Michaël Dingkuhn

Leaf K/Na ratio predicts salinity induced yield loss in irrigated rice

Salinity is a major constraint to irrigated rice production, particularly in semi-arid and arid climates. Irrigated rice is a well suited crop to controlling and even decreasing soil salinity, but rice is a salt-susceptible crop and yield losses due to salinity can be substantial. The objective of this study was to develop a highly predictive screening tool for the vegetative growth stage of rice to estimate salinity-induced yield losses. Twenty-one rice genotypes were grown over seven seasons in a field trials in Ndiaye, Senegal, between 1991 and 1995 and were subjected to irrigation with moderately saline water (3.5 mS cm-1, electrical conductivity) or irrigation with fresh water. Potassium/sodium ratios of the youngest three leaves (K/NaLeaves) were determined by flame photometry at the late vegetative stage. Grain yield was determined at maturity. All cultivars showed strong log-linear correlations between K/NaLeaves and grain yield, but intercept and slope of those correlations differed between seasons for a given genotype and between genotypes. The K/NaLeaves under salinity was related to grain yield under salinity relative to freshwater controls. There was a highly significant correlation (p < 0.001) between K/NaLeaves and salinity-induced grain yield reduction: the most susceptible cultivars had lowest K/NaLeaves and the strongest yield reductions. Although there were major differences in the effects of salinity on crops in both the hot dry season (HDS) and the wet season, the correlation was equally significant across cropping seasons. The earliest possible time to establish the relationship between K/NaLeaves under salinity and grain yield reduction due to salinity was investigated in an additional trial in the HDS 1998. About 60 days after sowing, salinity-induced yield loss could be predicted through K/NaLeaves with a high degree of confidence (p < 0.01). A screening system for salinity resistance of rice, particularly in arid and semi-arid climates, is proposed based on the correlation between K/NaLeaves under salinity and salinity-induced yield losses.

Growth and yield potential of Oryza sativa and O. glaberrima upland rice cultivars and their interspecific progenies

Abstract A recent breakthrough in generating fertile progeny from Oryza sativa × O. glaberrima crosses gives rice breeders access to a broader range of germplasm. Interspecific crosses might provide new solutions to the low productivity of upland rice systems prone to weed competition. Two field and one pot experiments conducted during 1995 and 1996 served to characterize growth and yield potential of CG14 ( O. glaberrima ), WAB56-104 ( O. sativa ) and their progeny. During the 1995 wet season and the 1996 dry and wet seasons, the lines were seeded in a well-drained upland field in Ivory Coast with supplemental sprinkler irrigation. A randomized complete-block design with three replications was used, with cultivar and nitrogen levels as sub-plots. Specific leaf area (SLA), leaf area index (LAI), leaf chlorophyll content (SPAD method) and tiller number were measured at 2-week intervals until flowering. Grain yield and yield components were measured at maturity. In all environments, CG14 produced two to three times the LAI and tiller numbers as WAB56-104. This was associated with a high SLA and low leaf chlorophyll content. Grain yields of CG14 did not respond to N inputs, although the sink potential did. The difference was mainly caused by grain shattering. The progenies had intermediate LAI, SLA and leaf chlorophyll content, but their grain yields, tiller numbers and resistance to lodging and grain shattering were similar to WAB56-104. Across lines, LAI and SLA were significantly correlated. A paddy field experiment confirmed the relationship between LAI and SLA for a wider range of rice cultivars and interspecific progenies. A pot experiment demonstrated that leaf net CO 2 assimilation rates ( A max ) followed a common linear function of areal leaf chlorophyll content across cultivars. The main common cause of differential LAI and A max appeared to have been genotypic patterns of SLA, which might be an important determinant of growth vigor and competitiveness with weeds. The possibility is discussed of combining, in a single line, high SLA during vegetative growth (for weed competitiveness) with low SLA during the reproductive growth phase (for high yield potential), to produce an efficient plant type for low-management conditions.

Sodium and potassium uptake of rice panicles as affected by salinity and season in relation to yield and yield components

Salinity is a major yield-reducing stress in many arid and/or coastal irrigation systems for rice. Past studies on salt stress have mainly addressed the vegetative growth stage of rice, and little is known on salt effects on the reproductive organs. Sodium and potassium uptake of panicles was studied for eight rice cultivars in field trials under irrigation with saline and fresh water in the hot dry season and the wet season 1994 at WARDA in Ndiaye, Senegal. Sodium and potassium content was determined at four different stages of panicle development and related to salt treatment effects on yield, yield components and panicle transpiration. Yield and yield components were strongly affected by salinity, the effects being stronger in the HDS than in the WS. The cultivars differed in the amount of salt taken up by the panicle. Tolerant cultivars had lower panicle sodium content at all panicle development stages than susceptible ones. Panicle potassium concentration decreased with panicle development under both treatments in all cultivars, but to a lesser extent in salt treated susceptible cultivars. Grain weight reduction in the early panicle development stages and spikelet sterility increase in the later PDS were highly correlated (p < 0.01) with an increase in panicle sodium concentration in both seasons, whereas reduction in spikelet number was not. The magnitude of salt-induced yield loss could not be explained with increases in sodium uptake to the panicle alone. It is argued that the amount of sodium taken up by the panicle may be determined by two different factors. One factor (before flowering) being the overall control mechanism of sodium uptake through root properties and the subsequent distribution of sodium in the vegetative plant, whereas the other (from flowering onwards) is probably linked to panicle transpiration.

Climatic determinants of irrigated rice performance in the Sahel — I. Photothermal and micro-climatic responses of flowering

Abstract In the Sahel, variable crop duration of irrigated rice poses serious timing problems for intensification of production. Photothermal effects on phenology have been studied to develop simulation tools for breeding and cropping systems research. Forty-nine genotypes were planted at monthly intervals in various rice-garden trials. Environment variability among seasons, sites and within the crop canopy was characterized to develop a field-based, photothermal model for flowering. Basic concepts were summation of heat units and a linear thermal response of development rate having upper (Topt) and lower (Tbase) response limits. Photoperiodism was modeled by a slope constant (CPP) and a basic vegetative phase (BVP). Photoperiodism and transplanting shock acted as modifiers of heat requirements (Tsum), thereby having greater effects on duration at low than at high temperatures. Tbase, Topt, Tsum, BVP and CPP were considered genotypic constants and calibrated by optimization. Daily input temperature for the model was the physiologically relevant temperature Tphys at the shoot apex. Tphys depended on apex submergence, water temperature and diurnal temperature patterns. Diurnal temperature segments exceeding the Tbase-Topt range were disregarded. Mean water temperature was below air temperature, particularly at high leaf area index and on dry days. Mean air temperature was closer to the minimum than to the maximum when amplitudes were high or days short. Minimum temperatures below 18°C at booting stage resulted in near total spikelet sterility and a specific delay in heading. The model was validated for a site thermally different from the site of calibration.

Salinity increases CO2 assimilation but reduces growth in field-grown, irrigated rice

Salinity is a major yield-reducing factor in coastal and arid, irrigated rice production systems. Salt tolerance is a major breeding objective. Three rice cultivars with different levels of salt tolerance were studied in the field for growth, sodium uptake, leaf chlorophyll content, specific leaf area (SLA), sodium concentration and leaf CO2 exchange rates (CER) at photosynthetic active radiation (PAR)-saturation. Plants were grown in Ndiaye, Senegal, at a research station of the West Africa Rice Development Association (WARDA), during the hot dry season (HDS) and the wet season (WS) 1994 under irrigation with fresh or saline water (flood water electrical conductivity = 3.5 mS cm-1). Relative leaf chlorophyll content (SPAD method) and root, stem, leaf blade and panicle dry weight were measured at weekly intervals throughout both seasons. Specific leaf area was measured on eight dates, and CER and leaf sodium content were measured at mid-season on the first (topmost) and second leaf. Salinity reduced yields to nearly zero and dry-matter accumulation by 90% for the susceptible cultivar in the HDS, but increased leaf chlorophyll content and CER at PAR- saturation. The increase in CER, which was also observed in the other cultivars and seasons, was explained by a combination of two hypotheses: leaf chlorophyll content was limited by the available N resources in controls, but not in salt-stressed plants; and the sodium concentrations were not high enough to cause early leaf senescence and chlorophyll degradation. The growth reductions were attributed to loss of assimilates (mechanisms unknown) that must have occurred after export from the sites of assimilation. The apparent, recurrent losses of assimilates, which were between 8% and 49% according to simulation with the crop model for potential yields in irrigated rice, ORYZA S, might be partly due to root decomposition and exudation. Possibly more importantly, energy-consuming processes, such as osmoregulation, interception of sodium and potassium from the transpiration stream in leaf sheaths and their subsequent storage, drained the assimilate supply.

Effect of drainage date on yield and dry matter partitioning in irrigated rice

Double cropping of rice is constrained by tight cropping calendars. In mechanized production systems such as in the Senegal River delta of Africa, drainage date is important for yield, maturity and field trafficability. Randomized complete-block field experiments were conducted at the WARDA research station at Ndiaye in Senegal during the hot-dry and wet seasons to determine effects of drainage date on soil drying, grain yield and assimilate partitioning. The soil was a slightly acid, shrinking clay. Pre-germinated seed of I Kong Pao rice was sown broadcast and the plots kept flooded until drainage at four different dates after flowering (treatments). Topsoil and grain moisture content and the dry weight of panicles, stems, green leaves and dead leaves were measured on six dates between 1st flowering and 2 wk after maturity. Soil moisture content decreased more rapidly under a green canopy than with a mature crop. Grain yield was reduced by 34 to 36% in both seasons when plots were drained 3 to 4 days after flowering (DAF) (P < 0.01, compared to drainage at maturity) but were not affected when drained 12 to 14 DAF or later. Grain yield reduction was due one-third to poor grain filling and two-thirds to unfilled spikelets. Maturity (20% grain moisture content) was advanced 10 to 11 d by early drainage (3 to 4 DAF), compared to drainage at maturity. Apparent mobilization of reserves based on dry weight decrease of vegetative organs after flowering was 32 to 42% and constituted 46 to 84% of panicle growth, depending on treatment. Early drainage reduced dry matter production during ripening but did not proportionally affect grain yield because of compensatory mobilization of stem reserves. Models are proposed on the buffering effect of reserves if assimilation during grain filling is variable.

Climatic determinants of irrigated rice performance in the Sahel -- III. Characterizing environments by simulating crop phenology

Abstract Agricultural development programs in most countries of the Sahel seek to increase the intensity of irrigated rice production. Variable crop duration and temperature stresses, however, limit yields and calendrial options. In an effort to characterize climatic constraints to rice double-cropping in the region, a field-based model simulating flowering was combined with models predicting maturity and chilling- and heat-induced spikelet sterility. Simulations were performed for key sowing dates, 38 sites and three genotypes based on 10–33 years of weather records. Sowing between mid-September and mid-November was associated with near-total spikelet sterility due to chilling in all Sahelian environments. Crop duration was longest in November, increasing by 5 days per degree latitude in the continental Sahel. Greatest annual variation in duration was observed in the coastal west and extreme north of the Sahel. Climatic gradients along the courses of the Senegal and Niger rivers were associated with different annual patterns of duration and sterility. Based on the genotypes available to farmers, local rice-rice cropping calendars left little room for alternative calendars. Achieving a greater flexibility for cropping calendars would require the introduction of alternative short-duration varieties. The model permits the zonation of the Sahel in terms of calendrial options and varietal requirements. For a greater spatial resolution of such studies in the future, however, the pooling and digitalization of more weather data is recommended.

RICE CROP DURATION AND LEAF APPEARANCE RATE IN A VARIABLE THERMAL ENVIRONMENT. I. DEVELOPMENT OF AN EMPIRICALLY BASED MODEL

Abstract Variable crop duration is a major constraint to rice double cropping in arid irrigated environments, such as the Sahel. Photoperiodism and low air and water temperatures during the cool season are the major causes of variability, and cultivars are needed whose photothermal response provides a more stable crop duration. A previous study analyzed cultivar photothermal constants on the basis of progress to flowering. The present study sought to identify, on the basis of leaf appearance rates, the phenological stages that are most sensitive photothermally, and to explore technical options to screen germplasm for stable crop duration. Three Oryza sativa , indica-type rice cultivars (Jaya, IKP, IR64) were sown in the field at 15-day intervals during the dry season of 1995 (11 sowing dates) and 1996 (5 sowing dates) in Ndiaye, Senegal, under full irrigation and wide spacing to reduce microclimate variability. Mean daily water temperature ( T w ) varied from 13 to 35°C. After seed soaking, the rate at which the first leaf ( L 1 ) appeared was linearly related with T w , with a base temperature ( T base ) of about 10°C. Appearance rates of the subsequent three leaves ( L 2 – L 4 ) had a similar T base , and presented a distinct temperature optimum ( T opt ) at about 23°C, beyond which development rates decreased. Errors were too large to determine differences among cultivars in thermal constants. No significant temperature response was observed for the leaf appearances between L 5 to the flag leaf ( L 12 to L 20 ). Crop duration to flowering varied by 45 (IR64) and 63 days (Jaya). These variations were associated with highly variable leaf numbers in all cultivars, including photoperiod-insensitive IKP. One-third of the variable duration was hypothesized to be due to a variable basic vegetative phase (BVP), caused by variable germination and leaf appearance rates, and two-thirds to variable duration of panicle induction after BVP. Water temperature was the main determinant of both sources of variability. A simulation model, describing these temperature and photoperiod effects on leaf number, growth duration and leaf appearance rates, was developed using the 1995 data, and satisfactorily validated with the 1996 data. The model was used to identify phenological-stage and cultivar-specific causes of variable crop duration.

Orchestration of transpiration, growth and carbohydrate dynamics in rice during a dry-down cycle

The regulation of carbohydrate metabolism and source-sink relationships among organs play a key role in plant adaptation to drought. This study aimed at characterising the dynamics of transpiration, development, growth and carbon metabolism, as well as the expression of invertase genes, in response to drought during a dry-down cycle. Three 1-month experiments were conducted in controlled environment using the rice genotype IR64 (Oryza sativa L., indica). Plant leaf relative transpiration and expansion rates decreased linearly when fraction of transpirable soil water (FTSW) dropped below 0.66 and 0.58, respectively. Hexose and starch concentration responses to FTSW in a given organ were generally linear and opposite: in source leaves, hexose concentration increased and starch decreased, and vice versa in sink leaves and roots. Sucrose remained constant in source leaves and increased slightly in sink leaves. Starch reserves built up during stress in sink organs were rapidly mobilised upon rewatering, indicating its involvement in a mechanism to ensure recovery. Expression of cell-wall and vacuolar invertase genes under stress increased in sink leaves, interpreted as a mechanism to maintain sink activity (cell wall) and osmotic adjustment (vacuolar). It is concluded that carbohydrate metabolism in sink organs under drought is highly regulated, and important for stress adaptation.

Response of rice varieties to soil salinity and air humidity : a possible involvement of root-borne ABA

In a phytotron experiment four rice varieties (Pokkali, IR 28, IR 50, IR 31785-58-1-2-3-3) grown in individual pots were subjected to low (40/55% day/night) and high (75/90%) air humidity (RH), while soil salinity was gradually increased by injecting 0, 30, 60 or 120 mM NaCl solutions every two days. Bulk root and stem base water potential (SWP), abscisic acid (ABA) content of the xylem sap and stomatal resistance (rs) of the youngest fully expanded leaf were determined two days after each salt application. The SWP decreased and xylem ABA and rs increased throughout the 8 days of treatment. The effects were amplified by low RH. A chain of physiological events was hypothesized in which high soil electric conductivity (EC) reduces SWP, followed by release of root-borne ABA to the xylem and eventually resulting in stomatal closure. To explain varietal differences in stomatal reaction, supposed cause and effect variables were compared by linear regression. This revealed strong differences in physiological reactions to the RH and salt treatments among the test varieties. Under salt stress roots of IR 31785-58-1-2-3-3 produced much ABA under low RH, but no additional effect of low RH on rs could be found. By contrast, Pokkali produced little ABA, but rs was strongly affected by RH. RH did not affect the relationships EC vs. SWP and SWP vs. ABA in Pokkali, IR 28, and IR 50, but the relationship ABA vs. rs was strongly affected by RH. In IR 31785-58-1-2-3-3 RH strongly affected the relationship SWP vs. ABA, but had no effect on ABA vs. rs and EC vs. rs. The results are discussed regarding possible differences in varietal stomatal sensitivity to ABA and their implications for varietal salt tolerance.