PubMed2004
Amomum villosum Lour. (Zingeraceae) is a perennial herb that occurs in the understory of tropical and subtropical forests, and is an important medicinal plant. A. villosum, native to Guangdong province, was introduced intentionally to Xishuangbanna, Yunnan province in 1963, and was planted under tropical rainforest. The income from planting A.villosum in rainforest is very important for minority in Xishuangbanna. But now A. villosum fruit yield has decreased greatly due to plant senescence. The senescence mechanism of A. villosum is not known clearly. A. villosum has only one main stem without branch. The leaf age can be estimated by its position at the stem. In this study we measured the variables of photosynthesis and chlorophyll fluorescence, the content of chlorophyll (Chl), Caroteniod, protein and malondialdehyde (MDA), and the activities of antioxidant enzymes of leaves at different positions in A.villosum. We want to know (1) the reasons of leaf photosynthesis decreasing during aging and senescence, and (2) the relationships between oxidative stress and aging/senescence. Leaf age, maximum net photosynthetic rates (P(max)), Chl and soluble protein content increased with the increase of leaf position in A. villosum. P(max) was biggest at the third leaf, while Chl and protein content reached its maximum values at the fifth leaf. They decreased at the 7th leaf, and began to decrease sharply at 9th leaf. MDA content was lower in the first to 7th leaves, and increased greatly at 9th leaf. AQY and F(v)/F(m) began to decrease at 9th leaf too. The results presented above suggested that the third to 5th leaves were mature leaves with vigorous physiological function, the 7th leaf was aging one, the 9th leaf began senescent, the 11th to 15th leaves were senescent. The decrease of Chl and protein content, and stomatal conductance might be the important reason of P(max) decreasing in aging and senescent leaves of A. villosum. NPQ, AQY, F(v)/F(m), Phi(PSII) and q(P) decreased with leaf aging and senescence, which indicated that thermal dissipation decreased, and photoinhibition of photosynthesis intensified. Furthermore, photodamage occurred at the late stage of senescence. But the reducing extent of AQY, F(v)/F(m), Phi(PSII) and q(P) was smaller than that of P(max), indicating that the electrons transported by PSII was more than those used by carbon assimilation. The excessive electron might induce production of reactive oxygen species (ROS). The excessive electron and then ROS were smaller in aging leaf than in senescent leaf. The ROS could be scavenged effectively by antioxidant enzymes and antioxidants in aging leaf, but not in senescent leaf, although the activities of antioxidant enzymes increased significantly. The ROS could results in membrane peroxidation, so MDA content increased, which could intensify leaf senescence further. The results above indicated that aging was not associated with oxidative stress, but senescence was in A. villosum.