[Effects of NO3- stress on photosynthetic rate, photochemical efficiency of PS II and light energy allocation in cucumber seedling leaves].

This paper studied the effects of different NO3- concentration on the photosynthetic rate, photochemical efficiency, and absorbed light energy allocation in cucumber seedling leaves. The results indicated that when the available NO3- concentration in the medium was low (14-98 mmol NO3- x L(-1)), an appropriate supplement of NO3- could enhance the capability of cucumber leaves in capturing light energy, and promote the photosynthesis. However, with further increase of NO3-, the photochemical efficiency of PS II decreased, electron transfer restrained, and net photosynthetic rate as well as the absorbed light energy used in photochemical reaction of PS II decreased. At the same time, the light energy used in antenna heat dissipation increased, while the photochemical efficiency decreased. After treated with 140 and 182 mmol NO3- x L(-1) for 6 days, the photosynthetic rate (P(n)) was decreased by 35% and 78%, respectively, maximal PS II efficiency at open centers in the absence of NPQ (F(v)/F(m)), antenna efficiency at open centers in the presence of NPQ (F(v)'/F(m)'), actual PS II efficiency (phi (PSII ) and photochemical quenching (q(P)) were lower, non-photochemical quenching (NPQ) was higher, and the deviation from full balance between PS I and PS II (beta/alpha - 1) was improved significantly, compared with the control. The fluctuant ranges of these chlorophyll fluorescence parameters were increased at higher NO3- concentration, compared with those at lower NO3- concentration. The absorbed light energy allocated to the photochemical reaction of PS II (P) was reduced by high light intensity and high NO3- concentration. Meanwhile, the proportion allocated in antenna heat dissipation (D) increased significantly. Antenna heat dissipation was the main way for excessive energy dissipation.