Influence of Vesicular-arbuscular Mycorrhizae on the Growth and Water Relations of Vegetable Crops

Abstract

Onion, leek, pepper, lettuce and tomato transplants grown in a soilless medium inoculated with the VAMF Glomus aggregatum (Schenck and Smith emend. Koske) were larger and had higher tissue phosphorus (P) concentrations than non-inoculated plants if P levels in the medium were low. At higher P concentrations, inoculation had little or a slightly negative effect on transplant growth. Increasing P concentrations in the medium increased transplant growth, but decreased root infection by the VAMF. Increasing VAMF inoculvim concentrations did not affect growth or P uptake but increased VAMF infection of the transplants. Daily application of low P fertilizer solutions produced larger transplants with more extensively infected root systems than did similar amounts of P supplied less frequently but at higher concentrations. Different crops required different combinations of P concentration and application interval to produce vigorous mycorrhizal transplants. The controlled-release fertilizer Osmocote (Sierra Chemical Co., Milptas Calif.) produced predictable and stable solution P concentrations in the soilless medium used for transplant production. Growth and VAMF infection of the transplants could be manipulated by altering Osmocote P concentrations in the transplant medium. Pre-transplant inoculation of peppers subsequently planted into P deficient soil improved early P uptake, vegetative growth and total fruit yields relative to plants inoculated at transplanting. In P deficient soils, maximum pre-transplant VAMF infection of peppers increased subsequent growth and fruit yields more than maximum pre-transplant growth. Extensive pre-transplanting infection improved post-transplant P uptake earlier than in less heavily infected plants. In contrast, pre-transplant growth of lettuce was more important than mycorrhizal infection in determining subsequent growth, at all soil P levels. In pots, G. aggregatum increased total dry matter yields, promoted early fruit set and improved fruit yields of peppers at solution P concentrations below 0.3 to 0.4 mg/liter. At higher P concentrations, VAMF infection had no beneficial or harmful effects. Tissue P requirements for dry matter production by mycorrhizal plants were lower than in non-mycorrhizal plants, suggesting that mycorrhizae may influence the efficiency of utilization of absorbed P in addition to increasing P uptake efficiency. In the field, inoculation of peppers increased tissue P concentrations, growth and fruit yields by 28, 120 and 350% respectively relative to non-mycorrhizal plants in a fumigated P-fixing soil with 0.03 mg/liter solution P. Inoculation had no significant effect at 0.30 mg P/liter. Under similar conditions, VAMF infection increased yields of lettuce by 16%. Although non-mycorrhizal lettuce and peppers had similar solution P requirements for maximum growth, lettuce was more tolerant of sub-optimal solution P concentrations and was correspondingly less responsive to infection by the VAMF. Moderate water stress increased mycorrhizal responsiveness of peppers growing in P deficient soil. VAMF colonization of pepper seedlings growing in P deficient media increased the hydraulic conductivity of their roots, possibly by improving seedlings tissue P status. Mature mycorrhizal peppers had higher rates of transpiration per unit leak area than similar size non-mycorrhizal plants. At wilting, mycorrhizal transplants and mature peppers had higher leaf water potentials at lower soil water potentials than non-mycorrhizal plants. The influence of mycorrhizae on the water relations of mature peppers was apparently related to the mycorrhizae improving P uptake by their host but the change in water relations of inoculated seedlings was generally independent of host P-status.