The capability of Phalaenopsis to acclimate its photosynthetic capacity and metabolic activity to cool night temperature conditions is crucial for improving orchid production in terms of efficient greenhouse heating. The extent to which Phalaenopsis possesses acclimation potential and the mechanistic background of the metabolic processes involved, have, however, not been studied before. Plants were subjected to a direct and gradual shift from a day to night temperature regime of 28/28-28/16°C, the cold stress and cold acclimation treatment, respectively. In comparison with the cold stress treatment, the cold acclimation treatment led to a higher malate accumulation and a reduction in leaf net CO(2) uptake. Consistently, the contribution of respiratory CO(2) recycling to nocturnal malate synthesis was calculated to be 23.5 and 47.0% for the cold stress and cold acclimation treatment, respectively. Moreover, the lower levels of starch measured in the cold acclimated leaves confirmed the suggested enhanced respiratory CO(2) recycling, implying that Phalaenopsis CAM operation evolved towards CAM idling. It is, however, plausible that this adjustment was not an effect of the low night temperature per se but a consequence of cool-root induced drought stress. Apart from that, at the start of the photoperiod, membrane stability showed a depression which was directly counteracted by an increased generation of glucose, fructose and sucrose. From these observations, it can be concluded that the observed plasticity in CAM operation and metabolic flexibility may be recognized as important steps in the low night temperature acclimation of Phalaenopsis.

Knowledge of the energy saving night temperature (i.e. a relatively cool night temperature without affecting photosynthetic activity and physiology) and a better understanding of low night temperature effects on the photosynthetic physiology of Phalaenopsis would improve their production in terms of greenhouse temperature control and energy use. Therefore, Phalaenopsis 'Hercules' was subjected to day temperatures of 27.5 degrees C and night temperatures of 27.0 degrees C, 24.2 degrees C, 21.2 degrees C, 18.3 degrees C, 15.3 degrees C or 12.3 degrees C in a growth chamber. A new tool for the determination of the energy saving night temperature range was developed based on temperature response curves of leaf net CO(2) exchange, chlorophyll fluorescence, organic acid content and carbohydrate concentrations. The newly developed method was validated during a complete vegetative cultivation in a greenhouse environment with eight Phalaenopsis hybrids (i.e. 'Boston', 'Bristol', 'Chalk Dust', 'Fire Fly', 'Lennestadt', 'Liverpool', 'Precious', 'Vivaldi') and day/night temperature set points of 28/28 degrees C, 29/23 degrees C and 29/17 degrees C. Temperature response curves revealed an overall energy saving night temperature range for nocturnal CO(2) uptake, carbohydrate metabolism, organic acid accumulation and photosystem II (PSII) photochemistry of 17.1 degrees C to 19.9 degrees C for Phalaenopsis 'Hercules'. At the lower end of this energy saving night temperature range, a high malate-to-citrate ratio switched towards a low ratio and this transition seemed to alleviate effects of night chilling induced photoinhibition. At night temperatures of 24 degrees C or higher, the degradation of starch, glucose and fructose indicated an increased respiratory CO(2) production. During the greenhouse validation experiment, the differences between the eight Phalaenopsis hybrids with regard to their response to the warm day/cool night temperature regimes were remarkably large. In general, the day/night temperature of 29/17 degrees C led to a significantly lower biomass accumulation and less leaves which were in addition shorter, narrower and smaller in size as compared to the day/night temperature regimes of 28/28 degrees C and 29/23 degrees C. During week 25 of the cultivation period, plants matured and flower initiation steeply increased for all hybrids and in each day/night temperature regime. Before week 25, early spiking was only sufficiently suppressed in the 29/23 degrees C and 29/17 degrees C temperature regimes for three hybrids ('Boston', 'Bristol' and 'Lennestadt') but not in the other five hybrids. Although a considerable biochemical flexibility was demonstrated for Phalaenopsis 'Hercules', inhibition of flowering after exposure to a combination of warm days and cool nights appeared to be largely hybrid dependent

Nowadays, a quest for efficient greenhouse heating strategies, and its related effects on the plant’s performance, exists. In this study, the effects of a combination of warm days and cool nights in autumn and spring on the photosynthetic activity and efficiency of Phalaenopsis were evaluated; the latter, being poorly characterised in CAM plants and, to our knowledge, not reported before in Phalaenopsis. 24-h CO2 flux measurements and chlorophyll fluorescence analysis were performed in both seasons on Phalaenopsis ‘Hercules’ exposed to relatively constant temperature regimes, 25.5/24.0°C (autumn) and 30/27°C (spring) respectively, and distinctive warm day/cool night temperature regimes, 27/20°C (autumn) and 36/24°C (spring), respectively. Cumulated leaf net CO2 uptake of the distinctive warm day/cool night temperature regimes declined with 10-16% as compared to the more constant temperature regimes, while the efficiency of carbon fixation revealed no substantial differences in both seasons. Nevertheless, a distinctive warm day/cool night temperature regime seemed to induce photorespiration. Although photorespiration is expected not to occur in CAM, the suppression of the leaf net CO2 exchange during Phase II and Phase IV as well as the slightly lower efficiency of carbon fixation for the distinctive warm day/cool night temperature regimes confirms the involvement of photorespiration in CAM. A seasonal effect was reflected in the leaf net CO2 exchange rate with considerably higher rates in spring. In addition, sufficiently high PAR levels in spring led to an efficiency of carbon fixation of 1.06-1.27% which is about twice as high than in autumn. As a result, only in the case where a net energy reduction between the temperature regimes compensates for the reduction in net CO2 uptake, warm day/cool night temperature regimes may be recommended as a practical sustainable alternative

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