Mesophyll structure during leaf development in Ballota acetabulosa

Abstract

summaryMesophyll structure and water relations were studied in expanding and expanded dorsiventral leaves of Ballota acetabulosa Benth., a wild perennial shrub from the Mediterranean. The spongy mesophyll was developed earlier than the palisade in expanding leaves and exhibited a larger internal exposed surface. By contrast, in fully expanded leaves the internal exposed surface was much larger in the palisade than in spongy mesophyll. The development of chloroplasts in cells of both mesophyll tissues, as well as their arrangement along the cell walls exposed to intercellular canals, coincided with the formation of intercellular spaces. Leaf expansion appears to be positively related to increasing internal exposed surface and declining water potentials. As a result, mature leaves possess a high ratio of internal exposed surface per leaf area and low values of turgor. It is considered that an increase in the internal exposed surface might be an adaptive feature for small‐leaved shrubs, grown under water deficiency.