S. Ciompi

The effect of nitrogen deficiency on leaf gas exchange and chlorophyll fluorescence parameters in sunflower

Abstract Fully expanded leaves of sunflower ( Helianthus annuus L.) growing with either complete or nitrogen-deficient nutrition solution were analysed for gas exchange and chlorophyll fluoresence after 24 days from the transplanting. Under N-stress the decline in photosynthesis and the rise in stomatal conductance determined at light saturation level were accompanied by an increase of the intercellular CO 2 concentration (about +60%). This indicates that the decline in A max (photosynthetic activity at light saturation level) was due to limitation related to a reduced mesophyllic activity, rather than to stomatal limitation. At low light intensity photosynthetic efficiency of PSII was not affected. In N-stressed plants starch content decreased significantly in leaves. The sucrose/starch ratio rose in N-stressed plants, indicating that the carbon partitioning trend had shifted towards sucrose. This result is in accordance with the lower shoot/root ratio observed in nitrogen-stressed plants.

Photosynthetic response of tomato plants to vascular wilt diseases

Greenhouse experiments were conducted to study the chronology of effects on gas exchange and chlorophyll-a fluorescence, visible symptoms and hyphal colonization in plants of the susceptible tomato cultivar Bonny Best inoculated with tracheomycotic fungi Fusarium oxysporum f. sp. lycopersici or Verticillium albo-atrum. The net photosynthetic rates and related parameters of healthy (uncolonized and asymptomatic) leaves of infected plants were affected by both the parasites. In the first uncolonized leaf, net photosynthesis was depressed in different ways: in Fusarium-infected individuals, the maximum detrimental effect was observed a week after inoculation, while in Verticillium-infected plants the most severe depression was detected 21 days after inoculation. The behaviour of the physiological parameters investigated, together with the data relative to chlorophyll fluorescence measurements highlighted the fact that the depression in photosynthetic activity was caused by different concomitant factors in Verticillium-infected plants and was due mainly to drought stress in plants inoculated with Fusarium.

In Vicia faba leaves Photoinhibition from Ozone Fumigation in Light Precedes a Decrease in Quantum Yield of Functional PSII Centres

Summary Treatment with a single pulse of O 3 (150 ppb, 4 h) on Vicia faba (cv. Gigante d’Ingegnoli) plants induced a decrease in photosynthetic rate at light saturation level but only slight changes in stomatal conductance and transpiration rate. The increase in intercellular CO 2 concentration implied that the predominant factor responsible for the reduction in light-saturated CO 2 uptake was a decrease in the carboxylation efficiency. A slight change in the photochemical efficiency of PS2 (F v /F m ) that decreased in treated plants more than did the quantum yield of functional centres (ΔF/F′ m ) lasted hours after treatment termination; photosynthetic efficiency decrease occurred after an increase in F 0 . Thermal dependence of fluorescence yield of open PS2 reaction centres was modified by O 3 . This eventually led to photosensitizing these leaves to photoinhibition but quantum yield of functional centres was again less sensitive to this combination. Increase in electrolyte leakage kinetics in distilled water indicated that, in the stressed plants, damage at the membrane level was occurring and this was confirmed also by the thiobarbituric reactive substance increase in ozonated leaves, taken as indicators of membrane lipid peroxidation.