Evidence from photosynthetic characteristics for the hybrid origin of Diplotaxis muralis from a C₃-C₄ intermediate and a C ₃ species

Artificial hybridization studies have been carried out between plants with different photosynthetic types to study the genetic mechanism of photosynthetic types. However, there are only few reports describing the possibility of natural hybridization between plants with different photosynthetic types. A previous cytological and morphological study suggested that a cruciferous allotetraploid species, Diplotaxis muralis (L.) DC. (2n = 42), originated from natural hybridization between D. tenuifolia (L.) DC. (2n = 22) and D. viminea (L.) DC. (2n = 20). These putative parents have recently been reported to be a C ₃-C₄ intermediate and a C₃ species, respectively. If this hybridization occurred, D. muralis should have characteristics intermediate between those of the C₃-C₄ intermediate and C₃ types. We compared leaf structures and photosynthetic characteristics of the three species. The bundle sheath (BS) cells in D. tenuifolia included many centripetally located chloroplasts and mitochondria, but those of D. viminea had only a few organelles. The BS cells in D. muralis displayed intermediate features between the putative parents. Glycine decarboxylase P protein was confined to the BS mitochondria in D. tenuifolia, but accumulated mainly in the mesophyll mitochondria in D. viminea. In D. muralis, it accumulated in both the BS and the mesophyll mitochondria. Values of CO₂ compensation point and its response to changing light intensity were also intermediate between the putative parents. These data support the theory that D. muralis was created by natural hybridization between species with different photosynthetic types.