Photosynthesis and leaf structure of F1 hybrids between Cymbidium ensifolium (C3) and C. bicolor subsp. pubescens (CAM)

Background and aims The introduction of crassulacean acid metabolism (CAM) into C-3 crops has been considered as a means of improving water-use efficiency. In this study, we investigated photosynthetic and leaf structural traits in F-1 hybrids between Cymbidium ensifolium (female C-3 parent) and C. bicolor subsp. pubescens (male CAM parent) of the Orchidaceae. Methods Seven F-1 hybrids produced through artificial pollination and in vitro culture were grown in a greenhouse with the parent plants. Structural, biochemical and physiological traits involved in CAM in their leaves were investigated. Key results Cymbidium ensifolium accumulated very low levels of malate without diel fluctuation, whereas C. bicolor subsp. pubescens showed nocturnal accumulation and diurnal consumption of malate. The F(1)s also accumulated malate at night, but much less than C. bicolor subsp. pubescens. This feature was consistent with low nocturnal fixation of atmospheric CO2 in the F(1)s. The delta C-13 values of the F(1)s were intermediate between those of the parents. Leaf thickness was thicker in C. bicolor subsp. pubescens than in C. ensifolium, and those of the F(1)s were more similar to that of C. ensifolium. This was due to the difference in mesophyll cell size. The chloroplast coverage of mesophyll cell perimeter adjacent to intercellular air spaces of C. bicolor subsp. pubescens was lower than that of C. ensifolium, and that of the F(1)s was intermediate between them. Interestingly, one F-1 had structural and physiological traits more similar to those of C. bicolor subsp. pubescens than the other F(1)s. Nevertheless, all F(1)s contained intermediate levels of phosphoenolpyruvate carboxylase but as much pyruvate, Pi dikinase as C. bicolor subsp. pubescens. Conclusions CAM traits were intricately inherited in the F-1 hybrids, the level of CAM expression varied widely among F-1 plants, and the CAM traits examined were not necessarily co-ordinately transmitted to the F(1)s.