Effect of Waterlogging on Mycorrhizas of Radiata Pine and Douglas Fir

Radiata pine (Pinus radiata D. Don) and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) seedlings were subjected to waterlogging periods of 1, 2, 4, 3, and 16 weeks. Phosphorus-32 uptake and succinic dehydrogenase activity of waterlogged mycorrhizas were measured and compared with non-waterlogged mycorrhizas. After 2 weeks of waterlogging Douglas fir mycorrhizas absorbed less P than control mycorrhizas; radiata pine mycorrhizas were not significantly affected. Waterlogging periods of 4 to 16 weeks reduced P uptake and succinic dehydrogenase activity in both species. INTRODUCTION Conifers with mycorrhizal root systems generally make much better growth in the field than non-mycorrhizal plants (White, 1941; Mishustin, 1967; Wilde, 1968) and a knowledge of the factors that affect mycorrhizas adversely is important. Gilmour (1958) found that Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) from Milton Nursery showed severe chlorosis and poor growth in the field. This condition was shown to be associated with lack of mycorrhizas. He noticed that mycorrhizal development in the nursery was very poor and suggested that this was due to the seasonal waterlogging which occurred there. Thus there is some evidence that mycorrhizas are adversely affected by waterlogging. This study was undertaken to find out the effect of various periods of waterlogging on mycorrhizas of two important forest species in New Zealand —radiata pine (Pinus radiata D. Don) and Douglas fir. According to Gill (1970), in previous work on the effect of waterlogging on tree roots, root vitality appears to have been judged qualitatively. Since it is very difficult to decide whether a root is dead, half-dead or alive, an attempt was made in this study to make an objective quantitative measurement of root vitality. In the presence of living tissue, colourless tetrazolium salts are reduced to coloured formazan and it has been shown that dehydrogenases are responsible for this reduction (Mattson et. al., 19Al). Succinic dehydrogenase activity has been demonstrated using tetrazolium salts (Nachlas et al, 1957; Avers and King, I960; Morrison and Kronheim, 1962). Hauser and Morrison (1964) showed that viable and non-viable pollen grains could be distinguished using succinic dehydrogenase activity (SDA) as the criterion of viability. Defendi and Pearson (1955) made a detailed study of the quantitative estimation of SDA and showed that 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyltetrazolium chloride could be used reliably for estimating the amount of succinic dehydrogenase present in N.Z. JI For . Sci. 2 (2): 222-6 No. 2 Gadgil — Effect of Waterlogging on Mycorrhizas 223 the tissues. In this study SDA as estimated by a method similar to that of Defendi and Pearson was used as a measure of the vitality of mycorrhizas. It has been established that the rate of uptake of phosphate by mycorrhizal roots is greater than that by non-mycorrhizal roots in radiata pine (Morrison, 1957; Mejstrik, 1970) and it has been suggested that the same is true for roots of Douglas fir (McComb and Griffiths, 1946). Plants with well developed mycorrhizas would be expected to absorb more phosphate than plants with poor mycorrhizas and in this study the effectiveness of mycorrhizas was estimated by measuring phosphorus-32 (P) uptake.