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Lop Phavaphutanon and Fred T. Davies Jr.

Growth and nutrient content of neem tree seedlings (Azadirachta indica A. Juss) were studied in response to the mycorrhial fungi Glomus intraradices Schenck & Smith and Long Ashton Nutrient Solution (LANS) modified to supply phosphorus (P) at 0.65 and 1.30 mM P. Three months after inoculation, an extensive mycorrhizal colonization was observed in mycorrhizal plants at both P levels. Shoot growth of mycorrhizal plants was similar at both P levels while the growth of nonmycorrhizal plants increased with increasing P supply. Mycorrhizal plants had greater leaf area, shoot dry weight and root to shoot ratio than nonmycorrhizal plants at the same P level. The length of nonsuberized roots increased with increasing P supply regardless of mycorrhizal colonization while the length of suberized roots was significantly increased by mycorrhiza. Mycorrhiza altered dry mass partitioning to root systems resulting in greater length and dry weight of suberized roots in mycorrhizal plants. Mycorrhiza also improved nitrogen, phosphorus, calcium and sulfur uptake but did not affect micronutrient uptake, except for enhancing boron.

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Lop Phavaphutanon*, Yonit Hebbe, Shlomo Cohen, and Joshua D. Klein

Compounds that modulate the synthesis of gibberellin (GA) can also enhance resistance to abiotic stress in treated plants. Seed treatments of 600 ppm trinexapac-ethyl (TE), which inhibits GA synthesis by blocking the transformation of GA20 to GA1 and foliar applications of 15 ppm paclobutrazol (Paclo), which inhibits the oxidation of ent-kaurene to kaurenoic acid, were applied separately or together to three varieties of hot pepper (Capsicum annuum L.) that are popular in Thailand. Greenhouse-grown plants were subjected to 7-10 days of drought, and then rewatered before transfer to a screenhouse. Khee Noo (an upright “bird type” pepper) was most sensitive to drought, compared to Bang Chang and Hot (“cayenne type”). In all varieties, both Paclo and TE treatments reduced the height of irrigated plants, but led to the retention of both plant size and pepper yield in droughted plants, compared to either irrigated plants or to untreated droughted plants. Treatment with Paclo provided the greatest retention of leaf relative water content (RWC) under drought conditions, with no advantage to the combination Paclo+TE treatment. Only Paclo treatment increased leaf thickness in Bang Chan and Hot, whereas both Paclo and TE had similar effects on increasing leaf thickness in Khee Noo. Khee Noo was the variety most responsive to Paclo or TE treatments, with increases in leaf thickness, epicuticular wax, and leaf pigments, all of which may better allow the plants to survive stress by storing leaf moisture, enhancing photosynthesis (chlorophyll), and preventing oxidative injury (carotenoids). Electrolyte leakage, indicative of membrane permeability and thus of susceptibility to stress, was diminished by 25% to 33% in leaves from plants treated with Paclo or TE.

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Fred T. Davies Jr., Sharon A. Duray, Sein Hla Bo, and Lop Phavaphutanon

The Neem tree is of ornamental, revegetation, biomass and medicinal value. The compound azadirachtin, which is derived from Neem seeds, is commercially used for insecticidal properties. In a 2×2 factorial experiment, Neem seedlings were either colonized with the mycorrhizal fungi Glomus intraradices or noninoculated and fertilized with full strength Long Ashton Mineral Solution at 11 or 22 ppm P. Mycorrhizal and P main effects were highly significant (p-value<0.001) with all growth parameters except R:S ratio. Mycorrhizal plants had greater leaf number, leaf area, leaf dry weight, shoot and root dry weight than noncolonized seedlings. The higher P (22 ppm) level plants had superior growth compared with low P plants. Leaf area and leaf dry weight were similar in mycorrhizal/low P plants and nonmycorrhizal/high P plants. These results suggest that mycorrhizal growth enhancement has important implications for Neem trees which are found in agriculturally poor soils in hot and arid regions.

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Fred T. Davies Jr., Sharon A. Duray, Lop Phavaphutanon, and Randy Stahl

The influence of P nutrition on gas exchange, plant development, and nutrient uptake of Capsicum annuum chile ancho `San Luis' and bell pepper `Jupiter' plants was studied. Plants were fertilized weekly using 250 ml of a modified Long-Ashton solution, containing 0, 11, 22, 44, 66 or 88 μg P/ml. Phosphorus stress was evident with both pepper cultivars at 0 and 11 μg P/ml, with reduced plant growth and development: leaf number and area and fruit, leaf, stem, root, shoot, and total plant dry weight. The root: shoot ratio was greatest at 0 μg P/ml, reflecting greater dry matter partitioning to the root system. Greater P stress occurred at 0 μg·ml–1 in `San Luis' compared to `Jupiter' (88% vs. 58% reduction in total plant dry weight compared to optimum P response). `San Luis' was also more sensitive to P stress at 11 μg P/ml than `Jupiter', as indicated by the greater reduction in growth responses. With increasing P nutrition, leaf tissue P increased in both cultivars with maximum leaf tissue P at 88 μg P/ml. In `San Luis', there were no differences in tissue P between plants treated with 0 and 11 μg P/ml, whereas the `Jupiter' plants treated with 0 μg P/ml had the lowest tissue P. Low P plants generally had the highest tissue N and lowest S, Fe, Mn, Zn, B, Mo, and Al. With both cultivars, gas exchange was lowest at 0 μg P/ml, as indicated by low transpiration (E), stomatal conductance (gs), and net photosynthesis (A). Internal CO2 (Cj) and vapor pressure deficit were generally highest at 0 μg P/ml, indicating that Cj was accumulating with lower gs, E, and A in these P-stressed plants. Generally, no P treatments exceeded the gas exchange levels obtained by 44 μg P/ml (full strength LANS) plants.

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Fred T. Davies Jr., Sharon A. Duray, Lop Phavaphutanon, and Randy Stahl

In two separate experiments, the influence of phosphorus nutrition on gas exchange, plant development, and nutrient uptake of Capsicum annuum chile ancho `San Luis' and bell pepper `Jupiter' plants were studied. Plants were fertilized weekly using 250 ml of a modified Long–Ashton solution (LANS) containing 0, 11, 22, 44, 66, or 88 μg P/ml. Phosphorus stress was evident with both pepper cultivars at 0 and 11 μg P/ml, with reduced plant growth and development: leaf number and area, fruit, leaf, stem, root, shoot, and total plant dry weight. The root: shoot ratio was greatest at 0 μg P/ml, reflecting greater dry matter partitioning to the root system. Greater phosphorus stress occurred at 0 μg P/ml in `San Luis' compared to `Jupiter' (88% vs. 58% reduction in total plant dry weight compared to optimum P response). `San Luis' was also more sensitive to phosphorus stress at 11 μg P/ml than `Jupiter' as indicated by the greater reduction in growth responses. With increasing P nutrition, leaf tissue P increased in both cultivars with maximum leaf tissue P at 88 μg P/ml. In `San Luis', there were no differences in tissue P between 0 and 11 μg P/ml plants, whereas 0 μg P/ml `Jupiter' plants had the lowest tissue P. Low P plants generally had the highest tissue N and lowest S, Fe, Mn, Zn B, Mo, and Al. With both cultivars, gas exchange was lowest at 0 μg P/ml, as indicated by low transpiration (E), stomatal conductance (gs), and net photosynthesis (A). Internal CO2 (Ci) and vapor pressure deficit were generally highest at 0 μg P/ml, indicating that Ci was accumulating with lower gs, E, and A in these phosphorus-stressed plants. Generally, no P treatments exceeded the gas exchange levels obtained by 44 μg P/ml (full strength LANS) plants.