Sun-induced fluorescence - a new probe of photosynthesis: First maps from the imaging spectrometer HyPlant.

Variations in photosynthesis still cause substantial uncertainties in predicting photosynthetic CO2 uptake rates and monitoring plant stress. Changes in actual photosynthesis that are not related to greenness of vegetation are difficult to measure by reflectance based optical remote sensing techniques. Several activities are underway to evaluate the sun-induced fluorescence signal on the ground and on a coarse spatial scale using space-borne imaging spectrometers. Intermediate-scale observations using airborne-based imaging spectroscopy, which are critical to bridge the existing gap between small-scale field studies and global observations, are still insufficient. Here we present the first validated maps of sun-induced fluorescence in that critical, intermediate spatial resolution, employing the novel airborne imaging spectrometer HyPlant. HyPlant has an unprecedented spectral resolution, which allows for the first time quantifying sun-induced fluorescence fluxes in physical units according to the Fraunhofer Line Depth Principle that exploits solar and atmospheric absorption bands. Maps of sun-induced fluorescence show a large spatial variability between different vegetation types, which complement classical remote sensing approaches. Different crop types largely differ in emitting fluorescence that additionally changes within the seasonal cycle and thus may be related to the seasonal activation and deactivation of the photosynthetic machinery. We argue that sun-induced fluorescence emission is related to two processes: (i) the total absorbed radiation by photosynthetically active chlorophyll; and (ii) the functional status of actual photosynthesis and vegetation stress.

Interaction of rilmenidine and clonidine with pre- and postjunctional alpha-adrenoceptors in rat and rabbit blood vessels and in rat kidneys.

In several blood vessels, the influence of the alpha 2-adrenoceptor agonists rilmenidine and clonidine was compared. In aortas of rat and rabbit and in the rabbit pulmonary artery, both compounds evoked contractions due to stimulation of postjunctional alpha 1-adrenoceptors. In the aorta of the rat, but not in that of the rabbit, removal of the endothelium markedly enhanced the contractions to rilmenidine and clonidine. At the alpha 1-adrenoceptors, clonidine was about 135 times more potent than rilmenidine. The activity of both substances at post- and prejunctional alpha 2-adrenoceptors was compared in the rabbit saphenous vein. Rilmenidine and clonidine evoked contractions of the vein by stimulating postjunctional alpha 2-adrenoceptors and decreased the stimulation-induced overflow of [3H]-noradrenaline by activating the prejunctional alpha 2-adrenoceptors. At the post- and prejunctional alpha 2-adrenoceptors, clonidine was about 30 times more potent than rilmenidine. These data illustrate that, although less potent than clonidine, rilmenidine is 5 times more specific for the alpha 2-adrenoceptors. In the isolated perfused rat kidney, rilmenidine and clonidine antagonized the vasoconstrictions induced by noradrenaline. Although the exact mechanism of this inhibitory response remains to be elucidated, our results indicate that rilmenidine may possess some interesting properties at the level of the renal circulation.