Vegetative traits can predict flowering quality in Phalaenopsis orchids despite large genotypic variation in response to light and temperature.

Phalaenopsis is an economically important horticultural ornamental, but its growth is slow and costly. The vegetative cultivation phase is long and required to ensure sufficient plant size. This is needed to develop high quality flowering plants. We studied the effects of temperature (27 or 31 °C) and light intensity (60 or 140 µmol m-2 s-1) on plant growth and development during the vegetative cultivation phase in two experiments, with respectively 19 and 14 genotypes. Furthermore, the after-effects of treatments applied during vegetative growth on flowering traits were determined. Increasing light intensity in the vegetative phase accelerated both vegetative plant growth and development. Increasing temperature accelerated vegetative leaf appearance rate, but strongly reduced plant and root biomass accumulation when temperatures were too high. Flowering was greatly affected by treatments applied during vegetative growth, and increased light and temperature increased number of flower spikes, and number of flowers and buds. Genotypic variation was large in Phalaenopsis, especially in traits related to flowering, thus care is needed when generalising results based on a limited number of cultivars. Plant biomass and number of leaves during vegetative growth were positively correlated with flowering quality. These traits can be used as an early predictor for flowering capacity and quality of the final product. Additionally, this knowledge can be used to improve selection of new cultivars.

CAM-physiology and carbon gain of the orchid Phalaenopsis in response to light intensity, light integral and CO2.

The regulation of photosynthesis and carbon gain of crassulacean acid metabolism (CAM) plants has not yet been disclosed to the extent of C3-plants. In this study, the tropical epiphyte Phalaenopsis cv. "Sacramento" was subjected to different lighting regimes. Photosynthesis and biochemical measuring techniques were used to address four specific questions: (1) the response of malate decarboxylation to light intensity, (2) the malate carboxylation pathway in phase IV, (3) the response of diel carbon gain to the light integral and (4) the response of diel carbon gain to CO2 . The four CAM-phases were clearly discernable. The length of phase III and the malate decarboxylation rate responded directly to light intensity. In phase IV, CO2 was initially mainly carboxylated via Rubisco. However, at daylength of 16 h, specifically beyond ±12 h, it was mainly phosphoenolpyruvate carboxylase (PEP-C) carboxylating CO2 . Diel carbon gain appeared to be controlled by the light integral during phase III rather than the total daily light integral. Elevated CO2 further enhanced carbon gain both in phase IV and phase I. This establishes that neither malate storage capacity, nor availability of PEP as substrate for nocturnal CO2 carboxylation were limiting factors for carbon gain enhancement. These results advance our understanding of CAM-plants and are also of practical importance for growers.

Crassulacean acid metabolism species differ in the contribution of C3 and C4 carboxylation to end of day CO2 fixation.

Crassulacean acid metabolism (CAM) is a photosynthetic pathway that temporally separates the nocturnal CO2 uptake, via phosphoenolpyruvate carboxylase (PEPC, C4 carboxylation), from the diurnal refixation by Rubisco (C3 carboxylation). At the end of the day (CAM-Phase IV), when nocturnally stored CO2 has depleted, stomata reopen and allow additional CO2 uptake, which can be fixed by Rubisco or by PEPC. This work examined the CO2 uptake via C3 and C4 carboxylation in phase IV in the CAM species Phalaenopsis "Sacramento" and Kalanchoe blossfeldiana "Saja." Short blackout periods during phase IV caused a sharp drop in CO2 uptake in K. blossfeldiana but not in Phalaenopsis, indicating strong Rubisco activity only in K. blossfeldiana. Chlorophyll fluorescence revealed a progressive decrease in ΦPSII in Phalaenopsis, implying decreasing Rubisco activity, while ΦPSII remained constant in phase IV in K. blossfeldiana. However, short switching to 2% O2 indicated the presence of photorespiration and thus Rubisco activity in both species throughout phase IV. Lastly, in Phalaenopsis, accumulation of starch in phase IV occurred. These results indicate that in Phalaenopsis, PEPC was the main carboxylase in phase IV, although Rubisco remained active throughout the whole phase. This will lead to double carboxylation (futile cycling) but may help to avoid photoinhibition.