| Gas exchange, chlorophyll (Chl) fluorescence, photosynthetic pigments, soluble proteins, Rubisco and antioxidant systems were characterized from the fully expanded 6th leaves in rice seedlings grown with either complete (CK) or nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period.As compared to that in control, total Chl content in N-deficient plants tended to decrease from the 5th d on, the trend of loss of Chl was accentuated as the treatment lasted. The speed of decline in total Chl was faster in the N-stressed plants. Consequently, the ratio of Car/Chi began to rise significantly on the 5th d. The soluble protein content, which was acutely reduced by N deficiency on the 5th d, exhibited the same pattern as Chl, while it declined slightly in the control plants. Rubisco content in the N-deficient plants was exceptionally decreased by 70 % on the 15th d. Amax, Gs and E in the N-deficient plants decreased by 32 %, 42 % and 17 % respectively on the 5th d in comparison to the control. As nitrogen deficiency progressed, the photosynthetic parameters mentioned above in the N-deficient plants decreased more dramatically. However, Ci in the N-deficient plants increased by 10 % on the average every 5 d. Analysis of light-response curves exhibited that the speed of decline in LCP was much faster than that in the control. Likewise, there was a rapid decline in the AQY in N-deficient plants and it was 28% as high as that in the control on the 20th day, whereas it remained rather stable in the control plants. The A-Ci curve illustrated that carboxylation efficiency in the N-stressed plants was significantly decreased. Analysis of A-Ci curves showed that the CO2 compensation point in the nitrogen deficient plants was also reduced by 25 %. Besides, the light saturation point was also significantly diminished on the 20th d, which could be seen from the series of light-response curves.The results showed that FO, Fm, Fv/Fm and Fv7Fm' were not affected on the 5th d significantly, whereas PS was reduced significantly as compared to that in the control. Meanwhile, both q and ETR decreased significantly. There was a correspondingly higher qN in the N-stressed plants than that in the control plants. It corresponded with the results that the ratio of Car/Chi rose markedlyin the N-deficient plants compared to that in the control one. Besides, PS II excitation pressure (1-qp) also increased significantly (P<0.01) at the same time and remained rather stable; Meanwhile, the Ex also reached the peak in N-deficiency plants. Not until on the 15th d did Fm as well as Fv/Fm and Fv7Fm' exhibit significant decrease in the N-stressed plants in comparison to those in control. As nitrogen deficiency continued, Fv/Fm, Fv7Fm', qP, and ETR in the N-deficient plants lowered more rapidly as compared to that in control. Interestingly, not until on the 20th d did F0 in the N-stressed plants decline considerably compared to that in control.On a FW basis, the activities of SOD declined in N-deficient plants, while the activities of POD and CAT increased initially. With nitrogen deficiency progressing, the activities of the three key antioxidant enzymes concertedly fell. Accordingly, the lowered capacity to scavenge the free radicals compounded the peroxidation of biomembrane, which could be seen in over-accumulation of MDA in the N-deficient plants. However, on a soluble protein basis, the activities of SOD as well as CAT and POD increased strikingly than that in the control. The activities of three antioxidant enzymes reached the peak on the 15th d and then fell quickly.Analyses of linear regression showed that there exists a significant correlation between excitation pressure and Rubisco and soluble protein. Our results showed that there was a lag phase of the increase in the PS II excitation pressure compared to the decrease of Rubisco content. Therefore, it is very likely that the increased excitation pressure is the result of a saturation of the electron transport chain due to the limitati...
|
PDF Full Text Request