Chlorophyll fluorescence emission, pigment composition and photosynthetic rate of shade-grown cotton (Gossypium hirsutum L.) plants were measured immediately after suddenly exposing to full sunlight and at regular intervals there after within 15 d. Photoinhibition occurred in shade-grown cotton leaves immediately after exposed to full sunlight. The chlorophyll fluorescence parameter Fv / Fm and ΦPSⅡ , which reflect the efficiency of PSⅡ, obviously decreased in shade-grown leaves, much lower than that of the full sunlight-grown leaves. On the contrary, Fo value was sharply increased. Neither of these parameters could completely recover till next morning. The photoinhibition was chronic and continued for about 4 d, while the Fv / Fm and the net photosynthetic rate ( Pn ) continued to decline, then began to increase gradually 6 d later and turned stable after 10-12 d, appearing as an acclimation phenomenon. However, the final value of Fv / Fm and Pn did not reach the level as in those leaves grown in the full sunlight ever before. The final Pn was higher by 60% than that before exposure, but lower for more than 40% than that of the full sunlight-grown leaves. The most notable response of chloroplast pigment composition was a pronounced increase in the pool size of carotenoids in xanthophyll cycle over a period of 3 d. The results indicated that when shade-grown cotton seedlings were suddenly transferred to the full sunlight, the decline of Fv / Fm and Pn might associate with the damage of the PSⅡ reaction center. During the light acclimation, photoprotective mechanisms such as the xanthophyll cycle-dependent energy dissipation were increased, so that photodamage in leaves transferred from low to high light might be reduced.
杨兴洪1,2 邹琦1* 王玮1
摘要：研究了遮荫棉花(Gossypium hirsutum L.)突然由遮荫条件暴露在自然强光下时,叶绿素荧光发射、叶绿体色素组成、净光合速率(Pn)等在光照转换当天以及随后的适应过程中(光照转换后15 d内)的变化.遮荫棉花突然转到强光下,叶片发生了严重的光合作用光抑制,叶绿素荧光参数Fv/Fm和ΦPSⅡ急剧降低,且明显低于自然光照下生长的叶片,而Fo值却明显升高.这些参数即使在光照转换的次日清晨也不能完全恢复.Fv/Fm和Pn在光照转换以后的4 d内持续降低,在第6天以后开始逐渐升高,在10～12 d达到稳定值,表现出遮荫棉花叶片对光强变化的一定适应性,但Fv/Fm和Pn均未达到自然光照条件下生长的棉花叶片的相应值.最后的Pn值较遮荫下叶片增加60%,但同自然光照下生长的叶片相比只有后者的40%.试验结果还表明,光照转换以后叶片内叶黄素循环库逐渐增大,在较短的时间内(3 d)即可达到较高的水平.遮荫棉花突然转到自然强光下,叶片Fv/Fm及Pn的降低与PSⅡ反应中心的破坏有关,在对强光的适应过程中依赖叶黄素循环的热耗散等保护机制增强.光保护机制的逐渐完善有助于减轻叶片由遮荫转到强光下遭受的光破坏.