M.S. - Agronomy and Soil Science

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    Species-specific plant-soil interactions in a Micronesian mangrove forest
    ( 2000) Gleason, Sean Michael ; Agronomy and soil science
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    Effects of Exchangeable Soil Calcium, Magnesium and Calcium/Magnesium Ratios on Plant Nutrition and Growth of Lettuce on an Ultisol
    ( 1999) Zhang, Guo Qing
    Two greenhouse pot experiments were conducted to study the effects of exchangeable soil calcium, magnesium and calcium/magnesium ratios on nutrition and growth of lettuce (Lactuca sativa L.) on an Ultisol (Manana soil series) with low pH (4.35), Ca (0.57 cmolc kg-1) and Mg (0.60 cmoh kg-1) in Hawaii to obtain calibration data for Ca and Mg in Hawaii soils and test the current sufficiency recommendations for Ca and Mg for making fertilizer recommendations. The objectives of this study were; to identify the sufficiency levels of exchangeable soil Ca and Mg for growth of lettuce; to investigate the validity of an ideal Ca/Mg ratio for growth of lettuce and to determine the effects of soil Ca and Mg levels as well as Ca/Mg ratios on soil nutrients and the nutrition and growth of lettuce. Lettuce yield increased as soil Ca increased and also as plant Ca level increased. The Cate- Nelson method was applied to determine the critical levels of Ca, Mg and the Ca/Mg ratio in the soil and plant. A critical soil Ca level for lettuce was determined to be 1.9 cmolc kg-1 and is more reasonable and lower than the value of 5 cmok kg-1 that is currently recommended in Hawaii. Lettuce in the zero Ca treatment with 0.57 cmoh kg-1 soil Ca exhibited Ca deficiency symptoms in the Ca experiment. A critical plant Ca concentration for lettuce at maturity was also determined to be 4 g kg-1. Exchangeable soil cations interact with each other and application of a large amount of liming material can cause cation imbalance. In the Ca experiment, soil Mg, K and Na decreased as soil Ca increased. Application of Ca increased the soil Ca level, increased Ca uptake by the plant and reduced the uptake of Mg and Na but had no effect on the uptake of P. Soil Ca restricted K uptake at low Ca levels due to decreased ion selectivity and leakiness of membranes membranes when Ca was deficient. Lettuce growth was normal with all soil Mg levels in the Mg experiment. Lettuce yield also was not related to plant Mg level. A critical soil Mg level for lettuce could not be established, however, the soil Mg level of the zero Mg treatment, 0.67 cmoU kg’\ was apparently adequate for normal lettuce growth. Therefore, the sufficiency range for soil Mg recommended in Hawaii (2.5 to 3.3 cmok kg'^) appears too high. Lettuce in the zero Mg treatment did not show any Mg deficiency symptoms. A critical plant Mg concentration for lettuce at maturity also could not be determined, however, the plant Mg concentration of the zero Mg treatment, 4 g k g '\ was apparently sufficient for normal growth of lettuce. Interactions between soil cations also occurred in the Mg experiment where soil Ca, K, and Na decreased as soil Mg increased. Increased levels of soil Mg increased the uptake of Mg by the plant, and reduced the uptake of Ca and Na, but had no effect on the uptake of K and P. In the soil Ca/Mg ratios ranged from 0.11 to 7.70, lettuce growth was limited by a Ca/Mg ratio of around 0.11 and no yield reduction was observed in the Ca/Mg ratio range from 0.50 to 7.70, which is within the optimal range. This study provides evidence for the conclusion that plants can grow normally within a broad range of Ca/Mg ratios. Lettuce yield was related to both soil Ca/Mg ratio and plant Ca/Mg ratio. The lower critical level of the soil Ca/Mg ratio for lettuce was determined to be 0.5. However, caution should be used in interpreting yield response to the soil Ca/Mg ratio because soil Ca or Mg levels can also affect plant growth. The critical plant Ca/Mg concentration ratio for lettuce at maturity was also determined to be 0.5. Plant Ca/Mg ratios in lettuce were significantly related to soil Ca/Mg ratios.
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    Seed Quality of lsogenic Endosperm Mutants in Maize
    ( 1995) Zan, Guo-Hua
    Five studies were conducted based on ten near-isogenic corn hybrids converted to four endosperm genotypes — +, su, bt and sh2. Five different types of germination tests were conducted, including accelerated aging. Among the four endosperm genotypes, seed with wild type endosperm always had the best viability, followed in turn by su, bt and sh2 hybrids. Electrolyte leakage caused by accelerated aging (AA) was highly correlated with the deterioration of viability caused by AA. Seeds with poor germination ability suffered more from AA, indicating that seeds with good germination ability will have better storability. The accelerated aging, therefore, should be useful for predicting viability loss in seed storage. Pericarp thickness was influenced greatly by endosperm mutant genes. Pericarp thickness of sh2 hybrids at 36 days after pollination (DAP) were significantly greater than those at 18 DAP, while thinning trend of pericarp thickness was observed for wild type hybrids. There was no significant difference for bt and su hybrids from 18 to 36 DAP. A highly significant correlation was observed between seed weight and the difference of pericarp thickness harvested at 18 and 36 DAP. The data were interpreted as evidence for the effects of inner (endosperm) pressure on pericarp distension. The comparisons of eating quality among su, bt and sh2 endosperm mutants showed in general, that su had the worst and sh2 had the best eating quality considering sweetness and flavor, although the difference between bt and sh2 was not significant for many hybrids. The extensive genotypic variability for these eating qualities among the bt hybrids suggests that the allelic variation at loci other than bt is probably involved. Six germination-related characters were evaluated for bt and sh2 hybrids. These were seed weight, pericarp thickness, bubble volume, seed density, leachate conductivity and sweetness. In general, sh2 seed was sweeter with lower seed weight and density, and higher pericarp thickness, leachate conductivity and bubble volume (between endosperm and pericarp in mature seeds). These differences could be largely attributed to the efficiency of bt and sh2 genes in hindering the conversion of sucrose to starch. A method of measuring bubble volume through the change of soapy water volume was developed. The hypotheses were proved that bubble volume was determined by both shrinkage of endosperm and pericarp thickness, and thick pericarp affects germination rates of super sweet corn adversely through its effect on the formation of a large bubble volume. Bubble volume was observed to cause severe imbibition damage even with intact pericarps. The correlation between seed weight and germination was positive and significant at 5% level. The correlations of germination with pericarp thickness, bubble volume, seed density and conductivity were highly significant, as were the correlations among these four characters. However, germination was not significantly corrected with sweetness (organoleptic).
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