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Tropical Tree Seedlings Exposed to Elevated Nighttime Air Temperatures
Krause, G.H., Cheesman, A.W., Winter, K., Krause, B. and Virgo, A. 2013. Thermal tolerance, net CO2 exchange and growth of a tropical tree species, Ficus insipida, cultivated at elevated daytime and nighttime temperatures. Journal of Plant Physiology 170: 822-827.

The authors write that "information on thermal tolerance and the ability of plant species in the humid tropics to acclimate to altered high temperature regimes is scarce and largely restricted to a small number of important agronomic species." And, therefore, they decided to add to this "small number" of studied species, while simultaneously shifting their attention to a neotropical pioneer tree species, Ficus insipida.

What was done
Seeds of F. insipida were collected from mature trees and germinated in January 2012 in peat pellets until they developed 4 or 5 leaves, after which they were transferred into 2.2-L plastic pots filled with a commercial potting mix, given an initial dose of slow release fertilizer and thereafter watered to field capacity daily. This was done while the plants were maintained in controlled-environment chambers, where air temperature was held steady at 39°C during each 12-hour light period and at either 32 or 22°C during each 12-hour dark period. And at various times during their 39-day study, Krause et al. measured certain plant physiological parameters and processes, after which total plant biomass was determined.

What was learned
The five researchers report that "seedlings cultivated at 39/32°C exhibited much faster growth than seedlings grown at 39/22°C." In fact, they say that "total biomass accumulation was about three times higher in plants grown at the elevated nighttime temperature, as compared to biomass accumulation at 39/22°C," while their actual data indicate that total biomass accumulation was 3.25 times faster.

What it means
In discussing their findings, Krause et al. write that their most striking result was "the profound increase in biomass accumulation of plants under elevated nighttime temperature," which "contrasts with the widely assumed negative impact of increased nighttime temperature upon tropical tree growth and the general paradigm that warmer temperatures reduce tree growth in the tropics." Consistent with their findings, however, they note that "nighttime warming has been shown to stimulate growth in tobacco (Camus and Went, 1952), cotton (Koniger and Winter, 1993), and in another tropical pioneer tree species, Ochroma pyramidale (A. Cheesman and K. Winter, unpublished data)." They also note that "increased night-time temperatures have been implicated in increased growth of red oak seedlings in urban environments in upstate New York (Searle et al., 2012)." And if these results ultimately turn out to be representative of the majority of plants, it will bode well for earth's plant life, in that the warming of the last few decades of the 20th century typically has found minimum nighttime temperatures rising faster than maximum daytime temperatures.

Camus, G.C. and Went, F.W. 1952. Thermoperiodicity of three varieties of Nicotiana tabacum. American Journal of Botany 38: 521-528.

Koniger, M. and Winter, K. 1993. Growth and photosynthesis of Gossypium hirsutum L at high photon flux densities: effects of soil temperatures and nocturnal air temperatures. Agronomie 13: 423-431.

Searle, S.Y., Turnbull, M.H., Boelman, N.T., Schuster, W.S.F., Yakir, D. and Griffin, K.L. 2012. Urban environment of New York City promotes growth in northern red oak seedlings. Tree Physiology 32: 389-300.

Reviewed 2 October 2013