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Light quality and, in particular, its content of blue light is involved in plant functioning and morphogenesis. Blue light variation frequently occurs within a stand as shaded zones are characterized by a simultaneous decrease of PAR and blue light levels which both affect plant functioning, for example, gas exchange. However, little is known about the effects of low blue light itself on gas exchange. The aims of the present study were (i) to characterize stomatal behaviour in Festuca arundinacea leaves through leaf gas exchange measurements in response to a sudden reduction in blue light, and (ii) to test the putative role of Ci on blue light gas exchange responses. An infrared gas analyser (IRGA) was used with light transmission filters to study stomatal conductance (gs), transpiration (Tr), assimilation (A), and intercellular concentration of CO(2) (Ci) responses to blueless PAR (1.80 mumol m(-2) s(-1)). The results were compared with those obtained under a neutral filter supplying a similar photosynthetic efficiency to the blueless PAR filter. It was shown that the reduction of blue light triggered a drastic and instantaneous decrease of gs by 43.2% and of Tr by 40.0%, but a gradual stomatal reopening began 20 min after the start of the low blue light treatment, thus leading to new steady-states. This new stomatal equilibrium was supposed to be related to Ci. The results were confirmed in more developed plants although they exhibited delayed and less marked responses. It is concluded that stomatal responses to blue light could play a key role in photomorphogenetic mechanisms through their effect on transpiration.

In grassland plant communities, the ability of individual plants to regrow after defoliation is of crucial importance since it allows the restoration of active photosynthesis and plant growth. The aim of this study was to evaluate the effects of increasing defoliation intensity (0, 25, 65, 84, and 100% of removed leaf area) on sugar remobilization and N uptake, remobilization, and allocation in roots, adult leaves, and growing leaves of ryegrass over 2 days, using a 15N tracer technique. Increasing defoliation intensity decreased plant N uptake in a correlative way and increased plant N remobilization, but independently. The relative contribution of N stored before defoliation to leaf growth increased when defoliation intensity was severe. In most conditions, root N reserves also contributed to leaf regrowth, but much less than adult leaves and irrespective of defoliation intensity. A threshold of defoliation intensity (65% leaf area removal) was identified below which C (glucose, fructose, sucrose, fructans), and N (amino acids, soluble proteins) storage compounds were not recruited for regrowth. By contrast, nitrate content increased in elongating leaf bases above this threshold. Wounding associated with defoliation is thus not the predominant signal that triggers storage remobilization and controls the priority of resource allocation to leaf meristems. A framework integrating the sequential events leading to the refoliation of grasses is proposed on the basis of current knowledge and on the findings of the present work. [ABSTRACT FROM AUTHOR]

Abstract: Light quality plays a key role in higher plants’ morphogenesis. In most plant models, light is considered as a consumable resource and plants are assumed blind to light signals. However, prior to any effort for modelling photomorphogenetic mechanisms, it is necessary to characterise the spatial distribution of the Morphogenetically Active Radiation (MAR) over and within plant canopies. Measurements of local photosynthetic photon flux density (PPFD) and broadband irradiance (Es) are easy to carry out by using small sensors. Thus, the distribution of the MAR within a canopy can be estimated whenever the functional relationships between these measurements and the photon flux within any spectral band are known. The objective of this work was to determine these functional relationships from the light spectra received above and at various positions around a target plant within a growing sorghum crop. The MAR components considered in this work are related either to photon flux densities in various wavebands between 330 and 950nm or to the ratio between two photon flux densities. A part of the photon flux-related variables is strictly included in the PAR band and might be estimated from PPFD measurements using linear relationships. The other variables are related to both PPFD and Es by multiple linear relationships. The phytochrome photoequilibrium and the red to far-red ratios were related to the relative transmitted PPFD and to the ratio PPFD/Es within the canopy using a non-linear model. Models were validated against an independent set of data. We demonstrate that the MAR components within a sorghum crop can be accurately estimated with the functional relationships presented in this paper. [Copyright &y& Elsevier]

Sorbitol, together with sucrose and starch, is a major final product of photosynthesis in most species of the Rosaceae. However, little is known concerning the factors affecting the carbon fluxes into these three carbohydrates. The aim of this study was to investigate the effect of net photosynthetic CO2 assimilation rate (NAR) on primary carbon partitioning between sorbitol, sucrose, starch and other metabolites, and on the kinetic pattern of labelling in mature leaves of peach (Primus persica [L.] Batsch). A pulse-chase labelling experiment was conducted on a group of peach seedlings showing a range of NAR's that were obtained by using five photosynthetic photon flux density treatments. Mature leaves undergoing one of these treatments were subjected to a 4-min pulse of 14CO2 followed by a 2- to 120-min chase under 12CO2. The radioactivity in lipids, proteins and in a residual nonsoluble fraction was negligible. After a 2-min chase the organic ions were strongly labelled, accounting for up to 40% of the total incorporated radioactivity at NAR's below 10.4 µmol CO2 m-2s-1. After a 30-min chase, most label was found in sorbitol, sucrose and starch with very little radioactivity in the ionic fractions. The analysis of the partitioning of the newly-fixed carbon showed that at low NAR the 14C flux was mostly into sorbitol rather than into sucrose (ratio = 2.6) or starch. When the NAR was lower than 6.1 µmol CO2 m-2s-1, starch was lightly labelled. As the NAR increased above 10.4 µmol CO2 m-2s-1 the 14CO2 diverted into starch increased dramatically. This study demonstrates that at low NAR's the newly-fixed CO2 is mainly used for sorbitol synthesis. However, as NAR increases, the participation of sucrose and starch in CO2 partitioning are favoured. These results support the hypothesis that primary carbon partitioning depends on photosynthetic rate. [ABSTRACT FROM AUTHOR]