Photosynthetic Acclimation to Temperature and Drought in the Endemic Chelan Rockmat, Petrophytum Cinerascens (Rosaceae)

Article excerpt

DARRIN J. MOORE', ROBERT S. NOWAK2

Environmental and Resource Sciences, University of Nevada, Reno 89557

ABSTRACT.-Petrophytum cnerascens (Piper) Rydb. (Rosaceae) is a highly restricted endemic found on steep rocky outcrops and sandy soils along the Columbia River in eastern Washington. Our goal was to examine the plant's ability for photosynthetic acclimation to increased growth temperature. Plants collected from the field were subjected to three sets of day/night growth temperatures and to two different watering regimes. Leaf gas exchange was measured at step-intervals of leaf temperature. Data were fitted to individual temperature response curves, and the optimum temperature for assimilation (T^sub opt^), the high temperature compensation point (T^sub high^) and assimilation rate at the optimum temperature for assimilation (A^sub opt^) were calculated from the regressions. We analyzed the data using a 3 X 2 split plot experimental design with growth temperature as the main effect and watering treatment as the split plot factor. A.opt for plants grown at 30/16 and 34/20 C was significantly greater than that for plants grown at 38/24 C, but the main effects of growth temperature on ToP, and Thigh were not significant. The relative decrease in A^sub opt^, from the lowest growth temperattire to the highest was almost 60% for well-watered plants and over 20% for plants with periodic drought treatments. The lack of change in T^sub opt^ and T^sub high^, coupled with the decrease in At,pt with increased growth temperature, suggests that P cinerascens is not able to acclimate to increased global temperature and therefore may serve as a sensitive indicator species of global warming.

INTRODUCTION

Two basic mechanisms for plants to survive changes in global temperature are: (1) migration to areas with a suitable environment; and (2) acclimation or adaptation to the changed environment. For example, bristlecone pine (Pinus longaeva) which grows near tree line of the southern White Mountains of California, demonstrates migration to a suitable environment. The presence of dead trees above the current tree line indicates that climate was once warmer, but trees were not able to acclimate to lower temperatures (LaMarche and Mooney, 1967). In contrast, other plant species are able to acclimate to changes in temperature or other climatic factors such as drought. Photosynthesis in the coastal shrub Encelia californica acclimated to either higher or lower growth temperature in as little as 24 h (Mooney and Shropshire, 1967). In addition, the differential ability of plants to acclimate to temperature change was demonstrated in a study using two C4 grasses from the genus Bouteloua. The warm environment species B. eriopod ia acclimated to changes in growth temperature, whereas the cool environment species B. gracilis did not (Bowman and Turner, 1993).

Endemic plant species tend to be vulnerable to environmental changes. Although many endemic, threatened and rare species may be locally abundant, their populations are limited in number across the landscape Restricted distribution of plant species may be due to various factors, including geographic isolation or habitat specialization (Holsinger and Gottlieb, 1991). A geographically isolated plant is not able to disperse its seeds beyond some geographic barrier and thus may not be able to migrate to new areas when environmental conditions change. For a habitat specialist, species may go extinct during climate change because they are unable to tolerate changes in precipitation, temperature or other environmental factors. Because these habitat specialists occupy a small niche, such plants become extremely vulnerable to any change in environment. The continued existence of endemic plants during global climate change depends primarily on five factors: (1) the plant's potential to acclimate to environmental changes; (2) the plant's genetic variability; (3) environmental variables which limit plant distribution and dispersal; (4) the extent of direct disturbance by human activity and (5) the geographic dispersion of individual plants and of populations. …