The kinetics of P-700+ reduction in leaves: a novel in situ probe of thylakoid functioning

Abstract. The half time (t_1/2) of the reduction of P-700⁺ in the millisecond time frame is known to be limited by the reaction between plastoquinol and the cytochrome cytb₆f complex. This is considered to be the rate limiting reaction of thylakoid electron transport and measurements of it provide a means of analysing how thylakoid election transport is regulated in vivo. The half time for the reduction of photochemically oxidized P-700 has been measured in vivo using absorbance changes around 820 nm. The results showed that t_1/2 is independent of irradiance and decreases as photosynthetic induction progresses. Even with a constant t_1/2 the quantum efficiency of PSI declined as irradiance increased. The significance of the concept of photosynthetic control of electron transport is discussed in the light of these observations.     

Using combined measurements of gas exchange and chlorophyll fluorescence to estimate parameters of a biochemical C3 photosynthesis model: a critical appraisal and a new integrated approach applied to leaves in a wheat (Triticum aestivum) canopy

We appraised the literature and described an approach to estimate the parameters of the Farquhar, von Caemmerer and Berry model using measured CO₂ assimilation rate (A) and photosystem II (PSII) electron transport efficiency (Φ₂). The approach uses curve fitting to data of A and Φ₂ at various levels of incident irradiance (I_inc), intercellular CO₂ (C_i) and O₂. Estimated parameters include day respiration (R_d), conversion efficiency of I_inc into linear electron transport of PSII under limiting light [κ_2(LL)], electron transport capacity (J_max), curvature factor (θ) for the non‐rectangular hyperbolic response of electron flux to I_inc, ribulose 1·5‐bisphosphate carboxylase/oxygenase (Rubisco) CO₂/O₂ specificity (S_c/o), Rubisco carboxylation capacity (V_cmax), rate of triose phosphate utilization (T_p) and mesophyll conductance (g_m). The method is used to analyse combined gas exchange and chlorophyll fluorescence measurements on leaves of various ages and positions in wheat plants grown at two nitrogen levels. Estimated S_c/o (25 °C) was 3.13 mbar µbar^?1; R_d was lower than respiration in the dark; J_max was lower and θ was higher at 2% than at 21% O₂; κ_2(LL), V_cmax, J_max and T_p correlated to leaf nitrogen content; and g_m decreased with increasing C_i and with decreasing I_inc. Based on the parameter estimates, we surmised that there was some alternative electron transport.     

The use of light-induced absorbance changes at 820 nm to monitor the oxidation state of P-700 in leaves

Abstract The use of the light-induced absorbance change at 820 nm (ΔA 820) to monitor the oxidation and reduction of P-700 in irradiated leaves is examined. Results obtained from leaves irradiated with a range of wavelengths of light, poisoned with DCMU, or lacking PS I, are consistent with the proposition that the light-induced ΔA 820 can be used to monitor P-700 oxidation in leaves.     

The Use of Microwaves to Monitor the Freezing and Thawing of Water in Plants

Harbinson, J. and Woodward, F. I. 1987. The use of microwavesto monitor the freezing and thawing of water in plants.—J.exp. Bot. 38: 1325–1335. The use of radiation with a frequency of 10·687 GHz tomeasure the freezing and thawing of water in plants is discussed.Results showing the freezing and thawing of plant material arepresented. In its simplest form the technique offers a methodfor investigating the freezing and thawing of water in plants. Key words: Freezing, microwaves     

Field Measurements of the Gas Exchange of Woody Plant Species in Simulated Sunflecks

The photosynthetic responses of eight tree and shrub speciesto simulated sunflecks was measured in the field. The net carbonexchange (NCE) of Corylus avellana and Ulmus glabra increasedwith irradiance up to the maximum irradiance of 230 µmolm^–2 s^–1. The NCE of Fraxinux excelsior, Hedera helixand the sun and shade forms of Rhododendron ponticum saturatedat about 120 µmol m^–2 s^–1 whereas the NCEof Ilex aquifolium, Daphne laureola and Fagus sylvatica hadeffectively saturated at 27 µmol m^–2 s^–1. In all cases the quantum efficiency of NCE could be predictedfrom measurements of chlorophyll fluorescence and the maximumvalue for NCE from measurements of stomatal conductance. Therelationships were combined into a model for predicting NCE/irradiancecharacteristics. Corylus avellana L., Daphne laureola L., Fagus sylvatica L., Fraxinus excelsior L., Hedera helix L., Ilex aquifolium L., Prunus laurocerasus L., Rhododendron ponticum L., Ulmus glabra Huds., gas exchange, stomatal resistance, water use efficiency, chlorophyll fluorescence, quantum efficiency