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Cuttings of neem trees (Azadirachta indica) were grown for 65 days at four P levels: 0, 15, 30, and 60 mg P/kg soil. Half of the plants were inoculated with the vesicular–arbuscular mycorrhizal fungi (VAM) Glomus intraradices. VAM increased growth and net photosynthesis (A) at the lowest two soil P levels. Increased A was attributed to increased stomatal conductance (g) and greater leaf P concentration. Nonstomatal inhibition of A due to P deficiency also was observed in non-VAM plants at lower soil P levels. At higher soil P, VAM and non-VAM plants had comparable growth, A, g, and tissue concentration of P and other elements. VAM plants at 0 mg P/kg soil had similar growth and leaf P concentration when compared to non-VAM plants at 15 mg P/kg soil, yet had a 11% higher A, indicating a direct effect of VAM on gas exchange. As soil P increased, total VAM colonization and vesicle formation decreased, while the amount of extraradical hyphae increased. Arbuscule formation was highest at 0 and 15 mg P/kg soil. Apparently, arbuscules and extraradical hyphae play an important role in the enhanced growth and gas exchange of VAM plants at lower soil P levels.
Symbiotic mycorrhizal fungi increase the P uptake of agronomic, horticultural, and forestry crops. Little is known about the real-time dynamics of carbon balance (net gain of biomass resulting from photosynthesis less the respiratory costs) of plants colonized with mycorrhizae. Our objective was to determine the carbon balance of endomycorrhizal (VAM) chile pepper `San Luis' (Capsicum annuum L.) as a model system for predicting plant response to limited P availability under elevated CO2. The increase in atmospheric CO2 is expected to result in increased plant productivity and greater demand for soil P, however, the lack of available soil P may become the most important nutritional problem limiting crop productivity. Under current conditions, the limitation of soil-P availability is an enormous problem that affects 25% of the world's arable lands. We are quantifying the carbon costs paid by the mycorrhizal plant under varying levels of P deficiency over the life cycle of the plant. Preliminary results from this study under ambient CO2 conditions indicate that there is a lower maintenance respiration and higher growth efficiency with mycorrhizal pepper plants under low soil-P conditions.
Growth recovery of mycorrhizal (VAM) and nonmycorrhizal (non-VAM) neem plants after drought exposure were followed under low phosphorus conditions. Drought significantly decreased plant growth regardless of mycorrhiza. Relative growth rate of droughted plants was greater than nondroughted plants during the growth recovery period, and compensated the loss of growth during the previous drought. VAM increased plant growth and improved regeneration of new roots outside the original root balls, particularly in plants previously exposed to drought. New roots of VAM plants were readily colonized by the VAM fungi, while those of non-VAM plants remained uncolonized. VAM growth enhancement after drought exposure was associated with greater uptake of phosphorus and other nutrients, and improved root regeneration.
Seedlings of Capsicum annuum L. cv. San Luis were grown in pots containing a pasteurized mixture of sand and sandy loam soil inoculated or noninoculated with the V-A mycorrhizal (VAM) fungus Glomus intraradices Schenck et Smith. Long Ashton nutrient solution (LANS) was modified to supply P at 0, 11 or 44 μg·ml–1. Diurnal gas exchange measurements were taken 15, 30 and 50 days after the experiment was initiated. Plant growth, leaf elemental content, and mycorrhizal development were assessed 52 days after transplanting. Gas exchange and net photosynthesis were enhanced by mycorrhiza and full strength LANS fertilization (44 μg·ml–1). The symbiosis increased leaf nutrient content of P, K, Mg, S, Fe, Mn, Zn, Cu, B, Mo, and Al. Mycorrhizal plants had higher shoot dry weights, leaf number, leaf area, and fruit primordia than nonmycorrhizal plants with P at 0 and 11 μg·ml–1. Root colonization (arbuscules, vesicles, and internal and extraradical hyphae development) were higher with P at 0 and 11 μg·ml–1. The quantity of spores recovered in soil was independent of P treatments.
Mycorrhizal (VAM) and phosphorus (P)-supplemented nonmycorrhizal neem plants (non-VAM) of comparable size and tissue nutrition were subjected to a slowly developing drought. VAM and non-VAM plants responded to drought similarly. However, mycorrhiza compensated for low P supply, allowing VAM plants to have comparable growth, tissue P, and other physiological parameters as non-VAM plants, which received higher P supply. Drought decreased growth, transpiration (E), photosynthetic rate (A), stomatal conductance (gs ), and plant water status. Osmotic adjustment did not occur, but the relatively low osmotic potential of this species helped maintain turgor during drought. Plant water relations and A of stressed plants fully recovered in 24 hours after rehydration, while gs and E partially recovered. Instantaneous water use efficiency (A/E) increased during drought and recovery, except for a decrease at peak stress due to very low A. Carbon isotope discrimination (D) values of mature leaves remained constant regardless of mycorrhiza or drought. However, D decreased in expanding leaves that developed during a drought period, indicating an increased long-term water use efficiency of these leaves.