Salixspecies variation in leaf gas exchange, sodium, and nutrient parameters at three levels of salinity

Leaf gas exchange, Na+, and nutrient parameters were quantified under control (CTL), medium (MST), and high (HST) saline treatments for Salix discolor Muhl. (DIS), Salix eriocephala Michx. (ERI), and Salix interior Rowlee (INT). Net photosynthesis (Pn) increased from CTL to MST for all species but remained unchanged between MST and HST for surviving INT plants in HST. Salix interior had greater water-use efficiency (WUE) than DIS and ERI. Under CTL, INT had 13× the leaf Na+concentration of DIS and ERI, but INT Na+only increased by 17% across each saline treatment, whereas DIS and ERI leaf Na+increased 6.0× from CTL to MST. Salix discolor and ERI had greater cation to Na+ratios than INT; however, INT ratios were stable across saline treatments, whereas the DIS and ERI ratios dropped precipitously. Across species and treatments, Pnhad a significant positive relationship to total aboveground dry mass, supporting the sink regulation of Pntheory. Also, Pnshowed a significant positive relationship to WUE and leaf N and to leaf Na+, which probably reflects greater energy expenditure required to mitigate the higher levels of Na+. The saline tolerance of INT may be due to natural selection pressure in the arid regions of the southwestern United States, where it is believed to have its evolutionary origins.

An experimental study of hybridization and pollination in Salix (willow)

Experimental crosses between Salix discolor, S. petiolaris, S. bebbiana, and S. lucida resulted in the synthetic hybrids S. discolor × petiolaris and S. bebbiana × petiolaris and their reciprocals. The cross S. bebbiana × discolor and all crosses involving S. lucida were unsuccessful. Field experiments on the mode of pollination of Salix revealed the presence of wind-borne pollen of S. discolor, S. petiolaris, and S. bebbiana, and seed was set by these species in the absence of insect vectors. They are suspected to be both entomophilous and anemophilous. Salix lucida did not set seeds under these conditions and it is hypothesized that it is fundamentally entomophilous. Evidence is presented that suggests that natural hybridization between S. discolor and S. petiolaris may be restricted by differences in flowering time and by pollen competition.

LIFE HISTORY AND HABITS OF THE WILLOW BEAKED GALL MIDGE, MAYETIOLA RIGIDAE (DIPTERA: CECIDOMYIIDAE), IN MICHIGAN

The willow beaked gall midge, Mayetiola rigidae (Osten Sacken), is univoltine in Michigan. Adults emerge from bud galls on Salix discolor Mühl. and other willows on mornings of warm days in early April. Eggs are laid singly on or near the buds of the host. Head capsule measurements indicate three larval instars. The last two instars each possess a spatula. The first-instar larva emerges in late April and penetrates the soft bud tissues. The gall begins to develop at the beginning of the second instar in mid-May. The third instar appears in early July and continues to enlarge the gall until fall. Prior to overwintering, the larva lines the inner chamber of the gall with silk and constructs one to seven silken septa across the passageway. Pupation occurs in mid-March. The gall deforms the stem and occasionally a galled branch dies or breaks off.

LIFE HISTORY AND HABITS OF RHABDOPHAGA SP. (DIPTERA: CECIDOMYIIDAE), A GALL MIDGE ATTACKING WILLOW IN MICHIGAN

The taxonomic status of Rhabdophaga sp. cannot be determined until a complete revision of the genus occurs. Rhabdophaga sp. on Salix discolor Mühl. is univoltine in Michigan. Adults emerge in mid-April, and shortly afterward deposit numerous eggs on the setaceous undersurface of emerging willow leaves; larval eclosion occurs between 1 and 2 weeks later. Head capsule measurements reveal three larval instars. The first-instar larva bores into the stem until it reaches the pith. Gall development begins about mid-June shortly after the second instar appears. The third-instar larva overwinters in the gall and the pupa appears in early April. The prolate gall is found on the proximal ends of young willow shoots; heavily galled shoots usually die.