SEASON AND INTENSITY OF WATER-STRESS EFFECTS ON NEEDLE TOUGHNESS OF PONDEROSA PINE

The response patterns of needle toughness (a host plant trait correlat ed with resistance to pine sawflies (Hymenoptera: Diprionidae)) were i nvestigated under greenhouse conditions at varying levels of water str ess during different periods in the annual-growth cycle of ponderosa p ine (Pinus ponderosa Dougl. ex Laws.) seedlings. We performed a 2 (str ess level) x 4 (genotype) factorial design (study 1), and a 2 (season of stress) x 3 (stress level) x 6 (genotype) factorial design (study 2 ) to examine the influence of water stress on needle toughness. Modera te levels of water stress in study 1 produced seedlings with tougher n eedles than the lows-stress conditions. Seedlings subjected to high le vels of water stress during the shoot-growth period had the highest ne edle toughness amoung all treatments in study 2. In addition, signific ant differences in needle toughness among open-pollinated families in both studies suggest that needle toughness varies from tree to tree an d may in part be genetically regulated. We also found a significant li near relationship between root/shoot ratio and needle toughness and an inverse relationship between needle roughness and total seedling biom ass in study 1. These findings support in general a previously publish ed growth-differentiation balance concept and may also represent possi ble adaptations of ponderosa pine to herbivory and the relatively xeri c environment of the southwestern United States.