Seasonal patterns of free proline and carbohydrate levels in cherry laurel (Prunus laurocerasus) and ivy (Hederea helix) leaves and resistance to freezing and water deficit

Seasonal changes in the levels of water-soluble carbohydrates, free proline and tolerance to freezing and water deficit were studied in leaves of cherry laurel (Prunus laurocerasus) and ivy (Hedera helix). Frost and water deficit tolerance was estimated based on the measurement of electrical conductivity of aqueous media containing leaf discs that were previously treated with frost (–7°C) or polyethylene glycol solution (PEG 600), respectively. Carbohydrate content in leaves of examined species changed differently throughout the measurement period. In laurel leaves the highest carbohydrate level was found in February, while it was lowest in June. In contrast, in ivy leaves the highest carbohydrate content was recorded during summer (June, July), while the lowest in February. However, a lack of correlation between soluble carbohydrate level and membrane injury index was shown in this study. Free proline content in leaves of investigated species was the highest in early spring, i.e. April. The lowest level of free proline in laurel leaves was found in July, September and October, but in ivy from July to February. A negative correlation between proline content and membrane injury index observed in frost and PEG treated leaves of both species suggest that this amino acid may have resulted on membrane protection.