Ecosystem carbon fluxes of a ryegrass and clover fodder crop in a Mediterranean environment

A mixture of ryegrass (Lolium italicum A. Braun) and clover (Trifolium alexandrinum L.) was sown in Eboli (Salerno, Southern Italy) in September 2007. Crop growth, leaf and canopy gas exchange and ecophysiological traits were monitored throughout the growth cycle. The gross primary production (GPP) was not affected by air temperature (Tair); on the contrary the ecosystem respiration (Reco) decreased as Tair decreased while net ecosystem CO2 exchange (NEE) increased. When was normalized with leaf area index (LAI), GPP decreased with Tair, a likely response to cold that down-regulated canopy photosynthesis in order to optimize the light use at low winter temperatures. Net photosynthetic rates (PN), the effective quantum yield of PSII (?PSII) and photosynthetic pigment content were higher in clover than ryegrass, in relation to the higher leaf N content. The lower ?PSII in ryegrass was linked to lower photochemical quenching coefficient (qP) values, due to a reduced number of reaction centres, in agreement with the lowest Chl a content. This behaviour can be considered as an adaptive strategy to cold to avoid photooxidative damage at low temperature rather than an impairment of PSII complexes.