Abstract. It is suggested that increased levels of free cytosolic calcium ([Ca2+]cyt) may serve as the primary physiological transducer of chilling injury in plants. Numerous similarities between the effects of [Ca2+]cyt‐raising treatments on plants and the effects of chilling temperatures on chilling‐sensitive (CS) plants are noted. It is proposed that chilling temperatures may lead to increases in [Ca2+]cyt in CS plant cells by reducing the rate at which they exclude Ca2+ from their cytosol and that rapid cooling (coldshock) may cause rapid increases in [Ca2+]cyt due to the activation of voltage‐dependent cation channels. Chill‐induced increases in [Ca2+]cyt in the cells of CS plants may reflect either an inherent inability of such plants to maintain homeostatic levels of Ca2+ at low temperatures or a stress‐induced reaction which has evolved to enable such cells to cope more effectively with the short‐term hardships imposed by cold. Previous proposals concerning the physiological transduction of chilling injury are also discussed. It is argued that there is little evidence to suggest that the immediate effects of low temperatures on CS cells include either decreases in ATP levels, general increases in the passive permeability of membranes, or increased rates of fermentation.

Abstract. The different effects which fast versus slow cooling have on such fundamental plant processes as ion transport, protoplasmic streaming, phloem translocation, growth, cell motility, water absorption and membrane potential are reviewed. When plant cells are rapidly cooled to non‐injurious temperatures, many of the physiological ramifications of rapid‐cooling stimulation are only transiently observed. It is proposed that these transient rapid‐cooling‐induced responses, sometimes elicited by temperature drops of only a few degrees centigrade, are manifestations of temperature sensing. The hypothesis is advanced that graded potentials, produced in response to rapidly falling temperature, are associated with graded increases in cytosolic free calcium. These transient increases in cytosolic free calcium give rise to many of the physiological effects elicited by rapid cooling. Other effects, however, such as those associated with alterations in membrane composition, gene expression and post‐translational modifications of proteins, may persist longer. The questions of the possible physiological advantage of temperature sensing, and its implications for the study of chilling injury, are discussed.

Abstract. Rapid‐cooling pulses to non‐stressful temperatures cause strong, transient depolarizations in cortical cells of cucumber roots. The amplitudes of these electrical responses are graded according to the rate and amplitude of the cooling pulse. Such graded potentials are typical of sensory processes and indicate that plants possess the ability to sense temperature change. La3+, a blocker of Ca2+ channels, and ethylene glycol bis‐(β‐aminoethyl ether) N,N,N′,N′‐acetic acid (EGTA), a Ca2+ chelator, inhibit the electrical responses elicited by rapid‐cooling pulses. High external [Ca2+] enhances them. These results indicate the involvement of a plasma membrane‐associated Ca2+ channel in the process of temperature sensing by plants. Calmodulin antagonists prolong the repolarization phase of the electrical responses, suggesting a role for calmodulin in the recovery from stimulation.

Since its discovery in the early 1960's, abscisic acid (ABA) has received considerable attention as an important phytohormone, and more recently, as a candidate medicinal in humans. In plants it has been shown to regulate important physiological processes such as response to drought stress, and dormancy. The discovery of ABA synthesis in animal cells has generated interest in the possible parallels between its role in plant and animal systems. The importance of this molecule has prompted the development of several methods for the chemical synthesis of ABA, which differ significantly from the biosynthesis of ABA in plants through the mevalonic acid pathway. ABA recognition in plants has been shown to occur at both the intra- and extracellularly but little is known about the perception of ABA by animal cells. A few ABA molecular targets have been identified in vitro (e.g., calcium signaling, G protein-coupled receptors) in both plant and animal systems. A unique finding in mammalian systems, however, is that the peroxisome proliferator-activated receptor, PPAR gamma, is upregulated by ABA in both in vitro and in vivo studies. Comparison of the human PPAR gamma gene network with Arabidopsis ABA-related genes reveal important orthologs between these groups. Also, ABA can ameliorate the symptoms of type II diabetes, targeting PPAR gamma in a similar manner as the thiazolidinediones class of anti-diabetic drugs. The use of ABA in the treatment of type II diabetes, offers encouragement for further studies concerning the biomedical applications of ABA.

Foliar nyctinasty is a plant behaviour characterised by a pronounced daily oscillation in leaf orientation. During the day, the blades of nyctinastic plant leaves (or leaflets) assume a more or less horizontal position that optimises their ability to capture sunlight for photosynthesis. At night, the positions that the leaf blades assume, regardless of whether they arise by rising, falling or twisting, are essentially vertical. Among the ideas put forth to explain the raison d'être of foliar nyctinasty are that it: (i) improves the temperature relations of plants; (ii) helps remove surface water from foliage; (iii) prevents the disruption of photoperiodism by moonlight; and (iv) directly discourages insect herbivory. After discussing these previous hypotheses, a novel tritrophic hypothesis is introduced that proposes that foliar nyctinasty constitutes an indirect plant defence against nocturnal herbivores. It is suggested that the reduction in physical clutter that follows from nocturnal leaf closure may increase the foraging success of many types of animals that prey upon or parasitise herbivores. Predators and parasitoids generally use some combination of visual, auditory or olfactory cues to detect prey. In terrestrial environments, it is hypothesised that the vertical orientation of the blades of nyctinastic plants at night would be especially beneficial to flying nocturnal predators (e.g. bats and owls) and parasitoids whose modus operandi is death from above. The movements of prey beneath a plant with vertically oriented foliage would be visually more obvious to gleaning or swooping predators under nocturnal or crepuscular conditions. Such predators could also detect sounds made by prey better without baffling layers of foliage overhead to damp and disperse the signal. Moreover, any volatiles released by the prey would diffuse more directly to the awaiting olfactory apparatus of the predators or parasitoids. In addition to facilitating the demise of herbivores by carnivores and parasitoids, foliar nyctinasty, much like the enhanced illumination of the full moon, may mitigate feeding by nocturnal herbivores by altering their foraging behaviour. Foliar nyctinasty could also provide a competitive advantage by encouraging herbivores, seeking more cover, to forage on or around non-nyctinastic species. As an added advantage, foliar nyctinasty, by decreasing the temperature between plants through its effects on re-radiation, may slow certain types of ectothermic herbivores making them more vulnerable to predation. Foliar nyctinasty also may not solely be a behavioural adaptation against folivores; by discouraging foraging by granivores, the inclusive fitness of nyctinastic plants may be increased.

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