Samanea leaflets usually open in white light and fold together when darkened, but also open and close with a circadian rhythm during prolonged darkness. Leaflet movement results from differential changes in the turgor and shape of motor cells on opposite sides of the pulvinus; extensor cells expand during opening and shrink during closure, while flexor cells shrink during opening and expand during closure but change shape more than size. Potassium in both open and closed pulvini is about 0.4 N. Flame photometric and electron microprobe analyses reveal that rhythmic and light-regulated postassium flux is the basis for pulvinar turgor movements. Rhythmic potassium flux during darkness in motor cells in the extensor region involves alternating predominance of inwardly directed ion pumps and leakage outward through diffusion channels, each lasting ca 12 h. White light affects the system by activating outwardly directed K+ pumps in motor cells in the flexor region.

One hypothesis to explain dieback of red spruce (Picearubens Sarg.) trees in high-elevation red spruce stands is that atmospheric deposition may introduce nitrogen in excess of plant needs (N saturation), which may disrupt normal metabolism and foliage development in this harsh environment. Some authors suggest that direct foliar uptake of N might contribute to N saturation. To examine the importance of foliar uptake by red spruce, we exposed seedlings in a greenhouse to a mist that simulated cloud water and contained either ammonium or nitrate as 15N (99 at.% excess at 2 mg/L). After 50 h exposure to mist, seedlings were washed and then separated into four tissue types: new foliage, old needles, stems, and roots. Total and isotopic nitrogen contents were determined for each tissue type. The accumulation rate of 15N in each tissue type was very low. Extrapolating our data on the basis of a year suggests that N from cloud water supplies only a small fraction of the N required for new growth (less than 1.5% for the seedlings we used). We observed that both ammonium-15N and nitrate-15N accumulated in stem tissues and could not be removed after repeated washings, which suggests an alternative mechanism for long-term N retention by red spruce stands.

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Phytochrome, a membrane-localized biliprotein whose conformation is shifted reversibly by brief red or far-red light treatments, interacts with the rhythmic oscillator to regulate leaflet movement and potassium flux in pulvinal motor cells of Samanea. Darkened pinnae exposed briefly to red light (high Pf, level) have less potassium in motor cells in the extensor region, more potassium in motor cells in the flexor region, and smaller angles than those exposed to far-red light (low PI, level). Increase in temperature from 24 to 37°increases the differential effect of the light treatments during opening (the energetic phase) but not during closure, implying that phytochrome controls an energetic process. It seems likely that phytochrome interacts with rhythmically controlled potassium pumps in flexor and extensor cells. During nyctinastic closure of white-illuminated pinnae, exposure to far-red light before darkening results in larger angles than does exposure to red. As in rhythmic opening, the angles of all pinnae and the differential effect of the light treatments increases with increasing temperature.