Effects of drought on needle anatomy of Pinus canariensis

The needles of Pinus canariensis exhibit morphological drought adaptations, such as special stomata morphology or rigid scle-renchymatic hypodermis. In the present paper the hypothesis that drought exposure during needle elongation triggers formation of xeromorphic traits was tested. With onset of the elongation growth of the new flush, 5-year old P. canariensis seedlings were exposed to 53 days of reduced water supply under controlled climate chamber conditions. The treatment resulted in a decrease of needle water potentials from –1.5 MPa to –4 MPa in drought stressed plants, whereas no change was observed in water poten-tials of control needles. Needles of drought exposed trees remained shorter and thinner. Cross-sections of elongating needles taken at 1 cm distance from the needle base were taken repeatedly during the experiment. Cross-section areas of different needle tissues were measured in an image analysis system. In needles of drought stressed trees sclerenchymatic tissues in the hypo-dermis and adjacent to the vascular bundles increased due to large increases in the number – not the size – of sclerenchymatic cells. The ratio of assimilation parenchyma to the vascular bundle (supplied versus supplying tissue) decreased. We conclude that these changes, which adapt needles to drought conditions, are triggered by drought stress experienced while needles are growing. Key words: Pinus canariensis, xeromorphoses, needle anatomy, stress