Manfred Küppers

Ecological significance of above-ground architectural patterns in woody plants: A question of cost-benefit relationships

Different branching patterns and their repetitive expression during growth of woody plants can lead to different growth forms such as shrubs and trees, although they may also result in similar crown shapes. Recent work has shown that an integrated view of carbon gain, increment of biomass and its architectural arrangement in space is essential in assessing cost-benefit relationships of crown formation and structure, especially in situations where crowns compete for space and light.

Short-term and long-term effects of plant water deficits on stomatal response to humidity in Corylus avellana L.

Short-term (hours) changes in plant water status were induced in hazel (Corylus avellana L.) by changing the evaporative demand on a major portion of the shoot while maintaining a branch in a constant environment. Stomatal conductance of leaves on the branch was influenced little by these short-term changes in water status even with changes in leaf water potential as great as 8 bars. Long-term (days) changes in plant water status were imposed by soil drying cycles. Stomatal conductance progessively decreased with increases in long-term water stress. Stomata still responded to humidity with long-term water stress but the range of the conductance response decreased. Threshold responses of stomata to leaf water potential were not observed with either short-term or long-term changes in plant water status even when leaves wilted. It is suggested that concurrent measurements of plant water status may not be sufficient for explaining stomatal and other plant responses to drought.

Responses to Humidity by Stomata of Nicotiana glauca L. And Corylus avellana L. Are Consistent With the Optimization of Carbon Dioxide Uptake With Respect to Water Loss

Intact leaves of N. glauca and C. avellana were exposed to a range of humidities and their gas exchange monitored. Rates of transpiration and assimilation of carbon dioxide, and their sensitivities to changes in total conductance (leaf and boundary layer) were determined. The ratio of these sensitivities, δE/δA, remained substantially constant over the range of humidities. The results represent the first experimental support for a recent hypothesis that stomata vary their apertures in such a manner as to keep δE/δA constant, which optimizes carbon gain with respect to water loss.

Leaf gas exchange of beech (Fagus sylvatica L.) seedlings in lightflecks: effects of fleck length and leaf temperature in leaves grown in deep and partial shade

SummaryResponses of leaf gas exchange in shade and half-shade grown seedlings of the European beech, Fagus sylvatica L., to constant light conditions indicate different phases of photosynthetic induction: an immediate, a fast and a subsequent slow phase. The slow phase has both biochemical and stomatal components. The higher the induction, the higher the lightfleck utilization efficiency (LUE) attributable to a lightfleck. LUE can be higher than 100% compared to a theoretical instantaneous response. Lightfleck quantum yield (total carbon gain attributable to a lightfleck per incident quantum density in the fleck) is highest in short pulses of light. Post-illumination carbon gain initially increases with fleck length, levelling off above 20 s. The lower the induction, the longer carbon is fixed post-illuminatively (up to 84 s) but the less carbon is gained. Shade leaves are induced much faster than partial shade leaves. They utilize series of lightflecks to become fully induced, while half-shade (and sun) leaves depend on continuous high light. Half-shade leaves lose induction faster in low light between lightflecks. High as well as low temperatures strongly delay induction in half-shade but not in shade leaves. In general, shade leaves are much better adapted to the dynamic light environment of the forest understorey; however, their water-use efficiency during induction is lower.

Modelling photosynthesis in fluctuating light with inclusion of stomatal conductance, biochemical activation and pools of key photosynthetic intermediates

Abstract. Photosynthetic carbon gain in rapidly fluctuating light is controlled by stomatal conductance, activation of ribulose-1,5-bisphosphate carboxylase-oxygenase, a fast induction step in the regeneration of ribulose-1,5-bisphosphate, and the build-up of pools of photosynthetic intermediates that allow post-illumination CO2 fixation. Experimental work over recent years has identified and characterised these factors. A physiologically-based dynamic model is described here that incorporates these factors and allows the simulation of carbon gain in response to any arbitrary sequence of light levels. The model output is found to conform well to previously reported plant responses of Alocasia macrorrhiza (L.) G. Don. observed under widely differing conditions. The model shows (i) responses of net assimilation rate and stomatal conductance to constant light levels and different CO2 concentrations that are consistent with experimental observations and predictions of a steady-state model; (ii) carbon gain to continue after the end of lightflecks, especially in uninduced leaves; (iii) carbon gain to be only marginally reduced during low-light periods of up to 2 s; (iv) a fast-inducing component in the regeneration of ribulose-1,5-bisphosphate to be limiting for up to 60 s after an increase in light in uninduced leaves: the duration of this limitation lengthens with increasing CO2 concentration and is absent at low CO2 concentration; (v) oxygen evolution to exceed CO2 fixation during the first few seconds of a lightfleck, but CO2 fixation to continue after the end of the lightfleck whereas oxygen evolution decreases to low-light rates immediately. The model is thus able to reproduce published responses of leaves to a variety of perturbations. This provides good evidence that the present formulation of the model includes the essential rate-determining factors of photosynthesis under fluctuating light conditions.

Compensating effects to growth of carbon partitioning changes in response to SO2-induced photosynthetic reduction in radish

SummaryExposure of plants to SO2 reduced their photosynthetic performance due tio reductions in carboxylating capacity. Although the reduced carbon gain resulted in a lower growth rate of SO2-exposed plants over that of controls, their loss of potential growth was minimized because of proportional increases in allocation to new leaf material.

CO2-assimilation, Transpiration und Wachstum von Pinus silvestris L. bei unterschiedlicher Magnesiumversorgung

ZusammenfassungAuf magnesiumarmen, quarzreichen sandigen Kiesen westlich von Pressath (Oberpfalz, Bayern) wurde der Einfluß der Magnesiumdüngung auf den Kohlenstoff- und Wasserhaushalt von 7jährigen Kiefern untersucht. Bei Mg-Gehalten in den Nadeln unter 0,3 mg pro g Trockengewicht nimmt das Ausmaß der Gelbspitzigkeit der Nadeln sowie ihr Stärkegehalt deutlich zu, während die Chlorophyll- und Stickstoffgehalte sinken. Nettophotosynthese und Atmung sind, bezogen auf die Gesamtnadel, bei Mg-Mangel verringert. Ebenso ist die Lichtabhängigkeit der Stomataöffnung abgeschwächt. Hiervon abweichend wird die Reaktion der Stomata auf Luftfeuchte nicht vom Mg-Ernährungszustand beeinflußt. Da auch die Wassernachleitung in der Pflanze von der Mg-Versorgung unabhängig ist, haben Mangelpflanzen die gleiche Transpiration und die gleichen Wasserpotentiale wie gedüngte Pflanzen. Die verringerte CO2-Aufnahme der Mg-Mangelpflanzen führt aber zu einer Verschlechterung der «Produktivität der Transpiration» und der Tagesbilanz des CO2-Gewinns. Höhenzuwachs und Längenzuwachs der Zweige sind eingeschränkt. Längerfristig führt ein Absinken des Mg-Gehalts der Nadeln unter 0,3 mg pro g Trockengewicht zum Absterben der Pflanzen.SummaryThe effects of various levels of magnesium fertilization on young Scots pines, growing on Mg-deficient soils derived from quaternary gravels in Northern Bavaria, were investigated. Needle concentrations below 0.3 mg Mg per g dry weight were strongly correlated with increasing starch contents and severe needle tip chlorosis. Net photosynthesis and respiration, considering the affected needle as a whole, decreased. Maximum leaf conductance for water vapour as well as stomatal response to humidity were not affected by magnesium deficiency; however, stomatal response to light was reduced. Differences in Mg-nutrition did not affect water flow in the root/leaf pathway. Therefore, similar transpirational water losses were related to similar water potentials with both fertilized and Mg-deficient plants. Lower water-use efficiency and lower daily carbon balances resulted from a reduced CO2 uptake in Mg-deficient plants. Low carbon gain and a change in assimilate partitioning limited height and internodal growth. The plants died at Mg contents of needles lower than 0,3 mg per g dry weight.

Simulation of photosynthetic plasticity in response to highly fluctuating light: an empirical model integrating dynamic photosynthetic induction and capacity

Abstract An empirical model was developed to simulate photosynthetic responses of leaves to highly fluctuating light, with a special focus on the functional role of photosynthetic induction and capacity. Based on diurnal courses of light as input data, which were recorded at natural plant sites, we applied this model to simulate the corresponding course of net photosynthesis (output data) for leaves of two neotropical tree species. All six model input parameters (leaf-specific) were obtained via measurements of leaf gas exchange. The model was tested for leaves in their natural environments, characterized by frequent light-flecks. We compared measured carbon gains with computed ones, using a standard steady-state and our induction model. Simulation runs with the steady-state model can result in an immense overestimation of the true situation, by 13.4% at open sites [pioneer species Heliocarpus appendiculatus (Turczaninow)] and by 86.5% at low light environments of the understorey [mid to late successional species Billia colombiana (Planchon and Lindley)]. These significant overestimations, particularly in the understorey, are mainly the consequence of neglecting a dynamic photosynthetic induction under fluctuating light conditions. The model presented here resulted in clearly improved predictions; in open and understorey sites the true carbon gain of leaves was computed with a mean error of less than 7%. As most leaves at natural plant sites are exposed to light environments allowing for dynamic rather than steady-state CO2 assimilation, the significance of such induction models is evident and is discussed in relation to scaling-up from leaf to canopy and to the whole plant indicating a large potential for errors.

Carbon fixation in eucalypts in the field

SummaryThe rate of CO2 assimilation at light saturation and an intercellular CO2 concentration of 350 μl l-1 (photosynthetic capacity), measured in leaves of Eucalyptus pauciflora, E. behriana, E. delegatensis and Acacia melanoxylon, declined over the course of cloudless days under naturally varying environmental conditions as well as under constant optimal conditions for high CO2 uptake. Since the capacity did not recover during the light period, it was different from the “midday depression” of gas exchange. The change appeared to be caused neither by the diurnal variation of total leaf water potential, by photoinhibition of redox-reaction centres in photosystems nor by changes in the intrinsic properties of Ribulose-bisphosphate carboxylase-oxygenase. The decline was more pronounced in winter than in summer. It was related to the duration of illumination or the cumulative carbon gain. It was reversible in the following dark phase, and it did not occur on changeable days with short peaks of high light.Despite the decline in photosynthetic capacity, the initial slope of the CO2 response of net photosynthesis, as obtained at low intercellular CO2 concentrations, remained constant during the day, but declined at night when photosynthetic capacity recovered. In all cases stomatal conductance varied in parallel with photosynthetic capacity. The relevance of changes in photosynthetic capacity for the intercellular CO2 concentration is discussed.

Photosynthetic induction strongly affects the light compensation point of net photosynthesis and coincidentally the apparent quantum yield

Abstract. The CO2 assimilation of tree saplings in a Costa Rican premontane forest characterized by highly dynamic light was measured using a rapidly responding system for the analysis of leaf gas exchange. Experiments were carried out on light compensation point and apparent (incident) quantum yield of dynamic photosynthesis. Because of the large variability in photosynthetic induction by light, CO2 assimilation cannot be predicted using steady-state response curves.Under fluctuating light conditions the light compensation point and, coincidentally, the apparent incident quantum yield are observed to be highly variable and strongly affected by photosynthetic induction. As a consequence, actual light compensation points of net photosynthesis are considerably higher (up to 250%) and apparent quantum yields are considerably lower (by more than 80%) than those obtained in steady state, both in a low light environment typical for the understorey and at a high light regime in open sites. An explanatory model of induction-dependent whole leaf light compensation points and apparent quantum yields is offered on the basis of an assumed constant quantum yield and light compensation point at the chloroplast level.Most leaves under natural conditions are exposed to highly variable light, either caused by variable cloud cover or by self-shading of leaves in the crown periphery and their movement in turbulence. These effects must not be neglected when considering ecological consequences of low light on shade tolerance of species or when considering carbon balances; they are discussed in relation to their significant effects on carbon gain at a whole plant level.