Benno M. Eller

Road dust induced increase of leaf temperature

Abstract Only a minor part of the absorbed global radiation by plant leaves is used in photosynthesis. The rest is converted into sensible heat influencing energy balance and temperature of leaves. Dust cover on leaves increases their absorptivity and an increase of leaf temperature influencing productivity of the plants is to be expected. Spectral properties and temperatures of leaves covered with road dust and identical but clean leaves were measured. Absorptivity of dusty leaves is more than doubled for the waverange 700 to 1350 nm. Temperatures of dusty leaves are 2 to 4 °C higher than those of clean leaves. Absorbed incident global radiation was calculated in order to discover which waverange of incident radiation caused the temperature rise in dusty leaves. Absorbed energy in the wavelengths over 700 nm is more than doubled for dusty leaves and is the major factor causing overheating.

The correlation between crassulacean acid metabolism and water uptake in Senecio medley-woodii

The combination of a chamber for CO2 gas exchange with a potometric measuring arrangement allowed concomitant investigations into CO2 gas exchange, transpiration and water uptake by the roots of whole plants of Senecio medley-woodii, a species which exhibits Crassulacean acid metabolism. The water-uptake rate showed the same daily pattern as malate concentration and osmotic potential. The accumulation of organic acids resulting from nocturnal CO2 fixation enhanced the water-uptake rate from dusk to dawn. During the day the water-uptake rates decreased with decreasing organic-acid concentration. With gradually increasing water stress, CO2 dark fixation of S. medley-woodii was increased as long as water could be taken up by the roots. It was also shown that a reestablished water supply after drought caused a similar increase which in both cases ameliorated the water uptake in order to conserve a positive water balance for as long as possible. This water-uptake pattern shows that Crassulacean acid metabolism is not only a water-saving adaptation but also enhances water uptake and is directly correlated with the amelioration of the plant water status.

Availability of water controls Crassulacean acid metabolism in succulents of the Richtersveld (Namib desert, South Africa)

Features of Crassulacean acid metabolism (CAM) were studied in a variety of different succulents in response to climatic conditions between March 1977 and October 1983 in the southern Namib desert (Richtersveld). A screening in 1977 and 1978 revealed that nearly all investigated succulents performed a CAM, but overnight accumulation of malate declined gradually with decreasing soil water potential, tissue osmotic potential, and leaf water content. This was further substantiated by an extended period of insufficient rainfall in 1979 and 1980 which damaged the evergreen CAM succulents between 80 and 100%. In most of the species still living, neither CO2-gas exchange nor diurnal acid fluctuation, indicative of CAM, could be detected unless an abundant rainfall restored both CAM features. Plants persisted in a stage of latent life.Water supply is one necessary prerequisite for CAM in the Richtersveld. But even well-watered plants with CAM were sensitive to short-term water stress caused by high water-vapour partialpressure deficit (VPD) in the night, which reduced or prevented CO2 uptake and resulted in a linear relation between overnight accumulated malate and VPD. The results do not support the opinion that, for the Namib succulents, CAM is an adaptive mechanism to water stress since long-term and short-term water stress stopped nocturnal malate synthesis, but instead lead to the conclusion that nocuturnal CO2 fixation is only performed when the water status of the plant can be improved simultaneously.

Desert succulents and their life strategies

A succulent is a plant with water storing tissue, but succulence clearly is a quality that can be possessed to a higher or lesser degree. This paper gives a definition of succulence and discusses problems concerning Delfs measure of succulence. A new measure, Succulence Quotient, is proposed. It measures the amount of water that a plant can store at the expenditure of one gram of organic matter. We demonstrate this measure on a number of plants from the Namib desert, southern Africa, and compare it with the measured caloric values of the plant tissues. We also discuss life cycles of desert succulents in terms of utilizable and structural biomass and water. We contrast the concepts of growth form, life form and life strategy, and propose, on the basis of life cycle characteristics, an outline classification of life strategies of desert succulents. One detail of this outline classification is further worked out and illustrated for succulents from the Namib desert.

Die strahlungsökologische Bedeutung von Epidermisauflagen

Summary The definition of superimposed layers is given and the ecological significance of their main types such as hairs, wax bloom, and dust covers is summarised. The influence of such layers on the spectral properties of leaves is analyzed and compared using published and new data. Comparisions are made for the visible (400 to 750 nm) and the near infrared (750 to 1350 nm) wave lenghts. Wheigthed means of the spectral propcrties for a given distribution of spectral intensity of the global radiation are calculated. Epidermal hairs and wax bloom reduce the amount of absorbed global radiation mainly as the result of a much increased reflectivity in the visible wave range. For the near infrared region of the spectrum only little influence of hairs or wax bloom on the spectral properties of leaves is determinable. The lowered absorption of energy by leaves covered with hairs or wax bloom reduced heat input to the leaves and therefore a smaller rise of the leaf temperature above the air temperature is observed. The light reflected by the hairs or the wax bloom contributes to a better illumination of leaves in the shade of others. This diffuse reflected light may increase photosynthesis at least if the photosynthetic apparatus is not light saturated without this stray light. Dust layers, mainly if they are of a dark colour increase markedly the absorption of global radiation by the combination leaf-dust compared with a clean leaf. The major part of this increase occours for the infrared radiation. A higher energy input to the leaf from the absorbed additional global radiation and a rised leaf temperature are the result.

Purine alkaloid formation and CO2 gas exchange in dependence of development and of environmental factors in leaves of Coffea arabica L.

In the leaves of Coffea arabica L., purine alkaloid formation was estimated by analyzing the theobromine and caffeine content and by measuring the methylation rate of [2-14C]theobromine to [2-14C]caffeine in short-term experiments (6–24 h). At the same time, growth (in terms of dry weight and area), net photosynthesis (NPS), and dark respiration were determined. During leaf development, which was considered to be terminated when NPS was at a maximum (60–80 μmol g-1 s-1) and dark respiration at a minimum (5–7.5 μmol g-1 s-1), the content of theobromine and the velocity of caffeine formation were both found to decrease by a factor of more than 100. The close correlation between the theobromine content and the methylation rate is suspended when purine alkaloid formation is influenced by factors other than leaf development. Among these factors, temperature is the most effective: the velocity of caffeine biosynthesis is increased by raising the temperature and vice versa. Although the plants were well irrigated, a drastic decrease of NPS in the afternoon was observed under all environmental conditions tested. Light saturation was reached between 170–360 μmol m-2 s-1. The temperature optimum of NPS was shown to be very broad (24–33°C)m provided the adaptation time was sufficiently long.

Modulation of CAM and Water Balance of Senecio medley-woodii by Environmental Factors and Age of Leaf

Summary Investigations into CO 2 gas exchange, transpiration, and water uptake by the roots of whole plants of Senecio medley-woodii , a species which exhibits Crassulacean acid metabolism were made with the combination of a CO 2 gas exchange chamber and a potometric measuring arrangement.The water uptake during the night, concomitant with the nocturnal formation of malate, is a main factor which brings about the amelioration of the water status.The detailed analysis of the leaf water relations demonstrates how nocturnal malate accumulation and the daytime consumption of malate modulate the water potential of the leaves and thus the water uptake pattern.The closely connected modulation of nocturnal malate accumulation and the stomatal movement by the actual plant water status, allows S.medley-woodii to maintain a positive water balance of 5 to 7 mol m -2 even under conditions of slight drought stress.The extent of the daily stomatal opening depends on the nocturnal positive water balance.A cool and humid day and night resulted in the typical CAM pattern with a high CO 2 dark fixation and a high positive water balance.A thermo- (28/17°C; day/night) and hygroperiod (35%/80%) provoked a drastic reduction in CO 2 uptake during both the light and the dark period.The daily water balance remained positive.A high air temperature and a low relative humidity (28°C and 35 %) maintained day and night abolished the diurnal malate rhythmicity.The water balance under these conditions was negative. S.medley-woodii showed an accentuation of the diurnal variation of malate and of CO 2 dark fixation with leaf development but the transpiration rates of young leaves exceeded those of mature ones.Nevertheless young leaves had a greater positive water balance than mature and intermediate leaves.

Öffnen der Stomata bei hoher Temperatur im Dunkeln

SummaryTranspiration and absorption of water by the roots of Piper betle L. have been measured simultaneously with intact single-leaf plants. The degree of stomatal opening has been calculated from leaf temperature, air humidity and wind speed. The roots were aerated at intervals.In an atmosphere of unchanged composition, the stomata of Piper betle L. remain closed or only slightly open at a leaf temperature of 36–38° C in the dark and at night. However, if the temperature is raised above 40° C, they open rapidly after a short time lag to a degree normally met only under daylight conditions.Opening occurs independently of the length of the preceding dark period and hour of the night. The high degree of opening in the dark does not change considerably for many hours, and is more pronounced at high air humidity. At temperatures above 40° C the degree of opening is little influenced by the change from light to dark or from dark to light.The results give evidence that heat resistance could be strongly affected by stomatal reactions at high temperatures. The maximum temperature during the experiments did not exceed 45° C. To ensure that Piper betle L. had not been damaged by the high temperature the experiments were repeated with the same plants on several consecutive days, and the same results were obtained.Close agreement was found between the highest possible stomatal conductance derived from transpiration and that calculated from scanning electron microscopic analysis.Transpiration and absorption of water by the roots of Piper betle L. have been measured simultaneously with intact single-leaf plants. The degree of stomatal opening has been calculated from leaf temperature, air humidity and wind speed. The roots were aerated at intervals.In an atmosphere of unchanged composition, the stomata of Piper betle L. remain closed or only slightly open at a leaf temperature of 36-38° C in the dark and at night. However, if the temperature is raised above 40° C, they open rapidly after a short time lag to a degree normally met only under daylight conditions.Opening occurs independently of the length of the preceding dark period and hour of the night. The high degree of opening in the dark does not change considerably for many hours, and is more pronounced at high air humidity. At temperatures above 40° C the degree of opening is little influenced by the change from light to dark or from dark to light.The results give evidence that heat resistance could be strongly affected by stomatal reactions at high temperatures. The maximum temperature during the experiments did not exceed 45° C. To ensure that Piper betle L. had not been damaged by the high temperature the experiments were repeated with the same plants on several consecutive days, and the same results were obtained.Close agreement was found between the highest possible stomatal conductance derived from transpiration and that calculated from scanning electron microscopic analysis.

Die Bedeutung der Wachsausblühungen auf Blättern von Kalanchoë pumila BAKER für die Absorption der Globalstrahlung

Summary Spectral properties of leaves of Kalanchoe pumila Bakeb were measured for the waverange from 400 to 1350 nm. The significance of the wax bloom for the spectral properties and the absorption of global radiation was determined. The energy absorption of juvenile leaves is increased up to 55% if the wax bloom is removed. The major part of this additional energy input arises from an increased absorptivity in the visible part of the spectrum (400 ... 750 nm), as the result of lowered reflection at the upper surface. For the infrared waverange the wax bloom of both the upper and the lower surface contribute to increase the reflectivity of the upper surface. If the wax bloom is removed only a slight increase of the absorptivity results since the effect caused by lowered reflection is nearly compensated by rised transmission values.

Responses of Crassulacean Acid Metabolism (CAM) to Increasing and Decreasing Water Stress in Plants in the Southern Namib Desert

CAM features in a variety of different succulents in response to climatic changes between March 1977 and April 1981 were studied in the southern Namib desert (Richtersveld). In 1977 and 1978 all investigated succulents performed a CAM. After an extended period of insufficient rainfall which damaged the succulents between 80 and 100% most species did not show a measurable CO2 gas exchange or diurnal acid fluctuation, indicative of CAM. But both features were restored quickly after an abundant rainfall in March 1981.