By using an in vivo hydroponic rice seedling culture system, we investigated the physiological and biochemical responses of a model rice japonica cultivar Nipponbare to salt stress using proteomics and classical biochemical methods. Yoshida's nutrient solution (YS) was used to grow rice seedlings. YS-grown 18-day-old seedlings manifested highly stable and reproducible symptoms, prominently the wilting and browning of the 3rd leaf, reduced photosynthetic activity, inhibition in overall seedling growth, and failure to develop new (5th) leaf, when subjected to salt stress by transferring them to YS containing 130 mM NaCl for 4 days. As leaf response to salt stress is least investigated in rice by proteomics, we used the 3rd leaf as source material. A comparison of 2-DE protein profiles between the untreated control and salt-stressed 3rd leaves revealed 55 differentially expressed CBB-stained spots, where 47 spots were increased over the control. Of these changed spots, the identity of 33 protein spots (27 increased and 5 decreased) was determined by nESI-LC-MS/MS. Most of these identified proteins belonged to major metabolic processes like photosynthetic carbon dioxide assimilation and photorespiration, suggesting a good correlation between salt stress-responsive proteins and leaf morphology. Moreover, 2-DE immunoblot and enzymatic activity analyses of 3rd leaves revealed remarkable changes in the key marker enzymes associated with oxidative damage to salt stress: ascorbate peroxidase and lipid peroxidation were induced, and catalase was suppressed. These results demonstrate that hydroponic culture system is best suited for proteomics of salt stress in rice seedling.

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Nitrogen fertilization is essential for increasing rice production to meet the food demands of increasing world's population. We established an in vivo hydroponic rice seedling culture system to investigate physio-biochemical/molecular responses of various rice japonica and indica cultivars to low nitrogen (N). Three-week-old seedlings grown in Yoshida's nutrient solution manifested stable and reproducible symptoms, such as reduced shoot growth and length under low N. Out of 12 genetically selected cultivars, 11 cultivars showed varied degrees of growth reduction response to applied N (4 and 40 ppm N for treatment and control, respectively), whereas one cultivar (no. 12) showed similar growth as the control though its leaf width was smaller than control. Leaves of a representative low N-responsive cultivar (BG90-2) were sampled for revealing protein profiles between low and normal (control) N application by two-dimensional gel electrophoresis (2-DGE). Forty-one proteins were identified with MALDI-TOF-MS and nESI-LC-MS/MS. Assignment of proteins into major (energy metabolism, photosynthesis and oxidative stress) and minor functional categories, revealed many novel low N-responsive proteins, including those having energy/photosynthesis-and defense/stress-and iron homeostasis-related functions. Results suggest the usefulness of proteomics in identifying novel N-responsive proteins and may provide potential markers for rice response to low N.

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The effects of NaCl‐induced stress on physiochemical factors such as inorganic cations, proline, malondialdehyde (MDA) and polyamines were investigated in the gramineous weed, Echinochloa crus‐galli Beauv. var. formosensis Ohwi (E. crus‐galli) and rice (Oryza sativa L. cv. Nipponbare). Growth inhibition at the 2nd leaf stage under salt stress was more severe in rice than in E. crus‐galli. Water content in the 2nd leaves was also more severely decreased in rice, indicating that E. crus‐galli was more salt‐tolerant. After NaCl treatment, Na+ accumulated in the 2nd leaves of both plant species but not in their roots. Proline accumulation in the 2nd leaves was significantly higher in salt stressed E. crus‐galli than in rice, suggesting the significance of proline production in the salt tolerance of this weed. Content of MDA of the rice increased more greatly with NaCl treatment than that in E. crus‐galli. NaCl treatment affected polyamine metabolism of both plant species, but the response of each plant to salt stress was somewhat different, especially in the leaves. Leaf putrescine and spermidine contents were high in non‐stressed plants in salt‐sensitive rice, although rather lower in E. crus‐galli in response to NaCl concentrations. These results indicate that an increase in proline and changes in polyamines relates to the salt tolerance of E. crus‐galli.

The phytotoxic activity of cafenstrole [1-(diethylcarbamoyl)-3-(2,4,6trimethylphenyl sulfonyl)-1,2,4-triazole] on rice and barnyardgrass (Echinochloa crusshoot, coronal roots and internode with adventitious roots, regardless of the emergence depth. In addition, the low phytotoxic activity on the growth of rice when the plants were transplanted below the herbicide-treated layer was attributed to the absence of the absorptive parts in the herbicide-treated layer.

The relationship between the fate of clomeprop in soil and its phytotoxic activity on the growth of radish (Raphanus sativus) seedlings was investigated in the laboratory. The phytotoxic activity of clomeprop in sea sand was much higher than in non-autoclaved soil, and the phytotoxic activity in non-autoclaved soil was higher than in autoclaved soil. The phytotoxic activity of 2-(2,4dichloro-3-methylphenoxy)propionic acid (DMPA), a hydrolysed metabolite of clomeprop, was higher than that of the latter under both soil conditions. Clomeprop was adsorbed on soil to a greater extent than DMPA. The concentration of clomeprop in soil water of non-autoclaved soil decreased with increase of the DMPA concentration in the soil water in a time-dependent manner. It is suggested that the phytotoxic activity of clomeprop applied to soil is induced mostly by the DMPA concentration in soil water after hydrolytic degradation by soil microbes.