|Abstract or Summary
- The photoperiodic responses of Douglas-fir (Pseudotsuga menziesii (Mirb,) Franco) seedlings grown from seed collected at Salmon Arm, British Columbia, Coconino National Forest, Arizona, and Southern Vancouver Island were studied. Plants were grown for 100 days under a 12-hour photoperiod at 200 C. The effective length of the daily light and dark periods were altered by interruption at various times with different durations of red and far-red light. Interruptions of darkness with 2 15, and 30 minutes of red light increased the duration of growth, epicotyl and leaf lengths, and leaf numbers of all plants. Responses to these treatments increased as exposure times approached the center of the dark period. Exposures given equal times before or after the middle of darkness produced equal responses. Magnitudes of response increased as exposure lengths increased. Both the nature and magnitude of response varied among seed sources. Responses to 30 and 55 minutes of far-red light unexpectedly resembled those to red light. Apparently, excessive far-red light reversed the normal photoreaction of phytochrome and caused the accumulation of the active form (Pfr) of the pigment. Fifteen-minute exposures accelerated the onset of dormancy and reduced the growth of all plants. Since responses to red light increased as exposure times approached the center of darkness, it appeared that each exposure produced similar Pfr concentrations and that dark conversion during the period of uninterrupted darkness determined the level mediating response. However, the dependence of response upon timing might also indicate that a particular Pfr concentration had to be present a specific length of time in order to produce a response. Consequently, responses were attributed to levels of Pfr activity not concentration. Since all plants were grown in uniform environments, some proportion of the variation among seed sources is heritable and reflects their adaptation to the three habitats. Salmon Arm plants required larger, critical levels of Pfr activity for continued growth than Coconino plants. This difference seemed an expected result since day lengths in northern areas are longer during the growing season. The critical requirement of Salmon Arm plants demonstrated that changes in day length are important in regulating their annual growth cycle. Such regulation would seem a prerequisite to survival for plants from an area of short growing seasons and large, abrupt seasonal changes in climate. Vancouver plants had the lowest and least critical requirement for Pfr activity, indicating that changes in day length were of lesser importance in the control of growth periodicity. Such control would seem to be, and apparently is, of lesser selective value for plants from a coastal area having long growing seasons and gradual transitions between seasons. Coconino plants resembled Salmon Arm plants in requiring a large, somewhat critical level of Pfr activity. This similarity reflects adaptation to the short growing season and seasonal extremes of climate characteristic of their mountainous, continental habitat. However, Coconino plants differed from the others in terms of their synchronous cessations and renewals of growth. This intermittent pattern of growth may reflect differences in the seasonal distribution of precipitation at the origins of the two continental sources. Summer rains constitute a greater portion of the annual rainfall in the Southern Rocky Mountains than in northern areas. Intermittent growth under the long days of the growing season could permit dormancy if moisture was limiting, but could favor growth resumption if summer rains caused conditions to remain or become favorable.