Photosynthesis and non-photochemical excitation quenching components of chlorophyll excitation in maize and field bean during chilling at different photon flux density

The influence of chilling (8 degrees C, 5 d) at two photon flux densities [PFD, L = 200 and H = 500 mu mol(photon) m(-2) s(-1)] on the gas exchange and chlorophyll fluorescence was investigated in chilling-tolerant and chilling-sensitive maize hybrids (Zea mays L., K383xK130, K185xK217) and one cultivar of field bean (Vicia faba L. minor, cv. Nadwislanski). The net photosynthetic rate (P-N) for the both studied plant species was inhibited at 8 T. PN of both maize hybrids additionally decreased during chilling. Changes in the quantum efficiency of PS2 electron transport (phi(PS2)) as a response to chilling and PFD were similar to PN. Measurements of phi(PS2)/phi(CO2) ratio showed that in field bean seedlings strong alternative photochemical sinks of energy did not appear during chilling. However, the high increment in phi(PS2)/phi(CO2) for maize hybrids can indicate reactions associated with chill damage generation. At 8 degrees C the non-photochemical quenching (NPQ) increased in all plants with chilling duration and PFD. The appearance of protective (q(1,p)) and damage (q(1,d)) components of q(1) and a decrease in q(E) (energy dependent quenching) took place. NPQ components of field bean and maize hybrids differed from each other. The amount of protective NPQ (q(E) + q(1,p)) components as part of total NPQ was higher in field bean than in maize hybrids at both PFD. On 5(th) day of chilling, the sum of q(E) and q(1,p) was 26.7 % of NPQ in tolerant maize hybrids and 17.6 % of NPQ in the sensitive one (averages for both PFD). The increased PFD inhibited the ability of all plants to perform protective dissipation of absorbed energy. The understanding of the genotypic variation of NPQ components in maize may have implications for the future selection of plants with a high chilling tolerance.