Drought is the major constraint to rice production in rainfed areas across Asia and sub-Saharan Africa. In the context of current and predicted water scarcity, increasing irrigation is generally not a viable option for alleviating drought problems in rainfed rice-growing systems. It is therefore critical that genetic management strategies for drought focus on maximum extraction of available soil moisture and its efficient use in crop establishment and growth to maximize biomass and yield. Extensive genetic variation for drought resistance exists in rice germplasm. However, the current challenge is to decipher the complexities of drought resistance in rice and exploit all available genetic resources to produce rice varieties combining drought adaptation with high yield potential, quality, and resistance to biotic stresses. The strategy described here aims at developing a pipeline for elite breeding lines and hybrids that can be integrated with efficient management practices and delivered to rice farmers. This involves the development of high-throughput, high-precision phenotyping systems to allow genes for yield components under stress to be efficiently mapped and their effects assessed on a range of drought-related traits, and then moving the most promising genes into widely grown rice mega-varieties, while scaling up gene detection and delivery for use in marker-aided breeding.
Grain yield of durum wheat (Triticum turgidum L. var. durum) under Mediterranean conditions is frequently limited by both high temperature and drought during grain filling. Genotype-by-environment (GE) interaction and genotype-by-trait (GT) data were analyzed for agronomic performance of durum wheat breeding lines. Data were obtained from 18 durum wheat breeding lines and two cultivar checks (Zardak and Sardari) for their agronomic performance under three different climate locations (moderate, warm and cold winters) and two moisture regimes (rainfed and two supplemental irrigations conditions) in two cropping seasons (2006 and 2007) in Iran. Analysis of GE interaction data based on multiple traits showed that the environment (combination of year-location- moisture regimes) effect was always the most important source of trait variation, accounting for 58.6 to 98.4% of the total variation. Biplot analysis of the studied traits revealed that (i) the locations tended to discriminate genotypes in dissimilar fashions, and (ii) the relationships among traits were not consistent over the locations, where they facilitated visual genotype comparisons and selection at each location.
Chilling is a major constraint in rice production in cool climates. In rice (Oryza sativa L.) plants, both the air temperature and the water (soil) temperature affect various growth processes independently, and low root zone temperature (thus, root temperature) can inhibit rice growth and yield. In this study, we investigated the effect of low root temperature on rice growth in relation to dry matter production and root water uptake. Plants were grown in hydroponic solutions at two temperatures, one equivalent to air temperature and the other 14ºC for 15 d starting 11 d after germination. Low temperature of the solution (low root temperature) inhibited dry matter production of rice plants by decreasing leaf area rather than photosynthetic rate. The response of leaf area was affected by changes in plant water status, that is relative water content (RWC) of stem was decreased by low root temperature resulting in reduced leaf area. The decrease in RWC caused by low root temperature was related to that in root hydraulic conductance (Kr). The responses of transpiration (E) and Kr to the low root temperature depended more on root surface area than on changes in hydraulic conductance per unit root surface area (Lpr). These results suggest that dry matter production under the low root temperature condition is controlled mainly by quantitative growth parameters such as leaf area and root surface area.
A two-band digital imaging system—one band for visible red (RED, 630–670 nm) and the other for near infrared (NIR, 820–900 nm)—was positioned 12 m above a 600-m2 rice field. The imaging system automatically logged bird’s-eye-view images at 10-min intervals from 0700 to 1700 JST daily during the 2008 paddy rice season. Radiometric corrections and midterm field validations for the pairs of two-band images utilized solar irradiance sensors and prior calibrations to calculate 0900-1500 JST daily-averaged reflectance factor (DARF). The DARF values in the RED and the NIR bands agreed with observations made with a portable spectroradiometer and showed smooth seasonal changes in terms of plant growth. During the before-heading period, NIR DARF values had a correlation coefficient (r) of 0.91 with the leaf area index (LAI). An empirically derived equation for LAI using NIR DARF values and the cosines of angles between the view and the planting row directions, and the view and the meridian directions, showed R2 values of 0.93, in estimations of LAI from the dataset (number of observations=52) acquired at four sample points centrally located within the viewing field. Validation indicated that the equation also worked well for the other six observation points spread across the viewing field, the data of which were not used in deriving the equation parameters. The DARF values observed in the NIR band calibrated with solar spectral irradiance sensors were useful for assessing rice LAI during the before-heading periods.
A two-year field experiment was conducted with japonica type high-yielding variety “Fukuhibiki, indica type high-yielding variety “Takanari, large grain type high-yielding variety “Bekoaoba and conventional japonica type varieties “Akitakomachi and “Hitomebore in the Tohoku region (colder area of Japan). The panicles of Takanari and Bekoaoba located lower in the canopy, and the flag leaf of those varieties intercept more solar radiation, because the flag leaf was long and the first internode of panicle did not emerge above the lamina joint of the flag leaf in Takanari and the panicle hung lower in Bekoaoba. The three high-yielding varieties produced a large sink size, due to the large potential sink size in Takanari, the very large grain size in Bekoaoba and the rather large number of spikelets per panicle and somewhat large grain size in Fukuhibiki. Based on dry matter production, the three high-yielding varieties had a high harvest index, although there were little varietal differences in top dry weight at maturity. Unhulled rice weight of the three high-yielding varieties was heavier than that of other varieties at 30 days after heading (30 DAH). However, there were no varietal differences in the increase in unhulled rice weight from 30 DAH to maturity (later than 50 DAH), probably due to the continual growth of the grains located lower until the late ripening period in all varieties. In conclusion, the traits related to high yield varied greatly among varieties. However, the three high-yielding varieties had a large sink size, heavy unhulled rice weight at 30 DAH and high harvest index.
To achieve super high yield of more than 10 t ha-1 in the Tohoku region (colder area of Japan), we conducted a two-year field experiment using a large grain type high-yielding rice variety Bekoaoba. Although high nitrogen application (HN) increased the top dry weight at the full heading stage only slightly, it increased the sink size (single grain weight x the number of spikelets per area), leaf area index and nitrogen content at the full heading stage and accordingly the dry matter production during the ripening period. As a result, the gross hulled rice yield was higher under HN than under standard nitrogen application (SN). Under HN, early topdressing increased not only the number of differentiated spikelets but also the number of degenerated spikelets. As a result, the sink size and gross hulled rice yield of the plants were not changed by the early topdressing. Under HN, the higher planting density increased the number of panicles per area but decreased the number of spikelets per panicle. As a result, the sink size and gross hulled rice yield were not changed with the planting density. In conclusion, HN produced the gross hulled rice yield of more than 9 t ha-1. However, the gross hulled rice yield could not be increased more than 10 t ha-1 by regulation of the timing of topdressing and/or the planting density. We discussed the strategies for super high-yielding rice in the Tohoku region.
A spectral and polarization image observation technique for detecting multiband polarimetric characteristics of reflected light from field-growing plants under daylight conditions was developed and the potential application of the method to in-situ assessments of wheat-leaf orientation at the heading stage was assessed. The developed digital imaging system corresponded to wavelength bands centered at 470, 550, and 647 nm, each with bandwidths of 10 nm. The instrument was fitted with a glass polarizer, which rotated from 0 to 360º in 15º steps, and polarized images of 1360×1024 pixels were captured at heading in wheat plots subjected to different fertilizer regimes at the jointing stage. Degree of polarization (DP) and mean brightness (MB) of the three bands were calculated from images of several pairs of top-dressed and non-top-dressed (non-dressed) plots, with a camera depression angle of 15–20º on two clear days. The relative azimuth angles between the view and insolation were approximately 135º (oblique front) and 180º (right in front), respectively. The mean DP for each plot area in the images varied between 0.3 and 1.4%. Although most of the top-dressed plots had significantly higher DPs than the non-dressed plots in the 550 nm band, few of the MB images in any band showed a clear difference between the top-dressed and non-dressed plots.
The use of cover crops is an effective technique to control weeds, which are one of the most serious problems for crop production without using herbicides. This study investigated the characteristics of cover crops for weed suppression at an organic farming field in a snowy-cold region, Hokkaido. Nine, three and two species of cover crops comprising both Poaceae and Leguminosae were grown in 2003, 2004 and 2005, respectively,at different sowing densities from 50 to approximately 4000 seeds m-2. The relationships between weed dry matter production and characteristics of cover crops, such as plant height and coverage, were investigated at 4 and 10 weeks after cover crop sowing (WAS). Correlation analysis of the weed dry weight with characteristics of cover crops revealed that the cover crop coverage at 4 WAS had a strong and stable effect on weed suppression. The cover crop coverage at 4 WAS was affected primarily by their seed weight when cover crops with a large variation in seed weight were used, and by the sowing density when cover crops with a small variation in seed weight were used. These results suggest that to achieve high weed suppression it is important to obtain higher coverage at the early growth stage of the cover crops with a heavy seed weight and high sowing density.