Simple unheated greenhouses covered with clear polyethylene, also known as high tunnels, in which plants are typically grown in the ground have become popular for extending the growing season for high-value horticultural crops. Although they are used principally to produce annual crops such as vegetables and cut flowers, increasing interest has focused on their use for perennial crops such as raspberries, blackberries, and ornamentals. Studies of temperature variation within the tunnel during the growing season have emphasized the rapid rise in air day temperature above ambient during the day and an equally rapid decrease at night. Spatial variation in temperature within the tunnel were much less marked, however, with air temperatures at the edge of a 10-m wide tunnel only ≈2 °C lower than in the center. For perennial crops, tunnel conditions during the off-season are also an important factor. In winter, air temperatures in the tunnel during sunny days rose above freezing even when ambient air temperatures stayed below freezing. Soil temperatures during the day and night fluctuated much less both inside and outside the tunnel and were significantly higher in the tunnel. Studies with nursery plants overwintered in similar structures indicate that spatial variation is again dwarfed by the overall air temperature fluctuation in these structures.
Hans Christian Wien
Joyous Suiyigheh Tata and Hans Christian Wien
The petals of some sunflower (Helianthus annuus) cultivars used as cut flowers are easily knocked off the flower; this ruins its appearance and destroys its market value. The objectives of this study were to characterize an abscission zone, if present, at the base of petals of sunflower florets in cultivars that differ with respect to petal drop and to determine if differences in the nature and/or development of the abscission zone among cultivars were correlated with differences in timing with respect to petal drop. In the first experiment, we measured the force required to pull petals from the flower head; in the second we investigated changes in the anatomy of the petal–achene juncture. Anatomical analysis revealed a differentiated region (the abscission zone) at the junction of the petal and achene consisting of cells with a different morphology from those above and below it. Cell division at the abscission zone of the short-lived cultivar occurred earlier than in the long-lived cultivar. These differences indicate that whereas the anatomical nature of the abscission zone is similar in the two cultivars, Procut Yellow Lite (PYL) and Procut Bicolor (PBC), the tempo of development differed. Specifically, the abscission layer reached full differentiation, or maturity, sooner in PBC, hence its earlier petal drop, than in PYL.