Chilling is an important constraint factor to the growing development and geographic distribution of plant species. Ipomoea cairica (L.) Sweet is one of the most notorious invasive weeds in South China, while it seems to be not invasive outside South China. Ipomoea nil (L.) Choisy (slightly invasive or non-invasive) and Ipomoea triloba L. (non-invasive) are two of the I. cairica's closely related species that are also originated from the tropical America. In the present study, we compared the chilling sensitivities of these three vine plants by testing their ecophysiological responses to low-temperature treatments (15 ℃, 10 ℃) in a hydroponic experiment, so as to illustrate the potential connections between the chilling sensitivity and invasiveness. We hypothesized that (i) the three alien plants were all sensitive to chilling stress and (ii) the chilling sensitivity of the non-invasive (or slightly invasive) plants I. triloba and I. nil were higher than that of the serious invasive species I. cairica.
Cuttings of I. cairica, I. nil and I. triloba collected from selected habitats in South China were planted in water to let the root grow for one week, and then cultivated in Hoagland nutrient solution for 4 weeks. Similarly sized plants of each species were placed in the light incubators with different temperatures (28 ℃, 15 ℃, 10 ℃) for 8 days. The chlorophyll fluorescence technique was used to monitor the photosynthetic apparatus of these Ipomoea plants every other day. After harvest the shoot biomass and root length of new growth for all plants were calculated. The accumulation of H₂O₂, malodialdehyde (MDA), proline and soluble carbohydrate in the leaves, as well as the root metabolic activity and membrane integrity were also tested as indications of physiological performance after different temperature treatments.
The chlorophyll fluorescence results showed that photosystem Ⅱ (PS Ⅱ) of all three Ipomoea species was partly damaged by chilling (15 ℃, resulted in increased initial fluorescence level (F₀), but decreased electron transport rate (ETR) and maximum photochemical efficiency (F_v/F_m)), especially for I. triloba, which always was most sensitive. There was also higher non-photochemical quenching (NPQ) under chilling stress in all three species, indicating increased thermal dissipation in PS Ⅱ. Compared to the control (28 ℃), all plants under chilling stress accumulated significantly more proline and soluble carbohydrate (P<0.01), both of which could improve their chilling tolerance. There were significantly less shoot biomass and root length of new growth, and higher levels of cell membrane integrity in roots under the 15 ℃ and 10 ℃ treatment for all three species (P<0.05), and that should be partly attributed to the accumulation of H₂O₂ and MDA, which were observed in all of the plants under chilling stress. These results clearly showed that at 15 ℃ there had been significant stress effects in these Ipomoea species. The first hypothesis of this research, i.e. the three species have high chilling sensitivities, was thus supported. As for the other aspect, when comparing the response intensities of the three Ipomoea species to chilling treatment, it is found that at the 10 ℃ treatment only I. cairica has continued to grow, while no growth was observed in I. nil and I. triloba during the periods of chilling stress, indicating that I. cairica has the highest chilling tolerances among the three species. The second hypothesis of this study was thus also supported. Besides, among the three Ipomoea species, I. triloba seems to be the most sensitive one to chilling as it usually had the highest physiological response intensities to chilling stress. The results from this study implied that the comparatively higher chilling tolerance of I. cairica in comparison to I. nil and I. triloba should be one of the reasons that in South China it is more invasive than the latter two species. The chilling sensitivity is an important key factor to the invasiveness of alien plants. Besides, the relatively high chilling sensitivity of I. cairica should be one of the most important reasons that it has not invaded other areas than South China. Therefore, it is necessary to pay more attention to the potential range expansion of I. cairica under the background of global warming.