Recent studies of physiological ontogeny in plants have revealed a a number of significant interrelationships among different plant organs. Considerable knowledge of the formative effect of one organ upon another
during the course of development has been obtained by exposing various
parts of the plant to contrasted environmental conditions such as differences in photo-period or temperature. In studies of this kind, however, observations have usually been confined to responses of the shoot with only incidental attention to the root. Though the effect of temperature upon development has been repeatedly studied, entire plants have commonly been exposed to different temperatures in such a manner that it has been difficult, if not impossible, to determine whether variations in formative response were primarily due to the effect of a given temperature upon the root or upon the shoot. In order to determine whether a given temperature exerted its characteristic formative and metabolic effects primarily through the root or through the shoot, a growth experiment employing a rapidly developing annual with tops and roots exposed to unlike temperatures was devised.

Ordinary hemp, Cannabis sativa, was selected as test material, not only
because of its experimental convenience and known sensitivity to temperature, but because of current interest in its response to the low temperatures of the northern latitudes in which commercial hemp production is at present being developed. A record of structural development was maintained from the time of germination to maturity, and the various formative responses were correlated with salt and water intake.

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There are some really fascinating results here. The temps for the test were H-86 deg F, L 60 Deg F. This is a VERY important body of data and a must read. The things that stand out to me are as follows:

  1. Low Top (Air)/ High Root (L/H) produced more bigger leaves.
  2. L/H produced thicker stems and longer internode spacing
  3. H/H produced taller plants with more nodes.
  4. The sex results are somewhat complicated but clearly show a connection to temp and seed sex results, and Low temps seems to favor female.
  5. L/H Dry weight of plant overall was substantially higher than either of the High temp (H/H, H/L) subjects. Even the L/L significantly outperformed the higher temps. The increase was in leaf and stem weight. They give the following analysis:

“ARNDT (1), DEMIDENKO and BARINOVA (6), KRAMER (12, 13, 14) and Maximov (16) essentially agree that soil water viscosity is increased at lowered soil temperature, rendering less water available to plants. Lowered absorbing capacity of the roots is attributed to decreased permeability of root cell walls and protoplasm. These factors, together with retarded physiological activity at low soil temp, result in slower root growth with the eventual production of a smaller root system.”

  1. Also, flowering occurred sooner with higher temps in both zones.
  2. Cooler root temps seem to impair nutrient transfer with a high level of minerals found in lower stem.

Sadly, they only used temps that were in effect extremes and so we have no data for the temperatures normally associated with cannabis today. But some interesting questions are raised. I assume the nutrients were “salts” i.e hydroponic, but they didn’t spell it out. I suspect if this was conducted with a true organic “Soil” grow, the L/H hot soil results would even be more pronounced, since bacteria tend to multiply exponentially with temp to an upper limit. This certainly suggests that there could be real benefits to warming the soil, and the oxygen depletion normally associated with warm roots / nutrients didn’t appear to be a problem.