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Abstract
In a 16-day study, the effect of increasing soil NaCl on leaf photosynthesis, chlorophyll a fluorescence, chloroplast ion compartmentation, variations of SOD (superoxide dismutase) and POD (peroxidase) isoenzymes and the relevance to salt resistance were investigated in seedlings of Populus euphratica Oliv. (P. euphratica) (salt-resistant) and rooted cuttings of P. “popularis 35–44” (P. popularis) (salt-sensitive). Initial salinity caused a rapid decline of net photosynthetic rate (Pn) and unit transpiration rate (TRN) in P. euphratica, resulting from the NaCl-induced stomatal closure. In a longer-term of salinity, CO2 assimilation in P. popularis was severely reduced whereas stressed P. euphratica maintained a relatively higher and constant level of Pn. Pn–Ci curves showed that salt stress (12 days) reduced CO2 saturation point (CSP), CO2 saturated Pn (CSP n ), and carboxylation efficiency (CE), but increased CO2 compensation point (CCP) in the two genotypes. Similarly, salinity lowered light saturation point (LSP), light saturated Pn (LSP n ), and apparent quantum yield (AQY) in both genotypes but the inhibitory effect of NaCl on light reaction was more pronounced in P. popularis, as compared to P. euphratica. Chlorophyll a fluorescence data indicated that a longer-term of salt stress (12 days) exhibited a marked influence on fluorescence parameters of P. popularis in both dark- and light-adapted states: (a) NaCl inhibited the maximal efficiency of PSII photochemistry (Fv/Fm) due to the salt-induced increase of Fo (the minimal fluorescence) and the marked decline of Fm (the maximal fluorescence); (b) salinity decreased coefficient of photochemical quenching (qP) but markedly elevated coefficient of nonphotochemical quenching (qN) in the light-adapted state. In contrast, there were no corresponding changes of chlorophyll a fluorescence in salinised P. euphratica. X-ray microanalysis results showed that salinity caused salt accumulation in the chloroplasts of P. popularis in which Na+ and Cl− increased up to 42 and 221 mmol dm−3, respectively. Great buildup of Na+ and Cl− in chloroplasts of P. popularis may exhibit direct and indirect restrictions on dark and light reactions. The activity of SOD isoenzymes (CuZn-SOD I and CuZn-SOD II) and POD isoenzymes in P. popularis decreased with increasing exposure period, and leaf malondialdehyde (MDA) content and membrane permeability (MP) increased correspondingly. In contrast to P. popularis, stressed P. euphratica maintained activity of SOD and POD isoenzymes and there was no significant increase of MDA and MP during the period of salt stress. In conclusion, P. euphratica plants exhibited a higher capacity to maintain the activity of anti-oxidant enzymes and restrict salt accumulation in the chloroplasts, the photosynthesis processes were less restricted consequently.
Highlights
Populus spp. are one of the most important tree species for afforestation in north China, but the productivity of fastgrowing poplars is usually restricted by soil salinity
We found that the increased membrane permeability in salt-sensitive P. popularis leaves was associated with salt-induced lipid peroxidation, which was likely due to the drastic decline of superoxide dismutase (SOD) and POD activity (Wang et al 2006)
Net photosynthetic rate (Pn), stomatal conductance (Gs) and unit transpiration rate (TRN) in P. euphratica rapidly declined after 1–4 h of salt stress, returned to control levels at 24 h, whereas there were no corresponding changes in stressed P. popularis plants (Fig. 1)
Summary
Populus spp. are one of the most important tree species for afforestation in north China, but the productivity of fastgrowing poplars is usually restricted by soil salinity. Comparative studies have been carried out to determine the salt tolerance of several poplar genotypes (Ma et al 1997; Fung et al 1998). Results showed that salt stress reduced photosynthesis of poplar leaves, but there was much variation in salt tolerance among genotypes of Populus. Salinised poplar trees transport Na+ and Cl– from roots to the shoots under NaCl stress, causing a great buildup of salt in leaves (Chen et al 2001, 2002b, 2003). Ma et al (1997) found that the reduction of photosynthesis under high salt treatment was not due to damage of the photosynthetic apparatus, but more likely due to the inhibition of dark reaction since chlorophyll a fluorescence was not reduced by salinity. The ion compartmentation in chloroplasts and the effects of salts on light and dark reaction needs to be investigated
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