Influence of vesicular-arbuscular mycorrhiza on Leucaena leucocephala growth, water relations and nutrient acquisition

Abstract

Knowledge of the dynamics of VA mycorrhizal plant systems is fundamental to the understanding of the relationships between VA mycorrhiza, plants and soil. Pot experiments were conducted to: 1) determine early physiological responses to VA mycorrhizal inoculation, 2) measure the acquisition and utilization of water and nutrients, dry matter production and assimilate partitioning in mycorrhizal and non-mycorrhizal plants, and 3) develop a model of VA mycorrhizal influence and its consequences to the physiology and ecology of VA mycorrhizal plants. Leucaena leucocephala seedlings, with and without the VA mycorrhizal fungus (Glomus aggregatum), were grown in a Wahiawa soil (Tropeptic Eutrustox) with soil P levels ranging from 0.005 to 0.429 mg L-1 of P in 0.01 M CaCl2 extract. Without mycorrhizal infection, leucaena plant growth was stunted under low soil P conditions. Even with high P fertilization, the growth of non-mycorrhizal plants was less than the growth of mycorrhizal plants. Daily pinnule sampling, pot weighing methods and multiple 5-day-interval harvests revealed a series of changes in nutrient uptake, dry matter production and water transpiration between mycorrhizal and non-mycorrhizal plants. The series of changes was as follows: 1) Five days after inoculation, plant roots had about 7% mycorrhizal infection. 2) At 10 days, root P concentrations were higher in mycorrhizal plants than in non-mycorrhizal plants. By 15 days after inoculation, increases in shoot P, K and S concentrations were observed in mycorrhizal plants. Shoot Mg and Ca concentrations in mycorrhizal plants were greater than in non-mycorrhizal plants at 20 and 25 days after inoculation, respectively. From 10 to 15 days after inoculation, the flux of P into mycorrhizal roots was greater than that into non-mycorrhizal roots. 3) Elevated nutrient contents in shoots of mycorrhizal plants was followed by superior growth rates. Mycorrhizal plants also allocated more assimilate to leaf growth than did non-mycorrhizal plants. Increased leaf growth was followed by increased transpiration. 4) Leaf area expansion rates and net assimilation rates were greater for mycorrhizal plants than for non-mycorrhizal plants. Greater dry weight was observed in mycorrhizal plants, supporting further growth of the mycorrhizal roots (positive feedback), and 5) Greatest soil volume was explored by the mycorrhizal roots. A scheme to explain these changes is pressed and used to describe processes involved in the soil-mycorrhiza-plant system. In contrast, the flux of P into non-mycorrhizal roots decreased during the period 10 to 15 days after transplanting. The resulting low P content in non-mycorrhizal plants further reduced relative leaf expansion rates, net assimilation rates and later reduced relative root expansion rates (negative feedback). Nevertheless, when non-mycorrhizal plants were subsequently inoculated they eventually attained a similar size and weight as mycorrhizal plants. The stunting of non-mycorrhizal plants thus appears to be reversible and probably is part of a survival strategy which reduces energy use while retaining the potential for mycorrhizal infection.