The allocation of biomass to different plant organs depends on species, ontogeny and on the environment experienced by the plant. In this paper we first discuss some methodological tools to describe and analyse the allocation of biomass. Rather than the use of shoot:root ratios, we plead strongly for a subdivision of biomass into at least three compartments: leaves, stems and roots. Attention is drawn to some of the disadvantages of allometry as a tool to correct for size differences between plants. Second, we tested the extent to which biomass allocation of plants follows the model of a ‘functional equilibrium’. According to this model, plants respond to a decrease in above-ground resources with increased allocation to shoots (leaves), whereas they respond to a decrease in below-ground resources with increased allocation to roots. We carried out a meta-analysis of the literature, analysing the effect of various environmental variables on the fraction of total plant biomass allocated to leaves (leaf mass fraction), stem (stem mass fraction) and roots (root mass fraction). The responses to light, nutrients and water agreed with the (qualitative) prediction of the ‘functional equilibrium’ theory. The notable exception was atmospheric CO2, which did not affect allocation when the concentration was doubled. Third, we analysed the quantitative importance of the changes in allocation compared to changes in other growth parameters, such as unit leaf rate (the net difference between carbon gain and carbon losses per unit time and leaf area), and specific leaf area (leaf area: leaf biomass). The effects of light, CO2 and water on leaf mass fractions were small compared to their effects on relative growth rate. The effects of nutrients, however, were large, suggesting that only in the case of nutrients, biomass allocation is a major factor in the response of plants to limiting resource supply.

The allocation of biomass to different plant organs depends on species, ontogeny and on the environment experienced by the plant. In this paper we first discuss some methodological tools to describe and analyse the allocation of biomass. Rather than the use of shoot:root ratios, we plead strongly for a subdivision of biomass into at least three compartments: leaves, stems and roots. Attention is drawn to some of the disadvantages of allometry as a tool to correct for size differences between plants. Second, we tested the extent to which biomass allocation of plants follows the model of a ‘functional equilibrium’. According to this model, plants respond to a decrease in above-ground resources with increased allocation to shoots (leaves), whereas they respond to a decrease in below-ground resources with increased allocation to roots. We carried out a meta-analysis of the literature, analysing the effect of various environmental variables on the fraction of total plant biomass allocated to leaves (leaf mass fraction), stem (stem mass fraction) and roots (root mass fraction). The responses to light, nutrients and water agreed with the (qualitative) prediction of the ‘functional equilibrium’ theory. The notable exception was atmospheric CO2, which did not affect allocation when the concentration was doubled. Third, we analysed the quantitative importance of the changes in allocation compared to changes in other growth parameters, such as unit leaf rate (the net difference between carbon gain and carbon losses per unit time and leaf area), and specific leaf area (leaf area: leaf biomass). The effects of light, CO2 and water on leaf mass fractions were small compared to their effects on relative growth rate. The effects of nutrients, however, were large, suggesting that only in the case of nutrients, biomass allocation is a major factor in the response of plants to limiting resource supply.

Given the close relationship between a plant's growth rate and its pattern of biomass allocation and the effects of abscisic acid (ABA) on biomass allocation, we studied the influence of ABA on biomass allocation and growth rate of wildtype tomato (Lycopersicon esculentum Mill. cv. Moneymaker) plants and their strongly ABA‐deficient mutant sitiens. The relative growth rate of sitiens was 22% lower than that of the wildtype, as the result of a decreased specific leaf area. The net assimilation rate and the leaf weight ratio were not affected. The mutant showed a much higher transpiration rate and lower hydraulic conductance of the roots. These two factors resulted in sitiens having a significantly lower leaf water potential and turgor. resulting in reduced leaf expansion and, consequently, a lower specific leaf area relative to the wildtype. Addition of ABA to the sitiens roots resulted in phenotypic reversion to the wildtype. We conclude that the influence of ABA‐deficiency on biomass allocation and relative growth rate is the result of altered water relations in the plants, rather than of a direct effect on sink strength of different plant organs.

In this paper we firstly show some general responses of biomass partitioning upon nitrogen deprivation. Secondly, these responses are explained in terms of allocation of carbon and nitrogen, photosynthesis and respiration, using a simulation model. Thirdly, we present a hypothesis for the regulation of biomass partitioning to shoots and roots.Shortly after nitrogen deprivation, the relative growth rate (RGR) of the roots generally increases and thereafter decreases, whereas that of the shoot decreases immediately. The increased RGR of the root and decreased RGR of the shoot shortly after a reduction in the nitrogen supply, cause the root weight ratio (root weight per unit plant weight) to increase rapidly.We showed previously that allocation of carbon and nitrogen to shoots and roots can satisfactorily be described as a function of the internal organic plant nitrogen concentration. Using these functions in a simulation model, we analyzed why the relative growth rate of the roots increases shortly after a reduction in nitrogen supply. The model predicts that upon nitrogen deprivation, the plant nitrogen concentration and the rate of photosynthesis per unit plant weight rapidly decrease, and the allocation of recently assimilated carbon and nitrogen to roots rapidly increases. Simulations show that the increased relative growth rate of the root upon nitrogen deprivation is explained by decreased use of carbon for root respiration, due to decreased carbon costs for nitrogen uptake. The stimulation of the relative growth rate of the root is further amplified by the increased allocation of carbon and nitrogen to roots. Using the simple relation between the plant nitrogen concentration and allocation, the model describes plant responses quite realistically.Based on information in the literature and on our own experiments we hypothesize that allocation of carbon is mediated by sucrose and cytokinins. We propose that nitrogen deprivation leads to a reduced cytokinin production, a decreased rate of cytokinin export from the roots to the shoot, and decreased cytokinin concentrations. A reduced cytokinin concentration in the shoot represses cell division in leaves, whereas a low cytokinin concentration in roots neutralizes the inhibitory effect of cytokinins on cell division. A reduced rate of cell division in the leaves leads to a reduced unloading of sucrose from the phloem into the expanding cells. Consequently, the sucrose concentration in the phloem nearby the expanding cells increases, leading to an increase in turgot pressure in the phloem nearby the leaf's division zone. In the roots, cell division continues and no accumulation of sugars occurs in dividing cells, leading to only marginal changes in osmotic potential and turgot pressure in the phloem nearby the root's cell division zone. These changes in turgor pressure in the phloem of roots and sink leaves affect the turgor pressure gradients between source leaf-sink leaf and source leaf -root in such a way that relatively more carbohydrates are exported to...

Even though the growth-promoting effects of gibberellins (GAs) in plants are well established, little is known about GA action on carbon metabolism and the available reports seem contradictory. We studied the effects of GA deficiency in mutants of tomato ( Solanum lycopersicum L.) on rates of carbon acquisition and the allocation of acquired carbon to growth and respiration of leaves, stems and roots. Carbon budgets were calculated from 24 h measurements of photosynthesis and respiration. The partitioning of nitrogen compounds to leaves, stems and roots, which strongly influences carbon budgets, was also studied. The GAdeficient mutants acquired less carbon per unit plant mass per day than did the wild type and used a larger fraction of it for root growth and root respiration. To find out to what extent these changes were just consequences of restriction of growth, the experiment was repeated at a low exponential nitrate addition rate, which forced all genotypes to grow at the same rate. Under these conditions, the low-GA mutants still photosynthesized and respired faster and partitioned more carbon to root growth than the wild type did. The reasons for the observed differences in carbon economies between the wild type and the low-GA mutants are discussed.