Susan C. Miyasaka

PLANT MECHANISMS AND MYCORRHIZAL SYMBIOSES TO INCREASE PHOSPHORUS UPTAKE EFFICIENCY

The tendency of phosphorus (P) to undergo adsorption and precipitation reactions in soil makes it one of the most immobile of the essential plant nutrients. As a result, the concentration of available P in soil solution is usually very low even though the total P content of soils is generally high. Consequently, P is second only to N in limiting crop productivity. In areas of the world such as the North America and Europe in which P fertilizers are applied liberally to agricultural fields, the loss of this nutrient through its transport in particulate and soluble forms from agricultural fields to lakes and rivers is of concern due to accelerated eutrophication. The need to provide crops with adequate P combined with the desire to minimize the adverse effect of P on the environment will require strategies to reduce P inputs and to maximize the efficiency of P uptake mechanisms in plants. Recent advances in molecular biology offer opportunities to manipulate plants to increase uptake efficiencies of P. Integration of arbuscular mycorrhizal fungi into cropping systems provides an alternative means of maintaining yields while reducing P inputs. To realize the potential of these new technologies, however, a thorough understanding of both soil and plant factors involved in P uptake, and the effects of cultural practices on arbuscular mycorrhizal associations are needed. A review of the literature was undertaken to summarize the relevant state of knowledge on P uptake processes in plants and the mechanisms by which P uptake efficiencies could be enhanced, with particular emphasis on arbuscular mycorrhizal fungi. *Journal Series No. 4468, College of Tropical Agriculture and Human Resources, University of Hawaii, Honolulu, HI.

Mulch and compost effects on yield and corm rots of taro

The potential of reducing losses due to corm rots on taro (Colocasia esculenta cv. Bun-long) using organic farming practices was evaluated in an Andisol under rain-fed conditions during successive seasons between 1996 and 1998. Treatments were: (1) inorganic fertilizer plus pre-emergent herbicide (control); (2) control with additional calcium (Ca) and alfalfa pellets; (3) chicken manure with silage mulch; (4) chicken manure with wood chip mulch; (5) chicken manure with macadamia nut compost and silage mulch; and (6) solarization plus chicken manure and silage mulch. Taro corms were harvested at 7, 9, and 11 months after planting (MAP). There were no significant effects of compost, Ca+alfalfa pellets, or solarization on taro yield. Plots that included mulch produced significantly greater fresh weight corm yields and higher percentage corm dry matter, but also showed a higher incidence of corm rots compared to non-mulched plots. Mulching appeared to increase soil moisture content which promoted both growth of taro and conditions conducive to corm rot. Although yields were increased in treatments with mulch, these increases in crop value were not sufficient to cover increased costs of production.

Arbuscular mycorrhizal fungi improve early forest-tree establishment.

Beneficial association of seedlings with arbuscular mycorrhizal fungi (AMF) is thought to improve early tree establishment through increased uptake of immobile soil nutrients such as phosphorus (P). Seedlings of Acacia koa (koa), a valuable hardwood that is endemic to the Hawaiian islands, was grown in the nursery with and without inoculation by the AMF Glomus aggregatum. Koa seedlings (with and without AMF) were planted in a former pasture unamended or amended with 4500 mg triple superphosphate per kg soil. Treatments were arranged in a randomised complete block design with five replicates per treatment. At 84 d after planting, AMF colonisation of roots was significantly greater if seedlings were inoculated with AMF. Between 84 to 134d after planting, total P content of pinnules was significantly greater in trees inoculated with AMF relative to uninoculated trees. At harvest, dry weight of stems was significantly increased by P fertilization or AMF inoculation. Dry weight of leaves also were significantly increased by inoculation. The results demonstrate that AMF inoculation of koa seedlings in the nursery could have significant beneficial effects on early tree establishment in the field.

Reactions of copper sulfate with wetland‐taro soils in Hawaii

Abstract The environmental impact of copper sulfate (CuSO4#lb5H2O) must be evaluated before the chemical can be registered as a pesticide to control the apple snail (Pomacea canaliculata) in Hawaiis wetlands. To help achieve this goal, we investigated the sorption‐desorption reactions of CuSO4#lb5H2O with six wetland‐taro soils (Tropaquepts) of Hawaii. Our results indicated that: (i) copper (Cu) was sorbed rapidly: 98.0–99.9% of the added Cu was removed from solution within one hour when the loading rate was less than or equal to 300 mg Cu kg‐1 [initial Cu concentration = 30.0 mg Cu L‐1 or 12 kg (ha‐cm)‐1 as CuSO4#lb5H2O which is 10 times the maximum recommended rate of pesticide applications, (ii) Cu sorption increased as soil pH increased from 5.0 to 8.0, and (iii) sorption capacity varied from 210 mg Cu kg‐1 in a Tropaquept from Kauai Island to 500 mg Cu kg‐1 in another Tropaquept from Maui Island, after seven days of incubation at soil‐solution pH 6.0 and total solution Cu concentration of 0.10 mg Cu...

Genetic Transformation of Taro

Taro (Colocasia esculenta (L.) Schott) is cultivated worldwide for its edible corms and leaves. It was the world’s fourteenth most-consumed vegetable and the fifth most-produced root crop in the world during 2010. However, various pests and diseases, especially fungal and oomycete diseases, are major problems causing steep declines in taro production. Conventional breeding of disease resistant cultivars is ongoing, although it is a lengthy process. Tissue culture and genetic transformation of taro are alternative options to improve yields, quality, and disease resistance. Compared with conventional breeding, genetic engineering has unique advantages, such as a much broader gene pool for selection of genes of interest and the capability of transferring only a few transgenes, thus maintaining all other desirable crop characteristics. Only a few reports are available on the regeneration and genetic transformation of taro. The first report of taro transformation described insertion of a reporter gus gene and a selection gene hpt into a Japanese taro cultivar via particle bombardment with a very low transformation efficiency. More recently, particle bombardment and Agrobacterium-mediated transformation methods have been used to transform a Chinese taro cultivar with a disease resistance gene chi11 from rice. The Agrobacterium-mediated method had much higher transformation efficiency than particle bombardment. Insertion of this rice chitinase gene into taro resulted in moderately increased disease resistance against the fungal pathogen Sclerotium rolfsii. These results demonstrate the potential usefulness of genetic transformation to increase disease resistance of taro, particularly in instances where there are no naturally occurring resistances within the taro germplasm or the elite taro cultivars are difficult to breed conventionally.