In agricultural systems, abiotic stresses, such as high light, high temperature and drought, are responsible for most of the reduction that differentiates yield potential from harvestable yield. As one of important oil crops in China, and even in the world, the high and stable yield of peanut (Arachis hypogaea L.) is very important to guarantee the food safety. Previous studies most focused on peanut yield, root activity, leaf area, net photosynthetic rate, chlorophyll content, activity of some enzymes, e.g. superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and so on. However, effects of drought and high temperature stresses on damaging mechanisms of peanut photosystems have been seldom mentioned, and both coordination between photosytem Ⅱ (PSⅡ) and photosytem Ⅰ (PSⅠ) and response of electron transfer components to two kinds of stresses were unclear. In the present work, we try to investigate the effects of high temperature and drought stresses on PSⅡ and PSⅠ activity and coordination between PSⅡ and PSⅠ, and further to provide the theoretical basis to peanut cultivation.
In the experiments, Luhua 14 was used as materials to investigate the responding mechanisms of peanut photosystems to high temperature and drought stresses. Detached leaves were exposed to high temperature (42 ℃) under high irradiance (1200 μmol · m^-2 · s^-1) (HH), drought (PEG-6000, 30%) under high irradiance (1200 μmol · m^-2 · s^-1) (DH), and high irradiance (1200 μmol · m^-2 · s^-1) (NH), respectively, and non-treatment leaves were as controls (CK). Relative to CK and NH, the maximal efficiency of PSⅡ (Fv/Fm) and absorbance at 820 nm decreased greatly in peanut leaves under HH and DH stresses, accompanied by the increase of relative variable fluorescence at the J-step (Vj), the obvious decrease of absorption flux per excited CS (t=m) (ABS/CSm), trapped energy flux per CS (t=m) (TRo/CSm) and active RCs per CS (t=m) (RC/CSm) calculated from the chlorophyll fluorescence transient curve. The obvious increase of 1-qP and the xanthophyll cycle-dependent energy dissipation (NPQ) were also detected in peanut leaves under HH and DH stresses. Additionally, the activity of SOD decreased in peanut leaves under HH and DH stresses, accompanied by the increase of malonaldehyde (MDA) and permeability of plasma membrane. These results showed that severe photoinhibition of PSⅡ and PSⅠ in peanut leaves was induced by HH and DH stresses. However, K-step was not induced in the rapid chlorophyll induction curve, which implied that peanut oxygen evolving complex (OEC) was not sensitive to high temperature and drought stresses, and donor side of PSⅡ reaction centers was more sensitive to high temperature and drought stresses relatively. The main factor caused the damage to peanut photosystems might be the accumulation of reactive oxygen species (ROS) induced by excess energy. First, the xanthophyll cycle could only dissipate part of excess energy; second, the water-water cycle could not dissipated energy efficiently under the stresses of HH and DH, which caused the accumulation of ROS greatly. HH and DH stresses had similar damaging effects on peanut photosystems, except that DH had more severe effect. Additionally, effects of HH and DH stresses on damaging site and damaging mechanisms were similar in peanut leaves.