Ramie (Boehmeria nivea L.) is an important crop that serves as fine fiber material, high protein feedstuff, and valuable herbal medicine in China. However, increasing salinity in soil limits the productivity. We investigated in a greenhouse experiment responses to salinity in two ramie cultivars, Chuanzhu-12 (salt-tolerant cultivar, ST) and Xiangzhu-2 (saltsensitive cultivar, SS), to elucidate the salt tolerance mechanism of this species. Salinity stress substantially reduced both chlorophyll and carotenoid contents. In addition, net photosynthesis, transpiration rate, stomatal conductance, intercellular CO2 concentration, and the ratio of intercellular CO 2 to ambient CO 2 were affected, less in ST. Nevertheless, salinity stress markedly improved water use efficiency and intrinsic water use efficiency in both species. Moreover, relative water contents, soluble proteins, and catalase activity were substantially impaired, while proline accumulation and superoxide dismutase activity were enhanced substantially, more in ST. Furthermore, noteworthy increase in peroxidase activity and decrease in malondialdehyde content was recorded in ST, whereas, in SS, these attributes changed conversely. Overall, the cultivar ST exhibited salt tolerance due to its higher photosynthetic capacity, chlorophyll content, antioxidative enzyme activity, and nonenzymatic antioxidants, as well as reduced lipid peroxidation and maintenance of the tissue water content. This revealed the salt tolerance mechanism of ramie plants for adaptation to salt affected soil.
Excessive nitrogen supply has resulted in environmentally negative impacts. In order to select and develop N-efficient ramie cultivars in increased N application environments, the morphological, physiological and biochemical responses of a ramie hybridization line (Chuanzhu 11) and its conventional parents (C9451 and R79-20) to N fertilizer were investigated under rain-fed conditions during three consecutive growing seasons. Two contrasting nitrogen levels (low nitrogen, LN: 20kg ha -2 ; high nitrogen, HN: 120kg ha -2 ) were used. Results indicated that high N substantially promoted the growth of ramie plants and led to remarkable increase in fiber yield in all cultivars during the three growing seasons. Such increase was also recorded in net photosynthesis, transpiration rate and stomatal conductance, as well as chlorophylls and carotenoids. However, high N supply caused no alteration or increase in intercellular CO 2 (C i ) and C i / C a (ambient CO 2 ) ratio, depending on species. Moreover, high N application significantly improved soluble protein and proline content while it reduced malondialdehyde content. The activities of superoxide dismutase and catalase also elevated, whereas peroxidase activity decreased by high N application in all cultivars. Furthermore, hybrid cultivar Chuanzhu 11 exhibited better performance as compared to its parents C9451 and H7920 due to improved growth, fiber yield, leaf gas exchange traits and enzymatic and non-enzymatic antioxidant systems under high N supply conditions. In conclusion, ramie hybrid cultivar Chuanzhu 11 was more efficient to absorb and utilize high levels of N. This meets the need for uptake and utilization of high concentration of N in increased N fertilizer environments.
The intercropping of potato and maize is widely practiced in China. In the potato/maize system, competition for light is an issue as the leaves of potato and maize become different strata within the canopy. The potato/maize intercropping trials using two potato varieties including Zhongshu 5 (early-maturing variety with erect branches) and Mila (mid-late maturing variety with spread branches) with the sole cropping potato as contro1 were carried out to determine the dynamic changes of LAI, SLW, Chl a+b, Chl a/b ratio, photosynthetical active radiation (PAR), gas exchange attributes in leaves at three position levels at tuber initiation stage and tuber expanding stage and yield. The results indicated that intercropping led to decrease LAI, SLW and Chl a/b ratio and increase Chl a+b. In addition, the gradual decrease in PAR, water use efficiency (WUE), stomatal limitation (L s =1-C i /C a , C a : ambient CO 2 concentration) and the increase in net photosynthetic rate (P n ), stomatal conductance (G s ), intercellular CO 2 (C i ) and transpiration rate (T r ) were observed from tuber initiation stage to tuber expanding stage. There was a continuous significant reduction in PAR, P n , G s and T r from the upper leaves to the middle and lower leaves in all treatments, with a lower reduction under intercropping than under sole cropping. The variations in P n , G s , C i , and L s indicated the decreased photosynthetic activity in the middle and lower leaves pertaining, to both stomatal and non-stomatal mechanisms. Intercropping also declined P n in the upper 第 2 期 黄承建等: 马铃薯/玉米套作对马铃薯品种光合特性及产量的影响 331 leaves and elevated P n in the middle and lower leaves. Furthermore, there were higher Chl a+b in vegetative growth stage and tuber initiation stage and lower Chl a+b in tuber expanding stage and starch accumulation stage in Zhongshu 5/maize than in Mira/maize system, with the opposite changing trend for SLW. Higher LAI and Chl a/b ratio at all developmental stages, higher PAR in the upper leaves and lower PAR in the middle and lower leaves were observed, and higher P n in the middle and lower leaves in Zhongshu 5 than in Mira in intercropping systems but the similar P n in the upper leaves. In comparison with Mira, there were lower G s , C i , and T r and higher WUE and L s in leaves at the same position levels in Zhongshu 5. In summary, intercropping led to substantial reduction in tuber yield at harvest time due to the change on the light environment and the photosynthetic characteristics of potato in potato/maize systems. Nonetheless, intercropping deteriorated the light environment for Zhongshu 5/maize system while improved the light environment for Mira/maize system, which led to lower LER in the former (1.24) than in the latter (1.40), showing the stronger intercropping superiority for Mira/maize system in the production.
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