Wolfram Beyschlag

Progress in Botany

In the past 55 years, enormous scientific progress was made in many fields of plant physiology and plant biochemistry. Throughout these years, our knowledge on the photosynthetic light processes, the chemical composition and H.K. Lichtenthaler (*) Botanical Institute 2 (Molecular Biology and Biochemistry of Plants), Karlsruhe Institute of Technology (KIT), University Division, Kaiserstr. 12, 76131 Karlsruhe, Germany e-mail: hartmut.lichtenthaler@kit.edu

Two different strategies of Mediterranean macchia plants to avoid photoinhibitory damage by excessive radiation levels during summer drought

The adaptive strategies to high radiation and water stress of the drought tolerant evergreen sclerophylls Quercus coccifera and Arbutus unedo are compared to those of the semi-deciduous Cistus spp. (C. albidus and C. monspeliensis). Cistus spp. partially avoided drought by a marked reduction of their transpirational surface through leaf abscission during summer, when predawn water potential declined below -5.5 MPa. Chlorophyll fluorescence measurements revealed a reversible diurnal decrease of maximum photochemical efficiency of PSII (F-v/F-m), which became more accentuated during summer drought in all species. An important strategy to avoid damage by excessive radiation levels in Cistus spp. was the structural regulation of light interception through leaf angle changes, from a more horizontal orientation in spring ( 70 degrees). Horizontal orientated leaves were highly susceptible to photoinhibition, and excessive radiation often resulted in irreversible photodamage followed by leaf abscission during summer, whereas vertical leaf orientation appeared to protect the leaf from severe photoinhibition. Still, these mechanisms were not fully successful in avoiding chronic photoinhibition, and predawn F-v/F-m values remained low in Cistus spp. during summer (only exhibiting a partial overnight recovery). Evergreen sclerophylls were less susceptible to photoinhibition, and the diurnal decline in F-v/F-m remained fully reversible during drought. Structural regulation of light interception was not found to be an important strategy in these species, and only small, though significant changes in leaf angle occurred. The ecological importance of the adaptive strategies of each functional group is discussed

Characteristic patterns of chronic and dynamic photoinhibition of different functional groups in a Mediterranean ecosystem

To analyse characteristic patterns of dynamic and chronic photoinhibition within a plant community, a new technique is proposed, which is based on the long- and short-term recovery time of maximum photochemical efficiency of PSII (F v/F m) after environmental stress. Chronic photoinhibition was determined as a sustainable decrease in predawn F v/F m, occurring during periods of prolonged stress, whereas dynamic photoinhibition was assessed from the fully reversible diurnal decline in F v/F m. Applied to a Mediterranean macchia ecosystem, this definition allowed the characterization of typical annual patterns of chronic and dynamic photoinhibition. Both types of photoinhibition were highest during summer drought. However, differences emerged among the ten dominant macchia species regarding their susceptibility to chronic photoinhibition during different seasons. Chronic and dynamic photoinhibition were dependent on leaf orientation. Semi-deciduous species avoided enhanced chronic photoinhibition through a reduction of excessive light interception by vertical foliage orientation during summer, whereas evergreen sclerophylls did not exhibit pronounced structural photoprotective mechanisms. Chronic and total photoinhibition were significantly correlated with predawn and midday water potentials, respectively, and a grouping of the macchia species into three functional groups is proposed according to this relationship.

Plant Competition for Light Analyzed with a Multispecies Canopy Model. I. Model Development and Influence of Enhanced UV-B Conditions on Photosynthesis in Mixed Wheat and Wild Oat Canopies

SummaryCompetition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290–320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B.

Stomatal patchiness in Mediterranean evergreen sclerophylls : Phenomenology and consequences for the interpretation of the midday depression in photosynthesis and transpiration.

Midday depression of net photosynthesis and transpiration in the Mediterranean sclerophylls Arbutus unedo L. and Quercus suber L. occurs with a depression of mesophyll photosynthetic activity as indicated by calculated carboxylation efficiency (CE) and constant diurnal calculated leaf intercellular partial pressure of CO2 (Ci). This work examines the hypothesis that this midday depression can be explained by the distribution of patches of either wide-open or closed stomata on the leaf surface, independent of a coupling mechanism between stomata and mesophyll that results in a midday depression of photosynthetic activity of the mesophyll. Pressure infiltration of four liquids differing in their surface tension was used as a method to show the occurrence of stomatal patchiness and to determine the status of stomatal aperture within the patches. Liquids were selected such that the threshold leaf conductance necessary for infiltration through the stomatal pores covered the expected diurnal range of calculated leaf conductance (g) for these species. Infiltration experiments were carried out with leaves of potted plants under simulated Mediterranean summer conditions in a growth chamber. For all four liquids, leaves of both species were found to be fully infiltratable in the morning and in the late afternoon while during the periods leading up to and away from midday the leaves showed a pronounced patchy distribution of infiltratable and non-infiltratable areas. Similar linear relationships between the amount of liquid infiltrated and g (measured by porometry prior to detachment and infiltration) for all liquids clearly revealed the existence of pneumatically isolated patches containing only wide-open or closed stomata. The good correspondence between the midday depression of CE, calculated under the assumption of no stomatal patchiness, and the diurnal changes in non-infiltratable leaf area strongly indicates that the apparent reduction in mesophyll activity results from assuming no stomatal patchiness. It is suggested that simultaneous responses of stomata and mesophyll activity reported for other species may also be attributed to the occurrence of stomatal patchiness. In Quercus coccifera L., where the lack of constant diurnal calculated Ci and major depression of measured CE at noontime indicates different stomatal behavior, non-linear and dissimilar relationships between g and the infiltratable quantities of the four liquids were found. This indicates a wide distribution of stomatal aperture on the leaf surface rather than only wide-open or closed stomata.

Seasonal changes in net photosynthesis rates and photosynthetic capacity in leaves of Cistus salvifolius, a European mediterranean semi-deciduous shrub

SummaryDuring five different periods between Nov. 1982 and Aug. 1983, the diurnal patterns exhibited in photosynthetic CO2 uptake and stomatal conductance were observed under natural conditions on twigs of Cistus salvifolius, a Mediterranean semi-deciduous shrub which retains a significant proportion of its leaves through the summer drought. During the same periods, net photosynthesis at saturating CO2 partial pressure was measured on the same twigs as a function of irradiance at different temperatures. From these data, photosynthetic capacity, defined here as the CO2- and light-saturated net photosynthesis rate, was obtained as a function of leaf temperature. C. salvifolius is a winter growing species, shoot growth being initiated in Nov. and continuing through May. Photosynthetic capacity was quite high in Nov., March and June, exceeding 40 μmol m-2 s-1 at optimum temperature. In Dec., photosynthetic capacity was somewhat reduced, perhaps due to low night-time temperatures (<5°C) during the measurement period. In Aug., capacity in oversummering shoots at optimum temperature fell to less than 8 μmol m-2 s-1, due to water trees and perhaps leaf aging. Seasonal changes in maximal photosynthetic rates under ambient conditions were similar, and like those found in co-occurring evergreen sclerophylls. Like the evergreens, Cistus demonstrated considerable stomatal control of transpirational water loss, particularly in oversummering leaves. During each measurement period except Aug. when capacity was quite low, the maximum rates of net photosynthesis measured under ambient conditions were less than half the measured photosynthetic capacities at comparable temperatures, suggesting an apparent excess nitrogen investment in the photosynthetic apparatus.

Does plant competition intensity rather depend on biomass or on species identity

In two experiments, we tested whether species specific traits or mainly biomass determines the competitive strength of plant individuals in resource-poor habitats. As measure of competition intensity, we calculated the log Response Ratio (lnRR) based on total biomass for three key species of early successional stages on inland dunes. Using seedlings of Corynephorus canescens and Hieracium pilosella in a pot experiment, competition intensity was significantly and positively correlated with the biomass of the respective competitors. In contrast, such a correlation was not detected in a controlled field experiment with adult plants of the two species and of Carex arenaria. However, in both experiments the strength of competitive interactions (measured as lnRR) significantly depended on the identity of the competing species. We conclude, that a biomass advantage over the competitors (which can for instance be achieved by earlier germination) seems to play a crucial role only for successful seedling establishment, while competitive interactions of neighbouring plants depend on species-specific biomass allocation strategies at both developmental stages. Um zu testen, ob artspezifische Eigenschaften oder hauptsachlich die Biomasse selbst die Konkurrenzstarke zwischen Pflanzen auf ressourcenarmen Standorten bestimmen, wurde der Log Response Ratio (lnRR) der Gesamtbiomasse fur drei dominante Arten fruher Sukzessionsstadien auf Sand bestimmt. In einem Topfexperiment, in dem Keimlinge der Arten Corynephorus canescens und Hieracium pilosella verwendet wurden, nahm die Konkurrenz mit zunehmender Biomasse umgebender Keimlinge signifikant zu. In einem zweiten Experiment unter kontrollierten Freilandbedingungen, dagegen, in dem ausgewachsene Pflanzen der gleichen Arten zusammen mit Carex arenaria gemessen wurden, war die Konkurrenzintensitat nicht signifikant von der Biomasse benachbarter Pflanzen abhangig. Artspezifische Eigenschaften hatten jedoch einen signifikanten Einfluss auf die Konkurrenzstarke sowohl von Keimlingen als auch von alteren Pflanzen. Wir folgern daraus, dass ein zeitlicher Vorsprung bei der Besiedlung offener Flachen, der mit hoherer Biomasse einher geht, fur einen Keimling einen entscheidenden Konkurrenzvorteil bietet. Mit zunehmendem Alter jedoch nimmt der Einfluss der Biomasse auf das Konkurrenzgeschehen ab und die artspezifische BiomasseAllokation gewinnt mehr und mehr an Bedeutung.

High competitiveness of a resource demanding invasive acacia under low resource supply

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via 15N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H.halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.

Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation

Four herbaceous plant species of a sand dune area and several herb species of an open early-successional patch were investigated for the occurrence of a simple relationship between aboveground biomass and plant cover. Without exception linear regressions of aboveground biomass on plant cover were found with slope factors depending on the growth form of the species. These results suggest that (early) growth of herbaceous plants in low and (temporarily) open vegetation is not affected by possible constraints caused by a decreasing ratio of plant cover to aboveground biomass. The obtained linear relationships could be used for rapid non-destructive determination of aboveground biomass by image-analysis of cover data.

Annual foraging of the leaf-cutting ant Atta colombica in a semideciduous rain forest in Panama

In a 1-y study of vegetation harvested by the leaf-cutting ants, Atta Colombica Guerin, daily harvesting activity of two nests was observed for 24 h at c. I-wk intervals (colony I: June 1993-June 1994; colony II: February-June 1994) on Barro Colorado Island. The average daily quantity of green leaves harvested by colony I was higher during the wet season (11.4 m(2) d(-1)) than during the dry season (9.0 m(2) d(-1)), but was highly variable between survey days. Total annual herbivory of green leaves was estimated to be 3,855 m(2) foliage area for colony I and 1,707 m(2) for colony II. Total dry weight of biomass harvested was higher in the dry season because most material collected during the wet season consisted of green leaves, while during the dry season, more than 50% of the total collected biomass was non-green plant material (stipules of Ficus sp., fruits, seeds, and flower parts of a variety of other species) which represented c. one third (ill kg y(-1)) of the total annual intake (370 kg y(-1)) of plant material. Total daily biomass intake was negatively correlated with daytime rainfall. The peak of daily foraging was affected by timing and duration of rainfall events. Highest input rates normally occurred between 15:00 and 16:00 h (colony I). Dry weight and surface area of harvested leaf fragments differed between plant species, with thicker leaves generally being cut into smaller pieces. Significant linear correlations were found between total daily harvest of fragments and the respective harvesting rate at the maximum of daily activity. High correspondence was found between estimates using this relationship and the measured daily leaf harvest of four other Atta colonies and of two colonies reported in published literature. The use of this relationship as a research tool is discussed.