POLYPLOLDS AND THEIR STRUCTURAL AND PHYSIOLOGICAL-CHARACTERISTICS RELATIVE TO WATER-DEFICIT IN BETULA-PAPYRIFERA (BETULACEAE)

The morphology and leaf anatomy, water relations, gas exchange, and gr owth characteristics of Betula papyrifera seedlings grown in a greenho use representing three ploidy levels (diploids, pentaploids, and hexap loids) have been compared in an effort to understand the role of polyp loidy in habitat differentiation and adaptation to water deficit. The polyploids are more tolerant of water deficit than their diploid relat ives. The polyploids have fewer stomata per unit area and smaller stom atal indices than the diploids. They also have thicker upper and lower epidermis and more pubescence, which tend to reduce water loss. The p olyploids have lower osmotic potential at saturation (Psi(tau,sat)) th an corresponding diploids, suggesting they are more able to maintain t urgor 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 ph otosynthesis in the diploids. They are more drought-avoiding than the polyploids as evidenced by closing their stomata to limit water loss a t a relatively higher tissue water potential (-1.425 MPa). The pentapl oids and hexaploids maintained, although to a small extent, photosynth esis when their tissue water potentials dropped to -1.86 MPa and -2.07 MPa, respectively.