Influence of different nitrogen forms application on rice photosynthesis: fluorescence with water-saving irrigation in black soil region of Songnen Plain, Northeast China

In order to study the response of the rice photosynthetic-fluorescence characteristics to the application of different nitrogen forms with water-saving irrigation, by using LI-6400XT-type photosynthetic apparatus and other equipment, the fluorescence parameters, stomatal resistance and photosynthetic-CO2 response curves of rice were measured at the critical stages under water-saving irrigation methods. Results showed that the change trend of ETR and photochemical fluorescence quenching coefficient (Q(p)) with different nitrogen forms were declining-rising-declining. Compared with CK (control treatment), ETR and Q(p) with NO3- treatment were better than the others, which indicated that this treatment was most advantageous to increasing ETR. The electron flow from PSII oxidation-lateral to PSII was enhanced. The potential quantum efficiency (F-v/F-m) was the lowest at tillering stage and the highest at heading stage. Compared with CK, at heading stage, F-v/F-m with NO3-, NH4+ NO3- and NH4+ treatments was increased by 1.68, 0.61 and 1.81%, respectively, while NO3- and NH4+ played a more important role in promoting the ability to capture light. The change trend of non-photochemical fluorescence quenching coefficient with different treatments was not obvious. During the growth period, the stomatal resistance (R-s) was changed dynamically, reaching the second peak at the jointing stage and the highest peak at the milk-ripe stage, and both were higher than CK. The R-s of different nitrogen forms was as NHNO3-, which showed that with different nitrogen forms, R-s of NO3- treatment was low, stomatal opening was correspondingly greater than the other nitrogen forms, and under the same moisture conditions, this treatment of stomatal opening was more beneficial for gas exchange and external CO2 flowing into the leaf cells, which could increase photosynthetic physiological response. By fitting the parameters of photosynthetic-CO2 response curve, it was concluded that the photorespiration rate (R-P) was greater than CK, but it was different for three nitrogen treatments during different periods. Rice light saturation point and apparent carboxylation efficiency () of NO3- treatment during three growth periods were more uniform, indicating that this treatment had a higher utilization rate for low concentration of CO2. Maximum photosynthetic rate (P-max) with NO3- and NH4+ treatments of the three growth periods was 29.396-31.208 and 28.969-31.371, respectively. The CO2 compensation point and curve angle () had no stable trend during the whole growth period. Therefore, the nitrogen forms could influence the photosynthetic characteristics of the rice leaves, and the result can provide theoretical guidance and scientific basis for increasing the efficiency of nitrogen utilization.