Oxidation-reduction potential dependence of low-temperature photoreactions of chloroplast Photosystem II

The light-induced free-radical signal of Photosystem II (observed after illumination at 77 °K) has been studied in chloroplasts as a function of the oxidation-reduction potential established prior to freezing. The intensity of the light-induced signal is unchanged in the potential region of +590 mV to +760 mV. At higher potential (+850 mV), there is a 30% decrease in signal intensity. The light-induced signal decreases to zero in the low-potential region, with a midpoint potential of +475 mV. These results are considered in terms of a Photosystem II reaction-center complex in which the light-induced free-radical signal arises from the oxidized form of the reaction-center chlorophyll, and this chlorophyll molecule is capable of being reduced at liquid-nitrogen temperature by a secondary electron donor which has a midpoint oxidation-reduction potential of +475 mV.     

Problems with the use of terracotta clay saucers as phosphorus-diffusing substrata to assess nutrient limitation of epilithic algae

1. We examined the diffusion properties of terracotta clay saucers, of types often used as components in phosphorus-diffusing substrata for investigating nutrient limitation of epilithic algae.
2. Laboratory experiments showed that phosphorus diffusion was low and inconsistent (0.06–2.6 mg P day⁻¹) through clay saucers filled with agar containing orthophosphate. Similarly, in situ release of phosphorus from two types of terracotta clay saucers (Australian and Italian) was variable (2–8 mg P day⁻¹; 5–25%) under three flow regimes (0, 0.1 and 0.3 m s⁻¹) over 30 days, with most phosphorus being released during the first day. Clogging of pores by agar appears to prevent the diffusion of phosphorus through the terracotta clay saucers. However, the two types of terracotta clay saucer also irreversibly sorbed large quantities of phosphorus (40–140 mg P).
3. Individual saucers can have markedly different physical and chemical properties both within and among terracotta types. Terracotta saucers also contain large quantities of iron, calcium and aluminium that are known binding agents for phosphorus. Variability in saucer composition and diffusion properties makes treatments difficult to replicate and weakens comparisons between studies that have used terracotta with different clay composition and manufacture.
4. We recommend that phosphorus-diffusing substrata should not be constructed from terracotta clay components.