GROWTH AND MORPHOGENESIS OF SUN AND SHADE PLANTS III. THE COMBINED EFFECTS OF LIGHT INTENSITY AND NUTRIENT SUPPLY

Effect of N supply on the carbon economy of barley when accounting for plant size

Nitrogen availability and ontogeny both affect the relative growth rate (RGR) of plants. In this study of barley (Hordeum vulgare L.) we determined which growth parameters are affected by nitrate (N) availability, and whether these were confounded by differences in plant size, reflecting differences in growth. Plants were hydroponically grown on six different nitrate (N) concentrations for 28 days, and nine harvests were performed to assess the effect of N on growth parameters. Most growth parameters showed similar patterns of responses to N supply whether compared at common time points or common plant sizes. N had a significant effect on the biomass allocation: increasing N increased leaf mass ratio (LMR) and decreased root mass ratio (RMR). Specific leaf area (SLA) was not significantly affected by N. RGR increased with increasing N supply up to 1 mM, associated with increases in both LMR and net assimilation rate (NAR). Increases in N supply above 1 mM did not increase RGR as increases in LMR were offset by decreases in NAR. The high RGR at suboptimal N supply suggest a higher nitrogen use efficiency (biomass/N supply). The reasons for the homeostasis of growth under suboptimal N levels are discussed.

A meta-analysis of plant responses to light intensity for 70 traits ranging from molecules to whole plant performance

By means of meta-analyses we determined how 70 traits related to plant anatomy, morphology, chemistry, physiology, growth and reproduction are affected by daily light integral (DLI; mol photons m^-2  d^-1 ). A large database including 500 experiments with 760 plant species enabled us to determine generalized dose-response curves. Many traits increase with DLI in a saturating fashion. Some showed a more than 10-fold increase over the DLI range of 1-50 mol m^-2  d^-1 , such as the number of seeds produced per plant and the actual rate of photosynthesis. Strong decreases with DLI (up to three-fold) were observed for leaf area ratio and leaf payback time. Plasticity differences among species groups were generally small compared with the overall responses to DLI. However, for a number of traits, including photosynthetic capacity and realized growth, we found woody and shade-tolerant species to have lower plasticity. We further conclude that the direction and degree of trait changes adheres with responses to plant density and to vertical light gradients within plant canopies. This synthesis provides a strong quantitative basis for understanding plant acclimation to light, from molecular to whole plant responses, but also identifies the variables that currently form weak spots in our knowledge, such as respiration and reproductive characteristics.

Shading and the capture of localized soil nutrients: nutrient contents, carbohydrates, and root uptake kinetics of a perennial tussock grass

The ability to exploit spatial and temporal heterogeneity in soil resources can be one factor important to the competitive balance of plants. Competition above-ground may limit selective plant responses to below-ground heterogeneity, since mechanisms such as root proliferation and alterations in uptake kinetics are energy-dependent processes. We studied the effect of shading on the ability of the perennial tussock grassAgropyron desertorum to take up nutrients from enriched soil microsites in two consecutive growing seasons. Roots of unshaded plants selectively increased phosphate uptake capacity in enriched soil microsites (mean increases of up to 73%), but shading eliminated this response. There were no changes in ammonium uptake capacity for roots in control and enriched patches for either shaded or unshaded plants. The 9-day shade treatments significantly reduced total nonstructural carbohydrate (TNC) concentrations for roots in 1990, but had no apparent effect on root carbohydrates in 1991 despite dramatic reductions in shoot TNC and fructan concentrations. Enrichment of the soil patches resulted in significantly greater phosphate concentrations in roots of both shaded and unshaded plants, with less dramatic differences for nitrogen and no changes in potassium concentrations. In many respects the shaded plants did surprisingly well, at least in terms of apparent nutrient acquisition. The effects of aboveground competition on nutrient demand, energy requirements, and belowground processes are discussed for plants exploiting soil resource heterogeneity.

Dry matter partitioning and root length/leaf area ratios in herbaceous perennial plants with diverse altitudinal distribution

Partitioning patterns in 22 exclusively low and 27 exclusively high altitude perennial herbaceous species were examined in order to test the hypothesis that plants of high altitudes allocate more dry matter to below-ground parts and in particular to storage organs, than typical low altitude plants. Our results raise some doubts about the general validity of this hypothesis. The mean fractions of total dry matter allocated to green leaves (22±2% s.e. at low and 24±2% at high altitude) and special storage organs (28±4% at both altitudes) do not differ significantly among sites. The mean relative portions of total dry matter allocated to above-ground plant parts amount to 57±3% at low and 42±3% at high elevation (P=0.002) and differ less than often assumed. The greater below-ground fraction at high altitude results from reduced stem and proportionally increased fine root compartments. At high altitude specific root length is increased by 50% and mean individual rooting density is tripled. Fine root length per unit leaf area is 4.5 times greater (P<0.001). However, interspecific variation in all these quantities is considerable and species with quite contrasting partitioning patterns coexist at both elevations. This suggests that the success of perennial herbaceous plants at high elevations does not necessarily depend on a large below ground biomass fraction. The increased fine root length at high altitude may substitute for reduced mycorrhizal infection. Figure 1 provides a graphical summary.