Seasonal dimorphism in the Mediterranean Cistus incanus L. subsp incanus

Mediterranean perennial species are described as being sclerophyllous, or summer deciduous, or seasonally dimorphic. Field observation in the coastal maquis of Castelvolturno Nature Reserve, southern Italy, showed that Cistus incanus L, subsp. incanus is a seasonally dimorphic species as it develops brachyblasts with small leaves in summer, and dolichoblasts with large leaves in winter. Field biometric data confirmed that winter shoots were 14-times longer than those developed in summer and had many more leaves. The area of single winter leaves was five-times that of summer leaves. Anatomical leaf structure also changed with the season: winter leaves were flat while summer leaves had a crimped lamina which was partially rolled to form crypts in the lower surface. Leaves were covered by considerably more trichomes in summer than in winter. Stomata were uniformly distributed along the lower epidermis of winter leaves but were only present in the crypts of summer leaves. In summer leaves, a palisade layer was often found on both sides of the lamina, the mesophyll cells were generally smaller and the intercellular spaces were reduced. Winter leaves had a dorsiventral structure and larger intercellular spaces. Seasonal dimorphism is generally reported to be an adaptation to summer drought. However, the morphology and anatomy of C. incanus L. subsp. incanus showed that the subspecies has not only developed a strategy to survive summer drought, but has evolved two different habits, one more xerophytic than the other, to optimize adaptation to the seasonal climatic changes occurring in Mediterranean environments. (C) 2001 Annals of Botany Company.