A greenhouse experiment was conducted to compare root system development of two upland rice cultivars, IRAT 13 and Senshou, during recovery from drought stress and to identify to plant traits that confer drought resistance. From 62 days after sowing (62 DAS), drought stress was given for 6 d followed by rewatering for 14 d. Root length density (RLD) and root diameter (thickness) were measured at the end of the stress and rewatering periods. Control plants were well-watered throughout the study. Cultivar IRAT 13 had thicker roots and higher relative RLD (ratio of RLD in drought-stressed plants to that in control plants) than under drought stress, and significantly higher root growth recovery after rewatering cultivar Senshou. Related plant traits such as evapotranspiration (ET), leaf and stem dry weights and weight of senescent leaves (dead leaves) in IRAT 13 were significantly more favorable for drought resistance compared to Senshou.
Several types of water soluble carbohydrates (WSC) were traced into plant parts of a winter wheat (Triticum aestivum L.) cultivar from heading to physiological maturity. Field grown plants were harvested at intervals of a few days and divided into grain, chaff, leaf blades, leaf sheaths, rachis+peduncle and culm internodes. Leaf blades and sheath showed the peak of WSC contents at about anthesis. Culm internodes accumulated fructan and sucrose during the early grain-filling phase, from a week after anthesis until the milk-ripe stage, then remobilized them during late and final grain filling phases, from the milk-ripe stage until maturity. The amount of sucrose known as short-term storage WSC was higher than fructan known as long-term storage WSC in each internode throughout the grain-filling phase. Chaff showed a large amount of fructan and fructose before anthesis, although it did very little sucrose. A pattern of sucrose accumulation in chaff was very different from that of fructan, unlike the other parts. These patterns of changes in WSC contents in plant organs roughly corresponded with four grain-filling phases, the initial, early, late and final phases.
To elucidate the physiological characteristics relating to better seedling establishment in submerged field under low-temperature conditions, we compared the time required to reach various growth stages after seed imbibition among rice (Oryza sativa L.) varieties differing in the performance of seedling establishment. Two experiments were conducted. In the field experiment, the percentage of seedlings with expanded second leaf at 30 days after sowing (PSSL) was measured and regarded as the index of establishment rate for each variety. In agar-bed experiment, the number of germinated seeds, the seedlings with the coleoptile elongated to the medium surface, and the seedlings with the 1 st leaf apparent were counted daily under a 16°C condition. A sigmoid model was applied to the above values to evaluate the time requirements. There was a significant correlation between the duration from 50% germination to 50% emergence in the agar-bed experiment and PSSL in the field experiments. However, the duration from imbibition to 50% germination, and that from 50% emergence to 50% appearance of the first leaf were not significantly correlated with PSSL. We found that fast growth of the coleoptile is an important characteristic for the varieties that can provide stable and excellent seedling establishment at low temperature.
The effects of dry matter production and the remobilization of nonstructural carbohydrates (NSC) on grain filling were investigated using the Indica-based Japonica crossed rice cultivar Takanari, which can bear a large sink. For three years, beginning in 1994, shade treatments were conducted with different nitrogen applications to develop large variations in plant growth. The percentage of ripened spikelets showed the greatest correlation with the total amount of carbohydrate supply per spikelet during 10 to 20 days after heading, calculated by adding the amount of dry matter increase to the amount of NSC decrease in leaf sheaths and culms during the period. Between the two components, the dry matter increase was more important. The NSC reserve played a role in compensating for the shortage of carbohydrate supply from assimilates after heading and showed a tendency to increase the percentage of ripened spikelets when dry matter production after heading was limited. However, the maximum ratio of compensatory translocation from the NSC reserve was estimated to be only 48%, because of smaller reservoir size compared with the demand. The NSC reserve at heading was not significantly increased by increasing the dry matter production before heading. It was concluded that to increase grain filling ability it is more effective to increase the dry matter production after heading than that before heading. Nitrogen application showed negative effects on the translocation of reserve NSC. It is important to optimize the nitrogen content to maximize the total source of carbohydrate supply.
The early vigor of hybrid rice (Oryza sativa L.) developed in temperate areas has been mainly attributed to its higher tillering rate. The objective of this study is to identify factors that contribute to greater vegetative growth of F1 hybrid rice recently developed in the tropics. We analyzed data from field experiments conducted from 1994 to 1997 in both dry and wet seasons at the International Rice Research Institute farm. Each experiment had one to four pairs of comparisons between best hybrids and best conventional varieties. Parameters used for comparison were tillering rate, leaf area growth rate (LAGR), and crop growth rate (CGR) between transplanting (TR) and midtillering (MT) and also between MT and panicle initiation (PI). In most cases, the hybrids exhibited greater CGR than the conventional varieties, either between TR and MT or between MT and PI or both. The difference in CGR was associated with the difference in LAGR between hybrids and conventional varieties. However, the tillering rate of hybrids was significantly lower than or equal to that of conventional varieties. Therefore, the higher LAGR, independent from tillering, contributed to greater CGR of recently developed F1 hybrid rice in the tropics. The difference in LAGR and CGR between hybrids and conventional varieties was not associated with tillering. The higher LAGR of the F1 hybrids than that of the conventional varieties could be attributed to their thinner leaf blades.
To clarify the effect of artificial dwarfing induced by uniconazole-P, a gibberellin biosynthesis inhibitor, on the snow tolerance of faba bean, we examined the snow damage, non-structural carbohydrate (NSC) concentration, and resistance to snow molds of the seedlings treated with uniconazole-P. Artificial dwarfing markedly decreased snow damage caused by continuous snow cover for 62 days. NSC concentration, which affects physiological snow tolerance and snow mold resistance, was lower in dwarfed plants than in non-dwarfed plants. However, there was no difference in the rate of decrease of NSC concentration between dwarfed and non-dwarfed plants under cold and dark conditions simulating snow cover. In inoculation tests, the lesions of Pythium iwayamai and Sclerotinia trifoliorum on the leaves deteached from dwarfed plants were shorter than those on the leaves from non-dwarfed plants. In artificially dwarfed plants, the leaves were thicker and darker in color, and mesophyll cells were larger and denser than those in the non-dwarfed plants. Our findings suggest that the artificial dwarfing caused by uniconazole-P increased the resistance of the leaves to snow molds, thereby increasing snow tolerance, without increasing NSC concentration. The microstructural changes that occur with dwarfing may be a factor in snow tolerance.
A pot experiment was conducted to study photosynthetic rate in terms of carbon dioxide exchange rate (CER), chlorophyll meter reading (SPAD value) and specific leaf area (SLA) of 12 F1 hybrids from crosses between japonica and indica rice and their parents, and to estimate the heterosis in F1 hybrids at active tillering, flowering and dough ripe stages. In all F1 hybrids and parent cultivars, CER was high at the active tillering stage with a small heterosis value. A positive heterosis for this trait was found at the flowering stage. The magnitude of heterosis for CER depended on the cross combinations. Although a positive heterosis for SPAD value was found at the active tillering stage, there was no correlation between heterosis for SPAD value and that for CER at the flowering stage, at which heterosis for CER was positive. Heterosis for SLA was high at the active tillering stage and gradually decreased thereafter. Heterosis for CER might be dependent not only on negative heterosis for SLA but also on other photosynthetic factors such as N content, soluble protein content and Rubisco activity.
A pot experiment was conducted to investigate the heterosis for morphological characters and to examine the relationship among some characters at active tillering, flowering and dough ripe stages of 12 F1 hybrids from crosses between japonica and indica rice. Heterosis for plant height, number of tillers, green leaf area and dry matter accumulation per plant was positive at all stages. The intensity of heterosis was higher at the active tillering stage for number of tillers, leaf area and dry matter accumulation per plant. A significant positive relationship was found between tiller number and leaf area per plant at active tillering and flowering stages. Significant positive relationships between leaf area and dry matter accumulation, and between tiller number and dry matter accumulation per plant were observed at all stages. A significant positive relationship was found between leaf area per plant at an early stage and dry matter accumulation per plant at a later stage, suggesting that early development of leaf area is an important factor for higher dry matter accumulation in F1 hybrids. Although heterosis for percent dead leaf blade was positive at the flowering stage due to early leaf senescence in some F1 hybrids, a larger leaf area was also found in F1 hybrids at this stage and leaves of F1 hybrids remained green longer compared to parent cultivars up to the dough ripe stage.
The cell elongatin-promoting activities of brassinolide (BR) and indoleacetic acid (IAA) in the lamina-inclination test using etiolated rice seedlings were compared at a low temperature. IAA promoted the lamina joint-cell elongation at 30°C, but the effect was markedly lowered with decreasing temperature and reached null at 15°C. On the other hand, BR and castasterone (CAS), a brassinosteroid, promoted this cell elongation even at 15°C. Homobrassinolide (HBR), a brassinosteroid, also promoted this cell elongation, although the activity was weak. The combination of BR and IAA synergistically promoted this cell elongation at a low temperature (15°C). Immersion of the dry seeds in 2+10-8 M BR for the first 24 h promoted the early growth of rice seedlings. This treatment also enhanced the germination rate and the growth after direct sowing in submerged paddy pots in a greenhouse or phytotron at a low temperature (15°C). Leaf spraying of BR on the rice seedlings at the 4th leaf stage increased plant height and the fresh weights of tops and roots even at a low temperature. BR sprayed on the completely expanded 4th leaves did not increase their blade length, but that sprayed on the expanding 5th and 6th leaves strikingly increased their blade length, and the effect tended to be stronger at a low temperature. These results suggest that BR promotes cell elongation in young rice seedlings under low-temperature stress, although IAA had no such effect, and that BR may promote germination and the early growth of rice seedlings at a low temperature in direct sowing in the submerged paddy field and in the rice nursery.
Deep rooting of wheat has been suggested that it influences the tolerance to various environmental stresses. In this study, the nodes from which the deepest penetrated roots had emerged were examined in winter wheat. The wheat was grown in long tubes with or without mechanical stress and in large root boxes. The length and growth angle of each axile root were examined to analyze the difference in the vertical distribution of the roots between the two wheat cultivars, one with a deep and one with a shallow root system. In Shiroganekomugi, a Japanese winter wheat cultivar with a shallow root system, the rooting depths of the seminal and nodal roots decreased as the rooting nodes advanced acropetally. Six out of nine deepest roots were seminal root in the non-mechanical stress conditions. In Mutsubenkei, a Japanese winter wheat cultivar with a deep root system, grown in root boxes, not only the seminal roots but also the coleoptilar and the first nodal roots penetrated to a depth of more than 1.3 m in the root box, and became the deepest roots. In both cultivars, the seminal roots became the deepest roots under the mechanical stress conditions. There were no clear tendencies in the root growth angles among the rooting nodes in the wheat root system. This indicates that the length of the axile roots can explain the differences in the rooting depths among axile roots in a wheat root system. On the other hand, the axile roots of Mutsubenkei elongated significantly more vertically than those of Shiroganekomugi. This suggests that not only seminal but also nodal roots exhibit strong positive gravitropism and penetrate deeply in a cultivar with a deep root system. In wheat cultivars, it is likely that the extent of its Root Depth Index results partly from the gravitropic responses of both seminal and nodal roots.
Many studies have confirmed that exogenous cytokinins increase tillering in grasses as wheat, oat and barley. In this study, the effects of benzylaminopurine (BA) on tillering of two rice cultivars, North Rose with low tillering ability and Sasanishiki with high tillering ability, were examined using hydroponics. Foliar spray of BA was applied to both cultivars at the 6-leaf stage. Root application of BA was used to Sasanishiki at the 8-leaf stage. Both root-applied and foliar-sprayed BA retarded the increase of tillers and resulted in significant decrease in tiller numbers per plant. The result indicated that primary tillers were much more affected than secondary tillers. Leaf production on the main stem was also temporarily inhibited with the exception of foliar spray to plants of cv.Sasanishiki. The effective concentration of BA on tillering was over 0.01 μM for root application and 100 μM for foliar spray. Magnitude of effect of root application was greater than that of foliar spray in cv.Sasanishiki. Foliar-sprayed BA mainly inhibited tillers than are at the stage of differentiation of tiller buds from appearance. Root-applied BA could further inhibit the growth of tiller buds into tillers. In conclusion, BA inhibited the tillering of rice plants rather than promoting it, which is inconsistent with general description of cytokinin roles.
To elucidate the possible participation of hemicellulose decomposition in lodging resistance, we studied the change of hemicellulose and cellulose content in the stems of rice during the ripening stage by methylation analysis and the expression of related genes by Northern blotting. In the rice stem in ripening stage, content of (1-3, 1-4)-β-glucan, a component of hemicellulose, decreased markedly although the content of arabinoxylan, a major component of hemicellulose, and cellulose showed little change during the same growth period. On the other hand, expression of the Gns 1 gene, which may encode (1-3, 1-4)-β-glucanase that catalyzes the degradation of (1-3, 1-4)-β-glucan, increased sharphy in the stem. The mechanism of decomposition of (1-3, 1-4)-β-glucan in rice stem and the possible association with lodging resistance is discussed.
The experiment was conducted in a farmer's field, Ban Fang district, Khon Kaen province in 1998-1999 to investigate the effect of seeding depth and soil mulching on the growth and yield of peanut grown after rice in the post-monsoon season of Northeastern Thailand. A split plot design was used with mulching and non-mulching as main-plots and seeding depths of 5, 10 and 15 cm as sub-plots. The results showed that deep seeding at 15 cm gave the highest leaf area index, highest total dry matter yield, highest pod number per plant, heaviest 100-seed weight and highest seed yield of peanut. This was attributed to the higher root length density at a deep soil layer provided by deep seeding. Peanut roots in the deep soil layer were capable of exploring a large soil volume for water which remained available throughout the crop cycle. Mulching application showed beneficial effects on growth and yield of peanut only with shallow seeding depth (5 cm).
The effects of mulches (rice husk, sawdust, ash and burnt soil) on soil temperature, moisture conservation of soil and to relate them with morpho-physiological attributes of onion were studied. All mulches except ash were effective in conserving soil moisture. Sawdust mulch retained soil moisture most effectively followed by rice husk and burnt soil. Almost at all stages of growth and at any time of the day, ash mulch retained the soil temperature most effectively and least effectively by sawdust. Dry matter accumulation, leaf area index (LAI), plant height and root lengths were significantly influenced by these mulches. Mulching caused a higher number of scapes but the effect on number of scapes, their fresh and dry weights were insignificant. Ash mulch induced early flowering while the other mulches significantly delayed it. Thus, the use of ash as mulch was better compared to the other three mulches. This might be due to the increased soil temperature under the ash mulch. Further work is required to elucidate the fertilising effect of the mulches.
Maize growth, root colonization by arbuscular mycorrhizal fungi (AM fungi) and by non-AM fungi, and soil properties were studied on a field of Humic Andosol in 1998. The field had been tilled with different intensities since November 1997. A plot was minimum-tilled with a rotary tiller (MT) and the other plot was tilled with a moldboard plow and disc-harrowed (T). The least-squares fit of cubic or quadratic surfaces to the data collected from 9-100 locations in each plot clarified that the dry weight of maize shoot and root colonization by fungi showed a clear spatial variability in the area of 17.5 m × 25 m. Inverse spatial variability patterns between AM fungi and non-AM fungi in both MT and T indicated that there was an apparent competition between them in colonizing maize roots. Maize growth showed a spatial variability pattern similar to that of AM fungal organs, in particular in T. This suggested that the spatial variability pattern of maize in T was more affected by the relationship between AM fungi and non-AM fungi. Site-specific control of AM fungal infection for improving maize growth in small fields was discussed.