Characterization of photosynthesis of flag leaves in a wheat hybrid and its parents grown under field conditions.

Two wheat cultivars, one with high yield and the other with a high tolerance against oxidative stress, were compared with a hybrid of these two cultivars by investigating their photosynthetic characteristics of flag leaves. From the beginning of flowering to the 17th day, CO(2) assimilation rate (P(max)) was maintained and there were no appreciable differences between the hybrid and its parents. P(max) showed no decrease at noon compared to that in the morning. From the 20th to the 30th day of flowering, P(max) decreased significantly, and this decrease was significantly less in the hybrid than in its parents. The actual photosystem II (PSII) efficiency (Phi(PSII)) and the maximal efficiency of PSII photochemistry (F(v)/F(m)) showed a significant decrease only on the 30th day after anthesis; this decline was much less marked in the hybrid relative to its parents, both in the morning and at noon. A decrease occurred in Phi(PSII) and F(v)/F(m) at noon when compared to that in the morning, but this decrease was less marked in the hybrid than in its parents. Rubisco activity decreased significantly from the 13th day of flowering and was higher in the morning than at noon both in the hybrid and its parents. However, the hybrid always showed a higher value of Rubisco activity. The activities of posphoenolpyruvate carboxylase and pyruvate phosphate dikinase showed similar changes to those in Rubisco activity, particularly from the 20th to 30th day. The results of this study suggest that the higher photosynthetic capacity of the flag leaf in the hybrid can help to accumulate more dry material, and may be the physiological basis for higher yield over its parents.

Tolerance of photosynthesis to photoinhibition, high temperature and drought stress in flag leaves of wheat: A comparison between a hybridization line and its parents grown under field conditions.

Photosynthesis and its tolerance to photoinhibition, high temperature and drought stress of flag leaves were investigated in a wheat (Triticum aestivum L.) hybridization line (1-12) and its parents (Jing-411 and Xiaoyan-54). From the beginning of flowering to the 10th day, light-saturated CO(2) assimilation rate (P(max)) showed no significant decrease and P(max) of 1-12 was comparable to that of its parents. From the 20th day, P(max) decreased significantly and this decrease was much less in 1-12 than in its parents, whereas no decrease in chlorophyll content was observed in 1-12 and its parents on the 20th day, indicating that photo-oxidative damage occurred in 1-12 and its parents but 1-12 is more resistant to photo-oxidative damage than its parents. To further characterize photo-oxidative damage, tolerance to photoinhibition, high temperature and drought stress was compared in 1-12 and its parents. When exposed to high light (1400µmolm(-2)s(-1)), the maximal efficiency of PSII photochemistry (F(v)/F(m)) decreased significantly with increasing exposure time and such a decrease was much less in 1-12 than in its parents. When exposed to higher temperatures (30-45°C) for 15min, F(v)/F(m) started to decrease at 42°C in 1-12 and its parents. The greatest decrease in F(v)/F(m) was observed in Jing-411. 1-12 and Xiaoyan-54 showed a comparable decrease in F(v)/F(m). Similar results were also observed in the actual PSII efficiency (Φ(PSII)), photochemical quenching (q(P)) and non-photochemical quenching (q(N)). During exposure of detached leaves to air under room temperature conditions, relative water content decreased with increasing exposure time. However, such a decrease was greatest in Jing-411 followed by Xiaoyan-54, and 1-12. There were neither changes in F(v)/F(m) nor q(N) during water loss in the line and its parents. However, there was a decrease in Φ(PSII) and q(P) and greatest decrease was observed in Jing-411 followed by Xiaoyan-54, and 1-12. Also, the decrease in P(max) was greatest in Jing-411, followed by Xiaoyan-54, and 1-12 during water loss. In addition, the activities of ribulose-1,5-bisphosphate carboxylase, phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, NADP-malate dehydrogenase and NADP-malate enzyme were significantly higher in 1-12 than in its parents. The results in this study suggest that high resistance to photo-oxidative damage of the flag leaves in 1-12 may be the physiological basis for its high yield when grown in north China. Our results also suggest that parents can be selected for improved biochemical and physiological traits and crossed to high yielding agronomically elite materials which can be selected for higher performance in yield.

Physiological characteristics of the primitive CO(2) concentrating mechanism in PEPC transgenic rice.

The relationship between carbon assimilation and high-level expression of the maize PEPC in PEPC transgenic rice was studied by comparison to that in the untransformed rice,japonica kitaake. Stomatal conductance and photosynthetic rates in PEPC transgenic rice were higher than those of untransformed rice, but the increase of stomatal conductance had no statistical correlation with that of photosynthetic rate. Under high levels of light intensity, the protein contents of PEPC and CA were increased significantly. Therefore the photosynthetic capacity was increased greatly (50%) with atmospheric CO(2) supply. While CO(2) release in leaf was reduced and the compensation point was lowered correspondingly under CO(2) free conditions. Treatment of the rice with the PEPC-specific inhibitor DCDP showed that overexpression of PEPC and enhancement of carbon assimilation were related to the stability of Fv/Fm. Labeling with(14)CO(2) for 20 s showed more(14)C was distributed to C(4) primary photosynthate asperate in PEPC transgenic rice, suggesting that there exists a limiting C(4) photosynthetic mechanism in leaves. These results suggest that the primitive CO(2) concentrating mechanism found in rice could be reproduced through metabolic engineering, and shed light on the physiological basis for transgenic breeding with high photosynthetic efficiency.

Characterization of photosynthesis, photoinhibition and the activities of C(4) pathway enzymes in a superhigh-yield rice, Liangyoupeijiu.

Characteristics of photosynthetic gas exchange, photoinhibition and C(4) pathway enzyme activities in both flag leaves and lemma were compared between a superhigh-yield rice (Oryza sativa L.) hybrid, Liangyoupeijiu and a traditional rice hybrid, Shanyou63. Liangyoupeijiu had a similar light saturated assimilation rate (Asat) to Shanyou63, but a much higher apparent quantum yield (AQY), carboxylation efficiency (CE) and quantum yield of CO(2) fixation (PhiCO(2)). Liangyoupeijiu also showed a higher resistance to photoinhibition and higher non-radiative energy dissipation associated with the xanthophyll cycle than Shanyou63 when subjected to strong light. In addition, Liangyoupeijiu had higher activities of the C(4) pathway enzymes in both flag leaves and lemmas than Shanyou63. These results indicate that higher light and CO(2) use efficiency, higher resistance to photoinhibition and C(4) pathway in both flag leaf and lemma may contribute to the higher yield of the superhigh-yield rice hybrid, Liangyoupeijiu.