Reduced Water Potential Research Articles

The Water Relations of the Alga Cyanidium caldarium in Soil

SUMMARY: The effects of water potential on the photosynthesis, growth and viability of natural populations and laboratory cultures of Cyanidium caldarium were measured. C. caldarium was very sensitive to reduced water potential, matric reduction being more harmful than osmotic. The distribution of this alga in different soil areas appeared to be related to the sensitivity to water stress, larger populations being found in areas with higher water potential. There are very steep gradients of water potential in the soils where Cyanidium caldarium is found. The water potential of the surface crust is much too low to allow growth and C. caldarium is found in a subsurface layer. The reduced light intensity below the soil surface interacts with the increased water potential to define the position of the algal layer.

The Effect of Reduced Water Potential on Soybean Mitochondria

The respiration of excised hypocotyls and of isolated hypocotyl mitochondria from soybean [Glycine max (L.) Merr., var. Wayne] was determined in various concentrations of sucrose and potassium chloride. Hypocotyl oxygen uptake declined with increasing solute concentration; no specific effects of either solute were apparent. Mitochondrial state III respiration was strongly inhibited as the solute concentrations were raised and there was in addition a specific inhibitory effect of the salt. State IV respiration, however, was unaffected by the presence of osmoticum. ADP/O ratios were also unaffected, except at high potassium chloride concentrations (470 mm). The primary effect of solutes was thus to limit the rate of substrate oxidation.Hydrostatic pressure did not reverse the decline in net phosphorylation accompanying reduced oxidation. It was inferred therefore that the inhibition was not due to lower water potential per se, but rather to some other effect of water or solute concentration.The effect of solutes on a mitochondrial enzyme, malate dehydrogenase, was also examined. Sucrose inhibited malate oxidation by both the mitochondria and the isolated enzyme in parallel, while potassium chloride was more inhibitory on the isolated enzyme. It was concluded that although the addition of solute lowers the water potential, the primary effects are exerted through specific effects of the solute on enzyme activity.

EFFECTS OF GROWTH REGULATING SUBSTANCES AND WATER POTENTIAL ON THE DEVELOPMENT OF SECONDARY XYLEM IN FRAXINUS

SummaryMarked variations in secondary vascular growth of detached Fraximis excelsior L. stems were obtained with changes in water potential and in IAA and GA concentrations in the culture medium. The width of new tissue, the number of fibres and the mean fibre diameter increased with IAA concentration up to 10 mg.1−1 and generally with GA concentration up to 100 mg.1−1, but by far the greatest influence was that of water potential wbich, especially between zero and −0.1 J.g−1, significantly depressed cell division and expansion. Expansion of cambial initials to a diameter of about 6 μ appeared to be necessary before active cell division commenced.Vessel development was particularly sensitive to IAA supply and the amount of IAA required for the formation of groups of vessels increased as the number of constituent vessels increased and as the water potential was reduced. Gibberellin bad no significant effect on the number of vessel groups formed, but appeared to increase the number of vessels formed in any group. Vessel formation may occur without cell division in an initially dormant cambium. Increasing the IAA supply led to increasing vessel cross sectional areas, but progressively less effectively as the water potential was reduced; the marked reduction in vessel lumen area between zero and −0.1 J.g−1 potential suggests that vessels formed at reduced water potentials may have different metabolic patterns than those formed at zero potential from an active cambium.The significance of the depressing effect on cell division and expansion of even a slight reduction in water potential is discussed in relation to wood development in trees growing under conditions of moisture stress.