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 ponder osa pine [Pinus ponderosa Dougl.]), to fast growing evergreen and deci duous trees (loblolly pine [Pinus taeda L.] and aspen [Populus tremula ides Michx.]) and annuals (radish [Raphanus sativus L.]). Several fact ors helped to explain the effects of ozone, the common stress in all R OPIS 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 leave s in response to ozone and allocated C to roots in response to water a nd N deficits. In contrast, red spruce had the lowest relative growth rate, and neither total plant biomass nor C allocation were affected b y ozone after four growing seasons. However, partitioning of C to foli ar starch reserves was reduced. Ponderosa pine and loblolly pine had i ntermediate relative growth rates. Ozone reduced total plant biomass a nd allocation to coarse roots in ponderosa pine, while total plant bio mass but not allocation was reduced in loblolly pine. Radish, aspen, a nd ponderosa pine all maintain low foliar starch reserves and experien ced ozone-induced foliar senescence. In contrast, red spruce and loblo lly pine maintain substantial foliar starch reserves, which were reduc ed 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 t o 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.