Abstract: To investigate whether hybrid of Brassica crops has heterosis in photosynthetic capacity and reveal the mechanisms involved. The reciprocal inter-subspecific hybrids between Brassica campestris ssp. chinensis ‘Aijiaohuang’ and B. campestris ssp. rapifera ‘Baimanjing’, and their parents were used to determine several photosynthetic processes by simultaneous measurement of leaf gas exchange and chlorophyll fluorescence parameters. Results showed that the net CO2 assimilation rates (Pn) of hybrids were significantly higher than their parents throughout the experiment, leading to a strong heterosis in photosynthesis. Hybrids also increased stomatal conductance (Gs) but had no influence on intercellular CO2 concentration (Ci) and stomatal limitation (l). The concentrations of total chlorophyll in the hybrids are between their parents resulting in a weak heterosis. Hybrids significantly increased the quantum yield of PSII electron transport (ΦPSII) which were accompanied by a similar increase in photochemical quenching (qP). Additionally, hybrids also had evident heterosis in the maximum carboxylation rate of Rubisco (Vcmax) and the capacity for ribulose-1,5-bisphosphate regeneration (Jmax), and the values were close to the high value parent ‘Aijiaohuang’ except for the significant higher Vcmax of the direct cross between ‘Aijiaohuang’ and ‘Baimanjing’.

Key words: Brassica, inter-subspecific hybrid, photosynthesis, chlorophyll fluorescence, heterosis