No abstract

Roots of Pisum sativum L. and Zea mays L. were exposed to different moisture gradients established by placing both wet cheesecloth (hydrostimulant) and saturated aqueous solutions of various salts in a closed chamber. Atmospheric conditions with different relative humidity (RH) in a range between 98 and 86% RH were obtained at root level, 2 to 3mm from the water-saturated hydrostimulant. Roots of Silver Queen corn placed vertically with the tips down curved sideways toward the hydrostimulant in response to approximately 94% RH but did not respond positively to RH higher than approximately 95%. The positive hydrotropic response increased linearly as RH was lowered from 95 to 90%. A maximum response was observed at RH between 90 and 86%. However, RH required for the induction of hydrotropism as well as the responsiveness differed among plant species used; gravitropically sensitive roots appeared to require a somewhat greater moisture gradient for the induction of hydrotropism. Decapped roots of corn failed to curve hydrotropically, suggesting the root cap as a major site of hydrosensing.

We have partially characterized root hydrotropism and its interaction with gravitropism in maize (Zea mays L.). Roots of Golden Cross Bantam 70, which require light for orthogravitropism, showed positive hydrotropism; bending upward when placed horizontally below a hydrostimulant (moist cheesecloth) in 85% relative humidity (RH) and in total darkness. However, the lightexposed roots of Golden Cross Bantam 70 or roots of a normal maize cultivar, Burpee Snow Cross, showed positive gravitropism under the same conditions; bending downward when placed horizontally below the hydrostimulant in 85% RH. Light-exposed roots of Golden Cross Bantam 70 placed at 700 below the horizontal plane responded positively hydrotropically, but gravitropism overcame the hydrotropism when the roots were placed at 450 below the horizontal. Roots placed vertically with the tip down in 85% RH bent to the side toward the hydrostimulant in both cultivars, and light conditions did not affect the response. Such vertical roots did not respond when the humidity was maintained near saturation. These results suggest that hydrotropic and gravitropic responses interact with one another depending on the intensity of one or both factors. Removal of the approximately 1.5 millimeter root tip blocked both hydrotropic and gravitropic responses in the two cultivars. However, removal of visible root tip mucilage did not affect hydrotropism or gravitropism in either cultivar.

Freeze-fracturing of untreated plasma membrane and inner wall surfaces of stelar tissue in corn roots demonstrated the association of globular complexes with the ends of nascent microfibrils. It is proposed that the granule complexes associated with the outer leaflet of the plasma membrane coordinate the assembly of the cellulosic microfibrils.

Scott, Tom K., and Winslow R. Briggs. (Stanford U., Stanford, Calif.) Auxin relationships in the Alaska pea (Pisum sativum). Amer. Jour. Bot. 47(6) : 492–499. Illus. 1960.—The distribution of “free” auxin in the 9‐day‐old ‘Alaska’ pea epicotyl was determined by short‐term ether extraction and by the standard agar diffusion technique. The apical bud appeared to be the only source of “free” auxin. In the upper (growing) internode “free” auxin as determined by diffusion was found to decrease significantly from apex to base, while “free” auxin as determined by extraction remained constant. Below this region, both diffusible and extractable auxin remain constant through one internode and then both decrease simultaneously to the base of the plant. In the growing region, a fraction of diffusible auxin must move from the transport system but remain readily extractable. Upon removal of the apical auxin source all “free” auxin will ultimately be found in the transport system from which it gradually disappears basally.