GENDER‐RELATED DIFFERENCES IN GAS EXCHANGE ARE NOT RELATED TO HOST QUALITY IN THE XYLEM‐TAPPING MISTLETOE, PHORADENDRON JUNIPERINUM (VISCACEAE)

Abstract

Male and female individuals of dioecious species often differ in morphology, physiology, growth, and habitat distribution. Where habitat distribution differences have been demonstrated, female plants generally occupy those habitats with greater resource availability (“rich” habitats). Gender‐specific habitat preferences are often presumed to be a consequence of greater resource requirements, per gamete, of female reproduction. Previous work has shown that Phoradendron juniperinum, a xylem‐tapping dioecious mistletoe that parasitizes Juniperus species in western North America, displays the opposite pattern: males are relatively more numerous than females in richer sites (i.e., branches with relatively high light and low evaporative demand within the host tree). We report here differences in host (“site”) quality and gas‐exchange properties between the sexes. To minimize environmental variation, all measurements were made on sunlit foliage between 9:00 a.m. and 2:00 p.m. Males had significantly higher photosynthetic rates (4.0 [SE = 0.2] μmol m‐2 sec‐1) than either females (2.9 [0.3] μmol m‐2 sec‐1) or nonreproductive individuals (3.0 [0.2] μmol m‐2 sec‐1). Female photosynthetic rates were not statistically different from those of nonreproductive individuals. No concomitant differences in stomatal conductance were observed. Gas exchange data were independently confirmed by significant differences in carbon isotope ratio (δ13C). Gender‐related differences were not related to host quality as measured by foliar N, foliar δ13C, or water potential of the host tree. The fate of the additional photosynthate in males is unknown, but we discuss the possibility that carbon costs of reproduction in males have been underestimated in past work.