Pea Seed Priming with Pluronic P85-Grafted Single-Walled Carbon Nanotubes Affects Photosynthetic Gas Exchange but Not Photosynthetic Light Reactions.

Nanotechnology is rapidly advancing towards the development of applications for sustainable plant growth and photosynthesis optimization. The nanomaterial/plant interaction has been intensively investigated; however, there is still a gap in knowledge regarding their effect on crop seed development and photosynthetic performance. In the present work, we apply a priming procedure with 10 and 50 mg/L Pluronic-P85-grafted single-walled carbon nanotubes (P85-SWCNT) on garden pea seeds and examine the germination, development, and photosynthetic activity of young seedlings grown on soil substrate. The applied treatments result in a distorted topology of the seed surface and suppressed (by 10-19%) shoot emergence. No priming-induced alterations in the structural and functional features of the photosynthetic apparatus in 14-day-old plants are found. However, photosynthetic gas exchange measurements reveal reduced stomatal conductance (by up to 15%) and increased intrinsic water use efficiency (by 12-15%), as compared to hydro-primed variants, suggesting the better ability of plants to cope with drought stress-an assumption that needs further verification. Our study prompts further research on the stomatal behavior and dark reactions of photosynthesis in order to gain new insights into the effect of carbon nanotubes on plant performance.

Manipulation of light quality is an effective tool to regulate photosynthetic capacity and fruit antioxidant properties of Solanum lycopersicum L. cv. 'Microtom' in a controlled environment.

Light quality plays an essential role in setting plant structural and functional traits, including antioxidant compounds. This paper aimed to assess how manipulating the light spectrum during growth may regulate the photosynthetic activity and fruit bioactive compound synthesis in Solanum lycopersicum L. cv. 'Microtom' to improve plant physiological performance and fruit nutritional value. Plants were cultivated under three light quality regimes: red-green-blue LEDs (RGB), red-blue LEDs (RB) and white fluorescent lamps (FL), from sowing to fruit ripening. Leaf functional traits, photosynthetic efficiency, Rubisco and D1 protein expression, and antioxidant production in fruits were analyzed. Compared to FL, RGB and RB regimes reduced height and increased leaf number and specific leaf area, enhancing plant dwarf growth. The RGB regime improved photosynthesis and stomatal conductance despite lower biomass, favoring Rubisco synthesis and carboxylation rate than RB and FL regimes. The RB light produced plants with fewer flowers and fruits with a lower ascorbic acid amount but the highest polyphenol content, antioxidant capacity and SOD and CAT activities. Our data indicate that the high percentage of the green wavelength in the RGB regime promoted photosynthesis and reduced plant reproductive capacity compared to FL and RB. Conversely, the RB regime was the best in favoring the production of health-promoting compounds in tomato berries.

Light Spectral Composition Influences Structural and Eco-Physiological Traits of Solanum lycopersicum L. cv. 'Microtom' in Response to High-LET Ionizing Radiation.

This study evaluated if specific light quality (LQ) regimes (white fluorescent, FL; full-spectrum, FS; red-blue, RB) during plant growth modified morphological and photosynthetic traits of Solanum lycopersicum L. 'Microtom' plants irradiated at the dry seed stage with 25 Gy 48Ca ions (IR). The irradiation reduced plant size while it increased leaf dry matter content (LDMC) and relative water content (RWC) compared to the control. FS and RB light regimes determined a decrease of plant height and a rise of RWC compared to FL plants. The irradiation under FS and RB regimes favoured the development of dwarf plants and improved the leaf water status. Under the FL regime, irradiated plants showed reduced photosynthesis and stomatal conductance. The opposite behavior was observed in RB irradiated plants in which gas exchanges were significantly stimulated. RB regime enhanced Rubisco expression in irradiated plants also inducing anatomical and functional adjustments (i.e., increase of leaf thickness and incidence of intercellular spaces). Finally, 48Ca ions did not prevent fruit ripening and the achievement of the 'seed-to seed' cycle, irrespective of the LQ regime. Overall, the present study evidenced that RB light regime was the most effective in optimising growth and photosynthetic efficiency of 'Microtom' irradiated plants. These outcomes may help to develop proper cultivation protocols for the growth of dwarf tomato in Controlled Ecological Life Support Systems (CELSS).

Isoprene emission aids recovery of photosynthetic performance in transgenic Nicotiana tabacum following high intensity acute UV-B exposure.

Isoprene emission by terrestrial plants is believed to play a role in mitigating the effects of abiotic stress on photosynthesis. Ultraviolet-B light (UV-B) induces damage to the photosynthetic apparatus of plants, but the role of isoprene in UV-B tolerance is poorly understood. To investigate this putative protective role, we exposed non-emitting (NE) control and transgenic isoprene emitting (IE) Nicotiana tabacum (tobacco) plants to high intensity UV-B exposure. Methanol emissions increased with UV-B intensity, indicating oxidative damage. However, isoprene emission was unaffected during exposure to UV-B radiation, but declined in the 48 h following UV-B treatment at the highest UV-B intensities of 9 and 15 Wm(-2). Photosynthesis and the performance of photosystem II (PSII) declined to similar extents in IE and NE plants following UV-B exposure, suggesting that isoprene emission did not ameliorate the immediate impact of UV-B on photosynthesis. However, after the stress, photosynthesis and PSII recovered in IE plants, which maintained isoprene formation, but not in NE plants. Recovery of IE plants was also associated with elevated antioxidant levels and cycling; suggesting that both isoprene formation and antioxidant systems contributed to reinstating the integrity and functionality of cellular membranes and photosynthesis following exposure to excessive levels of UV-B radiation.

Isoprene improves photochemical efficiency and enhances heat dissipation in plants at physiological temperatures.

Isoprene-emitting plants are better protected against thermal and oxidative stresses. Isoprene may strengthen membranes avoiding their denaturation and may quench reactive oxygen and nitrogen species, achieving a similar protective effect. The physiological role of isoprene in unstressed plants, up to now, is not understood. It is shown here, by monitoring the non-photochemical quenching (NPQ) of chlorophyll fluorescence of leaves with chemically or genetically altered isoprene biosynthesis, that chloroplasts of isoprene-emitting leaves dissipate less energy as heat than chloroplasts of non-emitting leaves, when exposed to physiologically high temperatures (28-37 °C) that do not impair the photosynthetic apparatus. The effect was especially remarkable at foliar temperatures between 30 °C and 35 °C, at which isoprene emission is maximized and NPQ is quenched by about 20%. Isoprene may also allow better stability of photosynthetic membranes and a more efficient electron transfer through PSII at physiological temperatures, explaining most of the NPQ reduction and the slightly higher photochemical quenching that was also observed in isoprene-emitting leaves. The possibility that isoprene emission helps in removing thermal energy at the thylakoid level is also put forward, although such an effect was calculated to be minimal. These experiments expand current evidence that isoprene is an important trait against thermal and oxidative stresses and also explains why plants invest resources in isoprene under unstressed conditions. By improving PSII efficiency and reducing the need for heat dissipation in photosynthetic membranes, isoprene emitters are best fitted to physiologically high temperatures and will have an evolutionary advantage when adapting to a warming climate.

Changes in photosynthesis, mesophyll conductance to CO2, and isoprenoid emissions in Populus nigra plants exposed to excess nickel.

Poplar (Populus nigra) plants were grown hydroponically with 30 and 200 µM Ni (Ni30 and Ni200). Photosynthesis limitations and isoprenoid emissions were investigated in two leaf types (mature and developing). Ni stress significantly decreased photosynthesis, and this effect depended on the leaf Ni content, which was lower in mature than in developing leaves. The main limitations to photosynthesis were attributed to mesophyll conductance and metabolism impairment. In Ni-stressed developing leaves, isoprene emission was significantly stimulated. We attribute such stimulation to the lower chloroplastic [CO2] than in control leaves. However chloroplastic [CO2] did not control isoprene emission in mature leaves. Ni stress induced the emission of cis-ß-ocimene in mature leaves, and of linalool in both leaf types. Induced biosynthesis and emission of isoprenoids reveal the onset of antioxidant processes that may also contribute to reduce Ni stress, especially in mature poplar leaves.

The impact of blue light on leaf mesophyll conductance.

Blue light has many direct and indirect effects on photosynthesis. The impact of blue light on mesophyll conductance (g(m)), one of the main diffusive limitation to photosynthesis, was investigated in leaves of Nicotiana tabacum and Platanus orientalis, characterized by high and low g(m), respectively. Leaves were exposed to blue light fractions between 0% and 80% of incident light intensity (300 micromol photons m(-2) s(-1)), the other fraction being supplied as red light. Leaves exposed to blue light showed reduced photosynthesis and unaltered stomatal conductance. The g(m), measured using the chlorophyll fluorescence-based method, was strongly reduced in both plant species. Such a reduction of g(m) may not be real, as several assumptions used for the calculation of g(m) by fluorescence may not hold under blue light. To assess possible artefacts, the electron transport rate measured by fluorescence (J(f)) and by gas-exchange (J(c)) were compared in leaves exposed to different fractions of blue light under non-photorespiratory conditions. The two values were only equal, a prerequisite for correct g(m) measurements, when the illumination was totally provided as red light. Under increasing blue light levels an increasing discrepancy was observed, which suggests that J(f) was not correctly calculated, and that such an error could also upset g(m) measurements. Blue light was not found to change the absorbance of light by leaves, whereas it slightly decreased the distribution of light to PSII. To equate J(f) and J(c) under blue light, a further factor must be added to the J(f) equation, which possibly accounted for the reduced efficiency of energy transfer between the pigments predominantly absorbing blue light (the carotenoids) and the chlorophylls. This correction reduced by about 50% the effect of blue light on g(m). However, the residual reduction of g(m) under blue light was real and significant, although it did not appear to limit the chloroplast CO(2) concentration and, consequently, photosynthesis. Reduction of g(m) might be caused by chloroplast movement to avoid photodamage, in turn affecting the chloroplast surface exposed to intercellular spaces. However, g(m) reduction occurred immediately after exposure to blue light and was complete after less than 3 min, whereas chloroplast relocation was expected to occur more slowly. In addition, fast g(m) reduction was also observed after inhibiting chloroplast movement by cytochalasin. It is therefore concluded that g(m) reduction under blue light is unlikely to be caused by chloroplast movement only, and must be elicited by other, as yet unknown, factors.