Filling in the gaps: evidence of leaf endodermis and vein sheath in gymnosperms

Botanical literature is filled with studies which tried to demonstrate that leaves of many gymnosperms have an endodermis with Casparian bands or a sheath of sclerified cells around leaf veins. Direct photographic evidence of an endodermis with Casparian bands (strips) is lacking for the leaves of many gymnosperms. Our goal was to confirm with direct evidence an endodermis with Casparian bands or a vein sheath in leaves of representative gymnosperms via histochemical staining and epifluorescence microscopy, while extending previous work by examining understudied petioles and leaf bases.

We sectioned leaves fresh with razor blades and viewed them unstained or stained, usually with berberine hemisulfate, counterstained with gentian violet, and phloroglucinol HCl. Bright-field, epifluorescence and/or laser confocal microscopies on a Zeiss LSM700 were used for viewing and imaging.

Most members of Pinaceae of Pinophyta (13 species of genera Cedrus, Pseudotsuga, Larix, Picea and Pinus) had endodermis with distinct Casparian bands in needles. We identified endodermis in the distal petiolar regions of nine of these same species. Sclerified vein sheaths or partial vein sheaths were observed in 10 species of 41 studied among Cycadophyta (Cycas), Ginkgophyta (Ginkgo) and the Pinophyta Podocarpaceae (Dacrycarpus, Dacrydium), Cupressaceae (Metasequoia), Taxaceae (Amentotaxus) and Pinaceae (Keteleeria, Abies, Tsuga).

Endodermis with Casparian bands is only characteristic of most species of Pinaceae; vein sheaths are found in three genera of Pinaceae and six genera of the other families of gymnosperms, but most gymnospermous leaves lack endodermis and vein sheath, particularly in petiolar and leaf base regions. The presence of endodermis with Casparian bands could have contributed to the adaptation of the Pinaceae to extreme environments; members of genera such as Picea and Pinus are the typical treeline species in many mountain ranges across the world.