ISHS Acta Horticulturae 327: II European Workshop on Thermo- and Photomorphogenesis in Plants (1992)

Leiv M. Mortensen, Roar Moe

The effects of various day and night temperature treatments on plant growth and morphogenesis were studied in growth rooms at 80 μmol m-2s-1 photon flux density for 16 h day-1. The experiment included Begonia x hiemalis, Begonia x tuberhybrida pendula, Calceolaria x herbeohybrida, Fuchsia x hybrida, Impatiens walleriana, Kalanchoe blossfeldiana, Pelargonium x hortorum (from cuttings and seed), Petunia x hybrida, (3 cvs.), Rosa sp. (2 cut and 2 pot cvs.), Salvia splendens, Senecio cruentus, Sinningia speciosa, Tagetes patula nana and Verbena elegans var. asperata. The following day/night treatments were given: 19/19, 21/15 (positive DIF) and 17/23°C (negative DIF). In addition 1.5 and 3.0 h drops to 12°C starting 45 minutes and 1.5 h before day, and an increase to about 30°C for the last four hours of the day (EOD) or the night (EON). The mean 24-h temperature was adjusted to be the same in all treatments. Plant height was not affected by temperature treatments in Senecio, Pelargonium (from cuttings), Rosa (two cut flower and two pot rose cultivars), Tagetes and Verbena. Positive DIF caused an increase in plant height in Kalanchoe only, whilst a negative DIF decreased the plant heights of Begonia, Calceolaria, Impatiens, Pelargonium (from seeds), Petunia and Salvia compared to the constant temperature. A short drop to 12°C had no effect on plant height whilst a longer drop (3 h) caused increased stem elongation in Kalanchoe and Petunia.

A high temperature at EON reduced the plant height in two species whilst a high temperature at EOD decreased the height in three species and increased it in another three.

The dry weight was generally not affected by negative or positive DIF in those species which responded to these treatments in respect of plant height. Time to flowering was more or less affected by the temperature treatments, in the different species.

CybeRose note: I include the following item for those of us who were not familiar with some of the terminology in the note above.

The Biology of Peatlands, 2ed (2013)
By Håkan Rydin, John K. Jeglum Direct measures [of light]

Chlorophyll takes up light at wavelengths between 400 and 700 nm, and radiation in this band is called photosynthetically active radiation (PAR). Each captured photon (light particle or quantum) can start a particular reaction in the cell chloroplast. To measure PAR, quantum sensors are used which filter out light outside this wavelength band and count the number of photons hitting it per unit area and time. This measure is called photosynthetic photon flux density (PPFD), and the unit is µmol m-2 s-1. The unit 'mol' is the same as is used to express numbers of molecules: 1 mol = 6.02 x 1023 (Avogadro's number).

Older instruments measured light in lux or W m-2, but only approximate factors are available to convert these measures to PPFD. The incoming radiation reaching the earth on a clear day is about 100 000 lux, or 1000 W M-2, and the corresponding PPFO is about 2000 µmol m-2 s-1 (Hall and Rao 1999). A common value on an open field is about 1000 µmol m-2 and a tree canopy can reduce this to values in the range of 1-100. There are several problems with direct measures using quantum sensors: