THE NATURE OF GROWTH DEPRESSIONS IN SUNFLOWER CAUSED BY VESICULAR–ARBUSCULAR MYCORRHIZAL INFECTION

S ummary When mycorrhizal (M) and non‐mycorrhizal (NM) plants were grown in a soil of low phosphorus (P) availability (5 μg g −1 ), mycorrhizal infection caused a transient depression in leaf area relative to NM plants that lasted for seven to eight weeks. When plants were grown in this same soil with added P, leaf areas of M and NM plants were similar. When plants were grown in a soil of moderate P availability (15 μg g −1 ), infection decreased leaf area throughout the experiment. High soil P levels were associated with lower degrees of infection. The degree of infection was positively correlated with the degree of depression of leaf area while the soil P level was negatively correlated with the ability to overcome the initial depression. When M and NM plants were grown in the soils of either low or moderate P availability, the relationship between relative leaf expansion rate (RLER) and tissue P concentration was the same for M and NM plants. Also, mycorrhizal infection did not affect plant N concentration to the same degree as it did P concentration. When grown in the soil of low P availability, dry matter in the shoot was shown to be partitioned differently for M and NM plants. M plants in vested mo re dry matter in leaves relative to stems compared to NM plants. Phosphorus‐amended NM plants also invested more dry matter in leaves relative to stems compared to unamended NM plants. Because of the possible difference in leaf dry weight: stem dry weight ratio between M and NM plants, final harvest shoot dry weights were not an adequate measure of the physiological status of the plants. Leaf expansion was shown to be a good indicator of M effects on plant growth.