Studies On Photosynthetic Characteristics Of Pak-choi (Brassica Campestris Ssp. Chinensis Var. Communis) Under Light Stress And Its Adaptation Mechanisms

Pak-choi(Brassica campestris ssp. chinensis var. communis)is subject to intense light stress, high temperature, heavy rain and insect threat in summer culture, so such cultural methods as bestrow and tridimensional are applied to keep from the stresses. In the changjiang river area, pak-choi is a main vegetable consumed in cities. But because of its intolerance of storage and transport, it is produced in suburb leaf vegetable base, and industrialized stereo culture becomes a development trend. These culture methods lead to an unstable irradiation environment, so it is necessary to sift out the shading tolerant varieties and the photooxidation tolerant varieties, and important to research the adaptation mechanisms. In this paper the photosynthetic characteristics of the pak-choi were studied, the photooxidation tolerance ability and the shading tolerance ability of regular cultivars were identified and classified, and the adaptation mechanism of the pak-choi to weak light was discussed from its photosynthetic characteristics, chloroplast ultrastructure, antioxidant enzymes metabolism and other aspects. The optimal ratio of NH₄+/NO₃- for the growth of the pak-choi under weak light was found through the research about the effects of weak light to the nitrogen metabolism ofpak-choi.The photosynthetic characteristics of pak-choi were studied. The results showed that, compared to other vegetables, the net photosynthetic rate (Pn) and light saturation point (LSP) of the Pak-choi were higher, while its light compensation point (LCP) was lower. As for the variety, 'Zhengda Kangre Qing NO.3', its LSP reached 1910.3μmol.m^-2·s^-1, and at this light its Pn was 20.2 CO₂μmol.m^-2·s^-1, the LCP was 30μtmol·m^-2·s^-1. The results also indicated that the pak-choi accumulated photosynthetic product in a large radiation range, it has strong photosynthetic capacity. The diurnal course of Pn presented two peaks and an obvious 'midday depression' phenomenon occurred.Thirty cultivars of pak-choi were treated with low light intensity and classified with the cluster analysis by three indices: change of dry weight, change of leaf thickness and change of Chl.a/b. The thirty cultivars were divided into three basic types, tolerant, middle-tolerant and susceptible. Among these cultivars, nine were susceptible, fifteen were middle-tolerant, and six were tolerant. The results showed that the correlation of three indices were significant. Comparatively, the cultivars 'Shulv', 'Zhengda Kangre Qing NO.3','Reyou NO.2', 'Kangre NO.605', 'Aiwang'and 'Huawang'were more tolerant.This paper studied the response of growth, reactive oxygen species metabolism and antioxidative enzyme activities of two different pak-choi cultivars ('Aiwang'and 'Lvyou') to weak light under weak light treatment and regular light recovery. The results show that under weak light treatment the growth of 'Aiwang'was retarded and the growth of 'Lvyou'was complete inhibited. The content of MDA, the O₂(?) produce rate and the content of H₂O₂ in two cultivars decreased at first and increased later. The POD activities increased under weak light treatment while the SOD, CAT and APX decreased firstly and increased later. The activities of all theses enzymes increased remarkedly after the recovery of growth light. The results suggested that, under weak light treatment and regular light recovery, 'Aiwang' was more efficient on the enhancement of activities of antioxidative enzymes to reduce the membrane lipid peroxidation, while 'Lvyou' was subject to a more grave membrane lipid peroxidation.Thirty cultivars of pak-choi were divided into five grades according to their photooxidation tolerance by the residual content of total chlorophylls under artificial photooxidation condition, whereas the tolerant ones included 'Qingyou NO.4', 'Shulv', 'Kangre NO.605', 'Lvyang Qing', 'Aikang NO.6'and 'Huawang', and the susceptible ones were 'Lvxing'and 'Aijiao Huang'. In order to study the characteristics of reactive oxygen metabolism between pak-choicultivars with different photooxidation tolerance, 'Kangre NO.605'and 'Lvxing'were used to compare the differences in the expriment. The results showed that the activities of antioxidant enzymes including superoxide dismutase (SOD), peroxidase (POD) and ascorbate peroxidase (APX) in the two cultivars were induced at the beginning of photooxidation stress, and then decreased in the sensitive 'Lvxing', but that of the tolerant 'Kangre 605'continued to keep at a high level. Additionally, the O₂(?) production rate and the content of H₂O₂ and malondialdehyde (MDA) in 'Lvxing'were higher than that in 'Kangre 605', which might be the cause leading to a lower level of chlorophylls. The results suggested that the intense antioxidant enzyme activities play an important role in 'Kangre 605'against photooxidation stressThe study on the effects of weak light (200～240μmol·m^-22·s^-1) on photosynthetic characteristics and ultrastructure of Chloroplast of two different pak-choi cultivars('Aiwang'and 'Lvyou')showed that under weak light, the chll.(a+b) and chll.b contents of 'Aiwang'increased obviously while 'Lvyou' increased inapparently. The net photosynthetic rate (Pn), light compensation point(LCP), light saturation point(LSP), apparent quantum yeild(AQY) and carboxytation efficiency(CE) of the two cultivars 'Lvyou' and 'Aiwang' were declined. Except LCP, the slope ratio of 'Aiwang's curve is lower. Fluorescence parameters show that grown under weak light, the PSâ…¡of 'Lvyou' was injured. However, 'Aiwang' was subjected to weak light by stable F_v/F_m, ETR,ΦPSâ…¡and qP. After treatment, cracks and foveolas appeared in the storma of 'Lvyou', most of the storma lameller fractured, the grana lameller disaggregated, and the mitochondria were nearly empty. Contrarily the chloroplast's structure of 'Aiwang' was flawless and the mitochondria growth was normal. The results suggest that shading led to damages of PSâ…¡, but different vatieties show different sensitivity.The Photosynthetic capacity, lamina mass per unit area, plastochron index, leaves nitrogen content, leaves nitrogen partitioning among different photosynthetic apparatus of two pak-choi cultivars('Aiwang' and 'Lvyou') were studied. 'Aiwang' showed strong resistance to shading whereas "Lvyou' was susceptible. The results showed that, the leaves nitrogen content per unit leaf area (N_A) of 2 cultivars were increased with the light intensitiy increase while the leaves nitrogen content per mass (N_m) were declined. The fraction of leaf nitrogen in thylakoid light harvesting systems (P_L) increased with the light intensity decline, whereas 'Aiwang' increased more. The partitioning coefficient for leaf nitrogen in bioenergetics (P_B) of 2 cultivars showed no significant change in different treatment while the P_B of 'Aiwang' is higher than 'Lvyou'. The partitioning coefficient for leaf nitrogen in Rubisco(P_C) of 2 cultivars reached peak at the 60% sunlight, whereas P_C of 'Aiwang' is higher than that of 'Lvyou' under low light. The results suggested that the tolerance of pak-choi to weak light was correlated to the leaves nitrogen regulation and partitioning among different photosynthetic apparatus.A controlled hydroponics experiment was conducted to study the effects of five NHn₄⁺/NO₃^- ratios (0/100, 25/75, 50/50, 75/25, and 100/0 respectively) on the nitrogen metabolism,the activity of the key enzymes(GS,NR),growth and photosynthetic of pak-choi under weak light condition. The results showed that, compared to the sole NO₃^- nutrition, the total nitrogen content in the leaves is significantly increased by the enhanced ammonium nutrition. And the best effect is achieved by the NH₄⁺/NO₃^- ratio of 25/75, under this ratio NR is enhanced and the decrease of GS under weak light condition is remarkably reduced. Compared to the sole NO₃^- nutrition, the fresh weight of pak-choi is remarkably increased by the enhanced ammonium nutrition. And the best effect is achieved by the NH₄⁺/NO₃^- ratio of 25/75, under which the decrease of AQY and CE under weak light condition is lower, and Rd ratio and Rp ratio is relatively lower.