Polyploids and their structural and physiological characteristics relative to water deficit in Betula papyrifera (Betulaceae)

The morphology and leaf anatomy, water relations, gas exchange, and growth characteristics of Betula papyrifera seedlings grown in a greenhouse representing three ploidy levels (diploids, pentaploids, and hexaploids) have been compared in an effort to understand the role of polyploidy in habitat differentiation and adaptation to water deficit. The polyploids are more tolerant of water deficit than their diploid relatives. The polyploids have fewer stomata per unit area and smaller stomatal indices than the diploids. They also have thicker upper and lower epidermis and more pubescence, which tend to reduce water loss. The polyploids have lower osmotic potential at saturation than corresponding diploids, suggesting they are more able to maintain turgor in the case of low tissue water potential than the diploids. The photosynthesis of the diploids is more sensitive to water deficit. A 2-hr artificial water stress treatment caused complete cessation of photosynthesis in the diploids. They are more drought-avoiding than the polyploids as evidenced by closing their stomata to limit water loss at a relatively higher tissue water potential (-1.425 MPa). The pentaploids and hexaploids maintained, although to a small extent. photosynthesis when their tissue water potentials dropped to -1.86 MPa and -2.07 MPa, respectively