Effect of near-infrared irradiation on photoconversion of the water-soluble chlorophyll-binding protein of Chenopodium album.

We investigated the effects of near-infrared irradiation on the photoconversion of Chenopodium album water-soluble chlorophyll-binding protein (CaWSCP) in the presence of sodium hydrosulfite and found a further photoconversion from CP742 to CP763, a novel form of CaWSCP. Interestingly, one-third of the absorption peak at 668 nm was recovered in CP763, but re-irradiation under oxidative conditions eliminated the photo convertibility of CaWSCP.

Not only light quality but also mechanical stimuli are involved in height convergence in crowded Chenopodium album stands.

In crowded stands, height is often similar among dominant plants, as plants adjust their height to that of their neighbours (height convergence). We investigated which of the factors, light quality, light quantity and mechanical stimuli, is primarily responsible for stem elongation and height convergence in crowded stands. We established stands of potted Chenopodium album plants. In one stand, target plants were surrounded by artificial plants that were painted black to ensure that the light quality was not modified by their neighbours. In a second stand, target plants were surrounded by real plants. In both stands, one-half of the target plants were anchored to stakes to prevent flexing by wind. The target plants were lifted or lowered by 10 cm to test whether height convergence was affected by the different treatments. Stem length was affected by being surrounded by artificial plants, anchoring and pot elevation, indicating that light quality, light quantity and mechanical stimuli all influenced stem elongation. Height convergence did not occur in the stand with artificial plants or in anchored plants. We conclude that light quality and mechanical stimuli are important factors for the regulation of stem growth and height convergence in crowded stands.

Plants in a crowded stand regulate their height growth so as to maintain similar heights to neighbours even when they have potential advantages in height growth.

BACKGROUND AND AIMS: Although being tall is advantageous in light competition, plant height growth is often similar among dominant plants in crowded stands (height convergence). Previous theoretical studies have suggested that plants should not overtop neighbours because greater allocation to supporting tissues is necessary in taller plants, which in turn lowers leaf mass fraction and thus carbon gain. However, this model assumes that a competitor has the same potential of height growth as their neighbours, which does not necessarily account for the fact that height convergence occurs even among individuals with various biomass. METHODS: Stands of individually potted plants of Chenopodium album were established, where target plants were lifted to overtop neighbours or lowered to be overtopped. Lifted plants were expected to keep overtopping because they intercept more light without increased allocation to stems, or to regulate their height to similar levels of neighbours, saving biomass allocation to the supporting organ. Lowered plants were expected to be suppressed due to the low light availability or to increase height growth so as to have similar height to the neighbours. KEY RESULTS: Lifted plants reduced height growth in spite of the fact that they received higher irradiance than others. Lowered plants, on the other hand, increased the rate of stem elongation despite the reduced irradiance. Consequently, lifted and lowered plants converged to the same height. In contrast to the expectation, lifted plants did not increase allocation to leaf mass despite the decreased stem length. Rather, they allocated more biomass to roots, which might contribute to improvement of mechanical stability or water status. It is suggested that decreased leaf mass fraction is not the sole cost of overtopping neighbours. Wind blowing, which may enhance transpiration and drag force, might constrain growth of overtopping plants. CONCLUSIONS: The results show that plants in crowded stands regulate their height growth to maintain similar height to neighbours even when they have potential advantages in height growth. This might contribute to avoidance of stresses caused by wind blowing.

Improved discrimination between monocotyledonous and dicotyledonous plants for weed control based on the blue-green region of ultraviolet-induced fluorescence spectra.

Precision weeding by spot spraying in real time requires sensors to discriminate between weeds and crop without contact. Among the optical based solutions, the ultraviolet (UV) induced fluorescence of the plants appears as a promising alternative. In a first paper, the feasibility of discriminating between corn hybrids, monocotyledonous, and dicotyledonous weeds was demonstrated on the basis of the complete spectra. Some considerations about the different sources of fluorescence oriented the focus to the blue-green fluorescence (BGF) part, ignoring the chlorophyll fluorescence that is inherently more variable in time. This paper investigates the potential of performing weed/crop discrimination on the basis of several large spectral bands in the BGF area. A partial least squares discriminant analysis (PLS-DA) was performed on a set of 1908 spectra of corn and weed plants over 3 years and various growing conditions. The discrimination between monocotyledonous and dicotyledonous plants based on the blue-green fluorescence yielded robust models (classification error between 1.3 and 4.6% for between-year validation). On the basis of the analysis of the PLS-DA model, two large bands were chosen in the blue-green fluorescence zone (400-425 nm and 425-490 nm). A linear discriminant analysis based on the signal from these two bands also provided very robust inter-year results (classification error from 1.5% to 5.2%). The same selection process was applied to discriminate between monocotyledonous weeds and maize but yielded no robust models (up to 50% inter-year error). Further work will be required to solve this problem and provide a complete UV fluorescence based sensor for weed-maize discrimination.

Leaf angle in Chenopodium album is determined by two processes: induction and cessation of petiole curvature.

Petiole curvature is important in regulating light interception by the leaf. To dissect the determination processes of leaf angle, we irradiated the lamina or petiole of Chenopodium album L. with either one or two spots of actinic light, after dark adaptation. When the abaxial side of the petiole was irradiated with blue light, the petiole curvature increased, and under continuous irradiation, the curvature continued for up to 6 h. The rate of curvature increased with increasing blue light intensity. The curvature induced by irradiation of the abaxial side with blue light ceased when the adaxial side of the petiole was simultaneously irradiated with either blue or red light. When an inhibitor for photosynthesis, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, was applied to the adaxial side of the petiole, the cessation of curvature caused by blue light was only weakly inhibited, while the cessation caused by red light was markedly inhibited. When the adaxial side of the petiole was irradiated alternately with red and far-red light, the far-red light antagonized the cessation of curvature caused by the red light. These results clearly show that the petiole curvature is controlled by two processes, the induction and the cessation of curvature. At least three photoreceptor systems, blue-light receptor, photosynthesis and phytochrome, are involved in the reactions.

Phytochrome-mediated long-term memory of seeds.

The question is how long phytochrome, stored within the cytoplasm of plant diaspores, may stimulate their germination. This question arose from the observation that soil cultivations in darkness for weed control gave inconsistent results. Namely, after a single nighttime or daytime cultivation during spring and summer, differences in weed emergence became hardly detectable after a period of six weeks. However, after nighttime and daytime cultivations in late autumn, emergence differences persisted for up to nine months. To examine whether this differing memory effect is phytochrome-mediated, seeds of Chenopodium album and Stellaria media were sown in pots with wet peat, either in daylight or after sunset. In the latter, seeds were irradiated with far-red light for one day prior to being covered and buried. For more than two years the far-red irradiated seeds produced significantly reduced emergence, indicating that germination and emergence of weeds in the field may be supported by maternal far-red absorbing seed phytochrome B(fr) over several months or even years. This conclusion allows refining of the strategy of lightless tillage.

The excess light energy that is neither utilized in photosynthesis nor dissipated by photoprotective mechanisms determines the rate of photoinactivation in photosystem II.

Photoinactivation of PSII is thought to be caused by the excessive light energy that is neither used for photosynthetic electron transport nor dissipated as heat. However, the relationship between the photoinactivation rate and excess energy has not been quantitatively evaluated. Chenopodium album L. plants grown under high-light and high-nitrogen (HL-HN) conditions show higher tolerance to photoinactivation and have higher photosynthetic capacity than the high-light and low-nitrogen (HL-LN)- and low-light and high-nitrogen (LL-HN)-grown plants. The rate of photoinactivation in the LL-HN plants was faster than that in the HL-LN, which was similar to that in the HL-HN plants, while the LL-HN and HL-LN plants had similar photosynthetic capacities [Kato et al. (2002b) Funct. Plant Biol. 29: 787]. We quantified partitioning of light energy between the electron transport and heat dissipation at the light intensities ranging from 300 to 1,800 micromol m(-2) s(-1). The maximum electron transport rate was highest in the HL-HN plants, heat dissipation was greatest in the HL-LN plants, and the excess energy, which was neither consumed for electron transport nor dissipated as heat, was greatest in the LL-HN plants. The first-order rate constant of the PSII photoinactivation was proportional to the magnitude of excess energy, with a single proportional constant for all the plants, irrespective of their growth conditions. Thus the excess energy primarily determines the rate of PSII photoinactivation. A large photosynthetic capacity in the HL-HN plants and a large heat dissipation capacity in the HL-LN plants both contribute to the protection of PSII against photoinactivation.