Len J. Wade

Rainfed lowland rice: physical environment and cultivar requirements

The physical environment of the rainfed lowland ecosystem is often characterised and grouped according to the surface hydrology of rice paddies and rice cultivars have been developed for each subecosystem. Rainfall is an important determinant of the yield of rainfed lowland rice, but other factors such as topography and soil fertility also affect grain yield and choice of cultivars. The growing environment and also rice yield vary greatly within small areas as well as across seasons. This causes great difficulty in determining the target population of environments for a rice breeding program. This paper reviews past work on characterising the variability in the physical environment, and rice production in the rainfed lowland ecosystem. It examines possible connections between this variability and slow progress in developing new cultivars that are widely adapted to the rainfed lowland rice ecosystem.

Genotypic Variation in Response of Rainfed Lowland Rice to Drought and Rewatering : I.Growth and water use

Abstract The lack of information on the dynamics of crop growth and water use has limited the capacity for indirect selection through physiological traits that confer drought tolerance in rainfed lowland rice (Oryza sativa L.). Shoot growth and transpiration in response to drought and rewatering were studied among eight diverse rice genotypes in three sets of pot experiments : one under severe stress development after panicle initiation (average transpiration of 15.0 mm d–1 ; experiment 1), and two under slow and progressive stress development during tillering (average transpiration of 2.1 and 7.6 mm d–1 in experiments 2 and 3, respectively). Higher transpiration generally caused a higher plant growth rate in two periods, though there was some contribution of water use efficiency. The first period was soon after ponded water was drained and watering withheld (early drought phase), when soil became aerobic, soil water was still readily available, and transpiration continued at a rate comparable to the well–watered treatment. The second period was after rewatering when transpiration was again not limited by soil water supply. In experiment 1, the effect of plant size before stress imposition was large and genotypic variation for response to drought and rewatering was small, except for KDML105, which tended to show smaller growth during drought and had a more rapid recovery after rewatering. During early drought phase in experiments 2 and 3, genotypes differed in relative amounts of tiller and leaf area production compared with the well-watered treatment. Genotypes with high seedling vigor before stress imposition and during the early drought phase, such as NSG19, KDML105, Mahsuri and IR58821, produced greater root length during the following more severe drought period and had a larger green leaf biomass at the end of the drought period in experiment 3. In these genotypes, transpiration increased sharply and leaf area expanded rapidly after rewatering, which caused superior drought recovery.

Genotypic Variation in Response of Rainfed Lowland Rice to Drought and Rewatering. III. Water extraction during the drought period

Abstract Soil water extraction was examined in relation to root system development and leaf osmotic adjustment in a pot experiment with eight rice genotypes (Oryza sativa L.). The time course of cumulative soil water extraction from layers between 5 and 45 cm estimated by time domain reflectometry (TDR) was similar to that of cumulative transpiration estimated from pot weighing for each genotype. The level of the TDR-estimated water extraction was 75% of the cumulative transpiration, and their coefficient of determination between the two was 96%. There was a 5-day difference (18 to 23 d) in transpiring 4 kg of water from the pots among genotypes, and the variation in daily transpiration rate was related to the extraction rate in the top-20 cm soil layers during the early half of the drought period, and to the extraction rate in the below-30 cm soil layers during the latter half of the drought period. The extraction rate in the subsoil was positively correlated with the average root length density at the corresponding depth during the latter half of the drought period, explaining 66% and 58% of the variation around the 30 cm and 40 cm depth, respectively. Mahsuri and IR58821 had higher water extraction rate from these subsoil layers than IR20 and IR62266 during the late drought period. Osmotic adjustment was higher in the genotypes that had a slower rate of transpiration and a lower pre-dawn leaf water potential at the end of the drought period. Among genotypes that extracted water rapidly, KDML105 had the highest osmotic adjustment. IR58821 and CT9993, known to have a deep and thick root system under well-watered conditions, had the lowest levels of osmotic adjustment. This study demonstrated under the simulated rainfed lowland conditions genotypic variation in water extraction rate from the deep soil layers during the late drought period, which was primarily related to proliferation of roots in these layers.

Genotypic Variations in Response of Lateral Root Development to Fluctuating Soil Moisture in Rice

Abstract Developmental plasticity in lateral roots may be one of the key traits for the growth of rice plants under soil moisture fluctuations. We aimed to examine responses in seminal root system development to changing soil moisture for diverse rice cultivars. Special attention was paid to the two different types of lateral roots ; the generally long, thick L type capable of branching into higher orders, and the non-branching S type. Plants were grown in half-split polyvinyl chloride tubes fixed with transparent acrylic plate for root observation under glasshouse conditions. When plants were grown first under drought conditions, then rewatered, the seminal root system development in terms of dry weight and total length was promoted as compared with plants grown under continuously well-watered conditions in IR AT 109 and Dular, drought tolerant cultivars. Promoted production of L type lateral roots mainly contributed to the development of the longer seminal root system. Plants exposed to soil submergence before they were grown under drought conditions did not show such promoted responses in these two cultivars. However, in KDML 105, a drought tolerant cultivar, the production of especially L type laterals was substantially promoted under drought and rewatered conditions. Honenwase was characterized by the shallow root system and great reduction in root system length when soil moisture becomes limited. These facts show that genotypic variations exist in the plastic response of rice seminal root system and that the L type lateral root plays a key role in manifestation of this plasticity.

Dry Matter Production and Root System Development of Rice Cultivars under Fluctuating Soil Moisture

Abstract Rice plants in the rainfed areas are mostly grown under fluctuating soil moisture. We examined responses in dry matter production, root development and water use to changing soil moisture in diverse rice cultivars. Rice plants were grown in polyvinyl chloride tubes under glasshouse conditions. Progressive drought right after planting greatly inhibited the shoot dry matter production, tiller development, nodal root development and water uptake in all cultivars tested. When the plants experienced soil submergence before being exposed to drought, all the cultivars exhibited higher dry matter production than their well-watered counterparts. Cultivar differences were clearly noted in the growth responses to rewatering after these plants were droughted. With well–watered control as basis, IRAT 109 and KDML 105 plants increased efficiency in converting available dry matter to increase their total root length by means of enhanced lateral root development. In the latter, however, the dry weight of roots also increased and so did root water uptake. In Dular, droughted plants did not show a clear response in terms of root development and water uptake to rewatering while its shoot growth was much more severely inhibited than the other cultivars. These findings suggest that phenotypic plasticity in the root system structure exhibited by promoted lateral root development and new nodal root production play a key role in the growth of rice under changing moisture level in the soil.

Growth and Water Use Response of Doubled-Haploid Rice Lines to Drought and Rewatering during the Vegetative Stage

Abstract We examined the responses of doubled-haploid lines (DHLs) of rice (Oryza sativa L.) to drought and rewatering in controlled rainfed lowland conditions, to test the the hypothesis that the DHLs would permit trait comparisons with less confounding by unrelated traits than had been reported previously. IR62266 and four DHLs derived from the cross between IR62266 and CT9993 (DHL-32, 51, 54 and 79) were grown in pot experiments in the greenhouse at the IRRI, Los Baños, Philippines. Genotypic variation in leaf and tiller development, transpiration, water use efficiency, osmotic adjustment and leaf water potential was examined in relation to dry matter production. Results revealed that greater seedling vigor through continued leaf expansion in early drought was associated with greater dry matter production after rewatering. A higher water use efficiency was related to a greater increase in dry matter production during drought. Leaf water potential was correlated strongly with dry weight, not only during drought, but especially on rewatering. Therefore, we found that the ability to continue leaf expansion, higher water use efficiency, and a greater osmotic adjustment for maintenance of leaf water potential as drought progressed were desirable traits for improved performance under drought and improved ability to recover on rewatering. These relationships could be analyzed precisely using such genetically-related materials as DHLs, with less confounding effects of plant size and genetic background.

Root growth and water extraction response of doubled-haploid rice lines to drought and rewatering during the vegetative stage

Abstract Doubled haploid lines (DHLs) of rice (Oryza sativa L.) were used to examine responses to drought and rewatering in controlled rainfed lowland conditions, in order to determine whether confounding by unrelated traits would be less than has been reported previously for contrasting cultivars that differ in genetic background. IR62266 and four DHLs derived from the cross between IR62266 and CT9993 (DHL-32, -51, -54 and -79) were grown in pot experiments in the greenhouse during the 2000 dry and wet seasons at IRRI, Los Baños, Philippines. There were two water regimes (well-watered and drought). Estimated water extraction obtained by time domain reflectometry (TDR) was similar to cumulative transpiration estimated from pot weighing for each genotype. Genotypic variation was observed in root traits and water extraction, with extraction slower in DHL-32 and faster in DHL-79, especially in deeper soil layers. An upper bound relationship between water extraction from a soil layer and root length density (RLD) in that layer was readily apparent over DHLs and soil depths, suggesting a critical value of RLD for water extraction of 0.30 cm cm−3 in these conditions. Because soils in the field would not be as homogenous as the puddled soils used in these greenhouse experiments, this critical RLD for water extraction from a soil layer is a reference for ideal conditions, and requires careful validation in the field. Use of DHLs permitted comparisons with reduced confounding by genetic background, with consequent improvements in precision.

Stomatal Responses in Rainfed Lowland Rice to Partial Soil Drying ; Evidence for Root Signals

Abstract The role of root signals in water deficit responses of rice (Oryza sativa L.) is important in the alternate flooding and drying conditions encountered in the rainfed lowlands, where the abundant roots in shallow soil layers may generate signals when droughted, with consequent reduction in stomatal conductance (gs) and growth, despite the likelihood of additional water in deeper soil layers. This study was conducted to confirm the presence of root signals, explore their nature and plant responses, consider the suitability of the methods, and discuss implications for adaptation under rainfed lowland drought. A split-root technique was used in greenhouse studies, whereby roots were divided into two sections: flooded and droughted. The decrease in gs and transpiration rate (Tr) due to drying of a portion of the roots, and their apparent recovery upon severing of this root portion, were consistent with the role for signals. The field study confirmed the evidence for root signals during progressive soil drying, whereby gs and Tr decreased before leaf water potential (ΨL) started to decline. The increase in leaf ABA concentration under field drought, and its strong association with soil moisture tension and gs, suggested its involvement in mediating stomatal responses during early drought in rice. The recovery in ΨL after severing of droughted roots in the greenhouse could be attributed to increased hydraulic conductance. These responses imply a role for both chemical and hydraulic signals in rice, which have important implications for adaptation and crop performance in contrasting rice ecosystems.

Constraints to High Yield of Dry-Seeded Rice in the Rainy Season of a Humid Tropic Environment

Abstract Dry seeding advances establishment and harvest of rainfed rice and may help the crop escape late-season droughts. Early establishment, however, may expose the crop to early and mid-season droughts and periods of low radiation during the grain formation phase. We conducted experiments in the 1992 and 1993 rainy seasons at Tarlac, Philippines, to investigate factors that may hinder the performance and yield of dry-seeded rice cv. IR72. The treatments included three seeding dates and three water regimes (totally rainfed, irrigated from seeding to complete emergence followed by rainfed, and fully irrigated). Drought stresses between panicle initiation and flowering, with a matric potential of -25 kPa to -60 kPa in the 0-10 cm soil layer, reduced final biomass by 20%–30%. The same stresses occurring during the vegetative stage delayed flowering 3-5 d, but did not reduce total biomass at harvest. High plant density of the dry-seeding culture (325-450 seedlings m–2) resulted in excessive vegetative growth (1600 -2200 tillers m–2 at maximum tillering stage). Inter- and intra-plant competition and low radiation (especially in typhoons) during anthesis and grain filling resulted in a high rate (40-70%) of tiller abortion, delay in flowering of later tillers, low percentage of filled spikelets (30-60%), and low yield (2.5–4.3 t ha–1), despite high biomass production (13-15 t ha–1). Selecting new varieties and devising cultural practices that ensure adequate plant population and weed competitiveness in drought years and avoid excessive vegetative growth in years with low radiation are research challenges to make full use of the potential of dry seeding to increase the productivity of rainfed lowland rice.

Progress in developing perennial wheats for grain and grazing

Abstract. n Dual-purpose cereals have been important for increasing the flexibility and profitability of mixed farming enterprises in southern Australia, providing winter feed when pasture dry matter production is low, and then recovering to produce grain. A perennial dual-purpose cereal could confer additional economic and environmental benefits. We establish that, at the end of a second growth season, selected perennial cereals were able to achieve up to 10-fold greater below-ground biomass than a resown annual wheat. We review and expand the data on available, diverse, perennial, wheat-derived germplasm, confirming that perenniality is achievable but that further improvements are essential through targeted breeding. Although not yet commercially deployable, the grain yields and dry matter production of the best performing lines approach the benchmarks predicted to achieve profitability. On reviewing the genomic composition of the most promising wheat-derived perennials, we conclude that the best near-term prospect of a productive breeding program for a perennial, wheat-derived cereal will utilise a diploid, perennial donor species, and the most promising one thus far is Thinopyrum elongatum. Furthermore, the breeding should be aimed at complete wheat–Th. elongatum amphiploids, a hybrid synthetic crop analogous to triticale. We advocate the generation of many primary amphiploids involving a diversity of Th. elongatum accessions and a diversity of adapted annual wheat cultivars. Primary perennial amphiploids would be inter-crossed and advanced with heavy, early-generation selection for traits such as semi-dwarf plant height, non-shattering heads, large seed size and good self-fertility, followed by later generation selection for robust perenniality, days to flowering, grain yield, forage yield, stability of grain yield across seasons, and disease resistance.