In this study, evidence is given that a number of isolated coupled plant mitochondria (from durum wheat, bread wheat, spelt, rye, barley, potato, and spinach) can take up externally added K ؉ ions. This was observed by following mitochondrial swelling in isotonic KCl solutions and was confirmed by a novel method in which the membrane potential decrease due to externally added K ؉ is measured fluorimetrically by using safranine. A detailed investigation of K ؉ uptake by durum wheat mitochondria shows hyperbolic dependence on the ion concentration and specificity. K ؉ uptake electrogenicity and the non-competitive inhibition due to either ATP or NADH are also shown. In the whole, the experimental findings reported in this paper demonstrate the existence of the mitochondrial K ؉ ATP channel in plants (PmitoK ATP ). Interestingly, Mg 2؉ and glyburide, which can inhibit mammalian K ؉ channel, have no effect on PmitoK ATP . In the presence of the superoxide anion producing system (xanthine plus xanthine oxidase), PmitoK ATP activation was found. Moreover, an inverse relationship was found between channel activity and mitochondrial superoxide anion formation, as measured via epinephrine photometric assay. These findings strongly suggest that mitochondrial K ؉ uptake could be involved in plant defense mechanism against oxidative stress due to reactive oxygen species generation.One of most outstanding problems in mitochondria bioenergetics concerns the mitochondrial permeability to metabolites, organic compounds, including vitamins, their derived cofactors, and metal ions. The mitochondrial inner membrane contains metabolite carriers (for review, see Refs. 1 and 2), responsible for shuttling substrates between matrix and cytosol and for catabolism dependent on matrix enzymes, as well as vitamin and cofactor translocators (for review, see Refs. 3 and 4). Moreover, the inner membrane also contains the cation carriers and channels that regulate cell and mitochondrial physiology. In particular, as regards K ϩ ion, in mammalian mitochondria, the transport properties are such that net potassium flux across the mitochondrial membrane determines mitochondrial volume (Refs. 5 and 6 and references therein). It has been shown that K ϩ uptake is mediated by diffusion leak, driven by the high electric membrane potential maintained by redox-driven electrophoretic proton ejection, and that regulated K ϩ efflux is mediated by the inner membrane K ϩ /H ϩ antiporter (see Ref. 7). There is also evidence for the existence of an inner membrane protein designed to catalyze electrophoretic K ϩ uptake into mammalian (5-12) and yeast (13, 14) mitochondria. As far as plant mitochondria are concerned, even though mitochondrial structure and function are expected to be strictly dependent on K ϩ transport across the mitochondrial membrane, the knowledge of K ϩ permeability is not established at present. Indeed, the presence of a powerful K ϩ /H ϩ antiporter, which partially collapses ⌬pH, thereby increasing ⌬⌿, 1 has been shown (Refs. 15 and 16 and ...

The quantum yield of photosynthetic electron transport (ΦPSII), evaluated by means of chlorophyll (Chl) fluorescence analysis, has proven to be a useful screening test for drought tolerance in durum wheat (Triticum durum Desf.). To explore the potential of this parameter further in detecting drought‐tolerant genotypes, three cereal species were studied; ΦPSII measurements were carried out under two different gas mixtures, at three points of the induction curve (to obtain the maximal ΦPSII and both the transient and steady‐state actual ΦPSII), and at three different water stress levels (moderate, severe and drastic). The species investigated were durum and bread wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.); two cultivars per species, characterized by different levels of drought tolerance, were tested. The two gas mixtures used were normal air (21% O2, 0.035% CO2 in N2) to monitor the whole photosynthetic process under physiological conditions, and CO2 enriched‐low O2 air (1% O2, 5% CO2 in N2) to monitor ΦPSII reduction under stress mainly related to Calvin cycle activity. When ΦPSII related to both assimilatory and non‐assimilatory metabolism was evaluated, the cultivar differences observed under normal Air were more representative of the agronomic performance upon drought stress than under high CO2‐low O2 air. Maximal ΦPSII showed no difference among either cultivars, gas mixtures or stress levels, the efficiency of excitation capture being highly resistant to drought. The ΦPSII evaluated during the transient yielded predictable values in respect of drought tolerance for durum wheat and barley cultivars, highlighting the key role of regulatory processes such as the Mehler peroxidase reaction and possibly also cyclic electron transport, in preventing overreduction under stress. The results clearly show that when Chl fluorescence analysis is used as a parameter in plant breeding, different experimental conditions should be used depending on the physiological mechanism that is bred or selected for.

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