The Control of Biomass Partitioning in Plants from “favourable” and “stressful” Environments: a Role for Gibberellins and Cytokinins

Plants which naturally occur on nutrient-poor sites have inherently lower growth rates when compared to plants from more favourable habitats. This difference in growth rate is maintained when the species are compared under conditions which are optimal for their growth. Genetic variation in plant growth rate is predominantly due to variation in the pattern of biomass allocation in leaves, stems and roots: fast-growing genotypes have more leaf area per unit plant weight, but the leaves have a low biomass density. Gibberellins are involved in some of these inherent differences in biomass allocation. Under nitrogen-stress, i.e. when exposed to nitrogen-poor conditions, a plant's growth rate is reduced, when compared to that at an optimum supply of nutrients. This is partly accounted for by a smaller investment of resources in leaves and a greater investment in roots. Rates of photosynthesis per unit leaf area may also be less at a growth-limiting supply of nitrogen, whereas carbon requirements for respiration, expressed as a proportion of carbon fixed in photosynthesis, are increased. This is largely due to increased carbon requirements for root respiration, due to the relative increase in size of the root system. Under nitrogen-stress, the leaves of inherently fast-growing as well as those of slow-growing Poa species accumulate less protein and more cell-wall components. This is associated with an increase in leaf mass per unit leaf area and a relatively greater investment in sclerenchymatic cells. The volume occupied by epidermal cells is reduced, despite their relative increase in number, due to the decrease in epidermal cell size. Cytokinins play a major role in the shift in biomass allocation from leaves to roots, as BULG. 25 dependent on nitrogen supply. There is no information to support a role of cytokinins in the changes in leaf anatomy. This paper reviews recent information on the mechanisms accounting for the pattern of biomass allocation between leaves and roots. The role of gibberellins and cytokinins in the control of biomass allocation is also discussed. We conclude that gibberellins account for part of the genotypic variation in biomass allocation, whereas cytokinins are responsible for part of the phenotypic changes upon nitrogen-stress.