Transpiration and Physico-chemical Properties of Leaves as Related to Drought Resistance in Loblolly Pine and Shortleaf Pine.

In the reforestation program now being carried on by various state and federal agencies, many millions of tree seedlings are planted each year. An extremely large number of these planted seedlings are killed within the first year or two after planting. One of the major causes of the fatalities is the inability of seedlings to resist drought. If the properties involved in drought resistance of seedlings were known, and if those species and age classes of planting stock possessing such properties could be identified or developed, much better survival could be obtained in plantations of tree seedlings. Drought resistance, the ability of a plant to endure permanent wilting without harm to subsequent development (11), has been induced in plants by repeated wilting and also by growing them in soil with a moisture content barely above the wilting percentage (27, 28). Observations on permanently wilted herbaceous plants (27) show that their transpiration rate may be less than one-fourth of the rate for turgid plants. Most conifers, however, do not show any external evidence of a wilted condition. Hence, in hardening conifers against drought, the wilting percentage of soil moisture must be previously determined to facilitate control of soil moisture conditions. Although attempts have been made to correlate drought resistance with rate of transpiration, there is much evidence (1, 2, 4, 21, 24, 32) to support the statement of MAXIMOV anld KRASNOSSELSKY-MAXIMOV (12) that the rate of transpiration when soil moisture is abundant cannot be used for judging drought resistance. Transpiration may be greatly reduced when soil moisture approaches the wilting percentage (15, 19) and the reduction may vary among plants of different morphological structure, but there appears to be no consistent relationship between drought resistance and rate of transpirationi at soil moisture contents near the wilting percentage. There is evidence of a correlation between drought resistance and bound water content of leaf tissue. KORSTIAN (9) found some indication of such a correlationi in a preliminary study of southeastern tree species. The work of NEWTON and MARTIN (17) is particularly outstanding in that a very definite correlation was obtained in cereal crops and grasses between the degree of drought resistance and the bound water content. NovIKOV (18) also