Abstract
With the aim of improving shade tolerance of cucumber, Cucumis × hytivus, a newly synthesized allotetraploid, was obtained by crossing a shade tolerant wild relative, Cucumis hystrix, with a cultivated cucumber, Cucumis sativus L. ‘BejingJietou.’ The results show that the new C. × hytivus only partly is an intermediate hybrid and it has not only chlorophyll deficiency, which recovers during leaf development, but also lower carotenoid content. Three light conditions with the combination of different light intensities and photoperiods were employed to investigate the photosynthetic response of these three Cucumis species to low light and long photoperiod. The consistent order of Pmax and DWS being lowest in C. hystrix, medium in C. × hytivus and highest in ‘BejingJietou’ suggests the three species to have genetically different photosynthetic efficiency, which relates well with the natural habitats of the parent species and the hybrid as intermediate. C. × hytivus appears to be inhibited by the low light levels to the same extent as the cultivated ‘BeijingJietou,’ which indicates neither improvement of shade tolerance nor hypothetical heterosis effect in C. × hytivus. However, unexpectedly, the PSII of C. hystrix was affected by the long photoperiod in the long term, suggested by the decrease of Fv/Fm. This sensitivity toward day length has not been passed on to C. × hytivus.
Highlights
Plants have evolved a number of mechanisms to acclimate to changing light levels, for example by changing the size of the light-harvesting complexes (LHCs) and leaf thickness to enable efficient capture and use of light (Lichtenthaler et al, 1981; Walters and Horton, 1994)
C. hystrix had a small plant size, whereas the hybrid C. × hytivus had medium size compared with the cultivated species ‘BejingJietou’ (Figure 1)
Pigments were extracted from plant tissue in cold 96% ethanol and the concentrations of the pigments were quantified by light spectroscopy (UV-VIS spectrophotometer, Shimadzu, Kyoto, Japan), light absorbance at 470, 648 and 664 nm according to Lichtenthaler (1987), including chlorophyll a (Chl a), chlorophyll b (Chl b), and total carotenoids
Summary
Plants have evolved a number of mechanisms to acclimate to changing light levels, for example by changing the size of the light-harvesting complexes (LHCs) and leaf thickness to enable efficient capture and use of light (Lichtenthaler et al, 1981; Walters and Horton, 1994). Cucumber (Cucumis sativus L., 2n = 14) is one of the most important vegetable crops in many countries and more than 75% (54.3 million tons) were produced in China (FAOSTAT 2013, data available at http:// faostat.fao.org/). During the winter in China, low irradiance is the major limiting factor for cucumber growth and yield in the protected production (Ma et al, 1998). The improvement of shade-tolerance in cucumber has been slow due to the narrow genetic base of cucumber (3–12% polymorphism; Knerr et al, 1989; Dijkhuizen et al, 1996). An interspecific cross was made in Cucumis with the shade adapted Cucumis hystrix Chakr.
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