Coniferous Root Exudate.

The literature on root exudate is replete with studies of factors influencing the quantity and cycle of its production. For the most part, this work has been done utilizing herbaceous species or vines, but citations are also available on some of the broad-leaved trees. Root exudate from conifers is rarely reported, but in the course of preparation of a thesis at the University of California it was convenient to test several young conifers for root exudate production. Exudate from decapitated conifers of several species, with sunflower's exudate under comparable conditions for comparison, is given in the accompanying table. The table gives the species, the volume of root exudate at each reading in cubic centimeters, and the time of each measurement. The data were collected as supplemental information to a series of transpiration studies and not as a designed experiment, which explains the different decapitating times for the various plants as indicated by 0.0 in table. The plants had been exposed to a series of humidities (90, 50, 70, 50, 30, 70, 90) at 750 F. for 22 days with 24 hours of light alternating with 24 hours of darkness. On October 4, the lights were turned off at 7: 30 A.M. and the 90%o humidity maintained. The plants were then cut off as fast as leaf area measurements could be made, which resulted in a difference of two and one-half days between the decapitation of the first plant and the last. The trees were approximately eight months old and grown in culture solution (1/10 normal Hoagland's Solution) with aeration. The sunflowers were seven weeks old at the time of decapitation after being grown in the same solution as the trees throughout their growth period. All plants, during the transpiration study and during the exudate measurements, were in the same culture solution and aerated. Most of the root systems of the plants were healthy at time of decapitation, although some had failed to develop the masses of new rootlets common to the others, possibly because of injuries incurred during the experiment. After decapitation, a short piece of rubber tubing was slipped over the top of the stump to catch any exudate. Exudate was measured by drawing it into a 5 cc. graduated cylinder. The tank containing the roots was immersed in a constant temperature bath held at 750 F. in a room that was windowless and dark most of the time. Exudate was obtained from all the species used, and with only one specimen (ponderosa pine) failing to give some signs of it. The data are chiefly notable, other than substantiating the existence of coniferous root exudate, for the performance of a couple of individual trees, especially 327