THE TROPIC RESPONSE OF PLANT-ROOTS TO OXYGEN - OXYTROPISM IN PISUM-SATIVUM L

Plant roots are known to orient growth through the soil by gravitropis m, hydrotropism, and thigmotropism. Recent observations of plant roots that developed in a microgravity environment in space suggested that plant roots may also orient their growth toward oxygen (oxytropism). U sing garden pea (Pisum sativum L. cv. Weibul's Apollo) and an agravitr opic mutant (cv. Ageotropum), root oxytropism was studied in the contr olled environment of a microrhizotron. A series of channels in the mic rorhizotron allowed establishment of an oxygen gradient of 0.8 mmol.mo l(-1).mm(-1). Curvature of seedling roots was determined prior to free zing the roots for subsequent spectrophotometric determinations of alc ohol dehydrogenase activity. Oxytropic curvature was observed all alon g the gradient in both cultivars of pea. The normal gravitropic cultiv ar showed a maximal curvature of 45 degrees after 48 h, while the agra vitropic mutant curved to 90 degrees. In each cultivar, the amount of curvature declined as the oxygen concentration decreased, and was line arly related to the root elongation rate. Since oxytropic curvature oc curred in roots exposed to oxygen concentrations that were not low eno ugh to induce the hypoxically responsive protein alcohol dehydrogenase , we suspect that the oxygen sensor associated with oxytropism does no t control the induction of hypoxic metabolism. Our results indicate th at oxygen can play a critical role in determining root orientation as well as impacting root metabolic status. Oxytropism allows roots to av oid oxygen-deprived soil strata and may also be the basis of an auto-a voidance mechanism, decreasing the competition between roots for water and nutrients as well as oxygen.