that it may play a key role in this stimulus-response signaling.Further investigation concerning regrowing mechanisms is now conducted.