Objective]This paper investigated the differences in photosynthesis characteristics and photo-protection mechanisms between Torreya grandis cv.‘Merrilli’and T. grandis by comparing the characteristics of gas exchange,leaf nitrogen content and chlorophyll fluorescence of the flat young leaves ( young leaves ) and fully grown leaves ( mature leaves) of two species. [Method]We measured the light response curve,photosynthetic pigment content,leaf nitrogen content and chlorophyll fluorescence parameters of young leaves (10 days after budding) and mature leaves (50 days after budding) of T. grandis‘Merrilli’and T. grandis (15 years old) in a natural forest. [Result]The,young leaves of T. grandis‘Merrilli’had significant lower Amax,LSP and higher SLW than that of the young leaves of T. grandis,and the relative difference value was 40. 3%,33. 1% and 29. 7%,respectively (P≤0. 05). However,the mature leaves of T. grandis‘Merrilli’had significantly higher Amax,LSP and SLW than those of T. grandis,and the relative difference value was 26. 3%,40. 9% and 44. 8%,respectively (P≤0. 05). Furthermore,T. grandis cv. ‘Merrilli’had significantly higher leaf nitrogen content than those of T. grandis,and the content was about 32. 5% and 44. 9% higher in young and mature leaf,respectively (P≤0. 05). However,the PNUE in T. grandis cv. ‘Merrilli’were about 54. 9% and 12. 8%lower in young and mature leaf compared to those of T. grandis,respectively. The Pn and Gs in young or mature leaves both of T. grandis and T. grandis cv.‘Merrilli’measured at midday ( at 13:00 pm) were all distinctly lower than those in the morning (at 9:00 am),the relative difference value of Pn was 33. 7%,26. 8%,35. 1% and 44. 4% (P≤0. 05), and the relative difference value of Gs was 27. 7%,23. 5%,38. 7%,45. 0% (P≤0. 05),respectively. No significant changes in Ci(P≥0. 05) were observed. The young leaves of T. grandis cv. ‘Merrilli’had significant higher quantum yield of △pH-and xanthophyll-regulated thermal dissipation (Y(NPQ),),and Car/Chl and Chl a/b ratios (P≤0. 05). The mature leaves of T. grandis cv.‘Merrilli’had significant higher quantum yield of fluorescence and light-independent constitution thermal dissipation (Y(NO)) than T. grandis (P≤0. 05). [Conclusion]The results indicate that young leaves of T. grandis cv. ‘Merrilli’( with lower Amax and LSP compared to T. grandis) mainly dissipate the excess energy through increasing Y(NPQ) and Car,and decreasing Chlb content under high irradiance (at 13:00 pm). The mature leaf of T. grandis cv.‘Merrilli’( with higher Amax and LSP) also needs to increase the Y( NO) to dissipated the excess energy. Significant lower PNUE in leaves of T. grandis cv.‘Merrilli’reminds us to pay more attention to the use of nitrogen fertilizer in future.%【目的】通过比较分析2个发育阶段的香榧和榧树叶片的气体交换参数、叶片氮含量及叶绿素荧光参数的差异，探讨香榧与榧树叶片光合特性及其光保护机制。【方法】以天然林中15年香榧和榧树的幼叶(即萌芽后10天)和成熟叶(即萌芽后50天)为研究对象，通过测定2个发育阶段香榧、榧树叶片的光响应曲线、光合色素含量、叶氮含量、比叶重(SLW)、叶绿素荧光参数的变化。【结果】1)香榧幼叶较榧树幼叶具有较低的最大净光合速率(Amax)、光饱和点(LSP)和较高的 SLW，相对差值分别为40.3%，33.1%和29.7%(P≤0.05)，而香榧成熟叶较榧树成熟叶具有较高的 Amax，LSP和 SLW，相对差值分别为26.3%，40.9%和44.8%(P≤0.05)。2)香榧幼叶和成熟叶较榧树叶片具有较高的氮含量，相对差值分别为32.5%和44.9%( P≤0.05)，却具有较低的光合氮素利用效率(PNUE)，相对差值分别为54.9%和12.8%(P≤0.05)。3)与9：00相比，13：00时(较高光强)香榧和榧树幼叶、成熟叶片的光合速率(Pn)和气孔导度(Gs)均显著降低，Pn 的相对差值分别为33.7%，26.8%，35.1%和44.4%(P≤0.05)，Gs 的相对差值分别为27.7%，23.5%，38.7%和45.0%，但胞间 CO2浓度(Ci)无显著变化(P≥0.05)。4)香榧幼叶较榧树幼叶具有较低的实际光化学量子效率(Y(II))(P≤0.05)，及较高的△pH 和叶黄素调节的热耗散量子效率(Y(NPQ))和类胡萝卜素/总叶绿素含量比值(Car/Chl)和叶绿素 a/叶绿素 b 含量比值(Chla/Chlb)(P≤0.05)；香榧成熟叶较榧树成熟叶具有较高的荧光和不依赖于光的非调节性热耗散量子效率(Y(NO))(P≤0.05)。【结论】具有较低 Amax和 LSP的香榧幼叶，在相同的高光强下较易发生光抑制，但较高光强下(13：00时)其光抑制程度小，主要是由于其能有效地通过增加 Y(NPQ)、Car的形式进行过剩光能的热耗散及通过降低 Chlb含量来减少光能的捕获；具有较高的 Amax和 LSP的香榧成熟叶还需通过 Y( NO)进行过剩光能的耗散。香榧叶片较榧树叶片具有较高的氮含量和较低的光合氮素利用效率( PNUE)，说明分配到非光合组分的氮素较多，因此香榧在生产中要注意氮肥的施用。
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