Abstract

Wei Li, Yu-Sen Zhao and Zhi-Qiang Zhou

The differences in chloroplast pigments, gas exchange and photosystemII (PSII) photochemistry as well as xanthophyll in seeding and sapling leaves of Taxus cuspidata grown in full sunlight were examined. Compared with the sapling leaves, the chlorophyll content, photosynthetic capacity and light intensity for saturation of photosynthesis were lower in seeding leaves. The response curves of PSII photochemistry demonstrated that both seeding and sapling leaves occurred a down-regulation of PSII photochemistry at high irradiance, more serious down-regulation being examined in seeding leaves. And the down-regulation of PSII photochemistry occurred significantly when measured at midday, indicating that photoinhibition occurred heavily in seeding leaves when exposed to high light. The actual PSII efficiency (ΦPSII) and the efficiency of excitation capture by open PSII centers drastically decreased with the increase of non-photochemical quenching (NPQ) at midday. The photorespiration rate in seeding leaves was lower than that in sapling leaves under high irradiance. The results indicated that the xanthophlly cycle was activated in both the seeding and sapling leaves at midday and an increase of de-epoxidation were observed, but a little higher level of de-epoxidation was measured in seeding leaves. The xanthophyll cycle may play an important role in the dissipation of excess light energy associated with NPQ to avoid photodamage. Our results suggested that photoinhibition occurred in seeding leaves significantly due to lower capacity of CO2 assimilation, photorespiration and the light intensity for saturation of photosynthesis, as well as the lower PSII photochemistry at high irradiance; therefore the T. cuspidata seeding could not adapt to growing at high irradiance.