ALLOCATION OF CARBON IN PLANTS UNDER STRESS - AN ANALYSIS OF THE ROPIS EXPERIMENTS

This paper addresses common patterns of plant C allocation in response to stress. The ROPIS studies used species ranging from slow growing, long-lived evergreen trees (red spruce [Picea rubens Sarg.] and ponderosa pine [Pinus ponderosa Dougl.]), to fast growing evergreen and deciduous trees (loblolly pine [Pinus taeda L.] and aspen [Populus tremulaides Michx.]) and annuals (radish [Raphanus sativus L.]). Several factors helped to explain the effects of ozone, the common stress in all ROPIS experiments, on allocation in these diverse species. Species with high relative growth rates readily changed allocation in response to stress. For example, radish and aspen allocated C to produce new leaves in response to ozone and allocated C to roots in response to water and N deficits. In contrast, red spruce had the lowest relative growth rate, and neither total plant biomass nor C allocation were affected by ozone after four growing seasons. However, partitioning of C to foliar starch reserves was reduced. Ponderosa pine and loblolly pine had intermediate relative growth rates. Ozone reduced total plant biomass and allocation to coarse roots in ponderosa pine, while total plant biomass but not allocation was reduced in loblolly pine. Radish, aspen, and ponderosa pine all maintain low foliar starch reserves and experienced ozone-induced foliar senescence. In contrast, red spruce and loblolly pine maintain substantial foliar starch reserves, which were reduced by ozone. However, they did not experience ozone-induced senescence. While fast growing species showed the greatest changes in allocation in response to stress, we do not suggest that rapidly growing plants are more sensitive to stress. We suggest they have a higher capacity to allocate C to compensate for the stress. Slower growing species rely more on C storage or multiple shoot growth periods within the growing season to respond to stresses that alter the pattern of C allocation in faster growing species.