K.M. Miézan

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.

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.

Increasing biodiversity of irrigated rice in Africa by interspecific crossing of Oryza glaberrima (Steud.) × O. sativa indica (L.)

The African rice Oryza glaberrima, traditionally cultivated since more than 3.500 years, is of poor agronomic performance but resistant/tolerant to various stresses and diseases. The introduction of these characters into O. sativa cultivars is difficult since crossing barriers cause spikelet sterility in F1. Backcrossing can restore fertility and recently facilitated the development of fertile O. glaberrima × O. sativa ssp. japonica hybrid progenies for rain fed systems. With the objective to gain access to African rice germplasm for improvement of irrigated rice, crosses were performed with eighteen O. glaberrima and twenty O. sativa ssp. indica accessions. In total about one hundred F1-hybrid grains were obtained. The F1 plants were all completely sterile and backcrossing (BC) to O. sativa was performed in order to restore spikelet fertility. Monitoring of Tog5681 × IR64 hybrid progenies under field conditions revealed a broad genetic diversity within the BC1 and BC2 populations. Some BC1 and BC2 progeny plants outperformed the O. sativa parent, indicating that the heterozygocity level and complementary gene action after two backcrosses are still sufficient to positively influence plant vigor. Spikelet fertility of progenies was highly variable, but almost complete fertility was already observed within the BC1F2 population. High spikelet fertility was preserved in one out of two analyzed BC1F3 families and inmost of the BC2F3 families. The ability to restore spikelet fertility within few generations and the potential of the genetic diversity present in interspecific progenies facilitates the development of plant types specifically designed for the African irrigated and lowland environment.

Rice crop duration and leaf appearance rate in a variable thermal environment. II. Comparison of genotypes

Abstract Rice crop duration in tropical-arid, irrigated environments, such as the Sahel, varies strongly among seasons and years. For rice double-cropping systems, cultivars are needed that have a stable duration under variable daylengths and temperatures. No efficient selection tools are currently available to screen for comparatively thermo- and photoperiod-insensitive cultivars, and little is known about the genetic diversity of rice in this respect. A previous study presented a model that disaggregates photothermal effects on rice phenology for the different crop development stages and the two main factors, daylength and temperature. The present study characterizes differences in the photothermal response of 18 rice lines for three major phases of their development, namely, germination, the appearance of early leaves, and the induction phase (IP) between the end of the basic vegetative phase (BVP) and panicle initiation (PI). A field experiment was conducted at Ndiaye in Senegal, using 11 staggered sowing dates at 15 day intervals. Three check cultivars were replicated four times per date, and the others were not replicated. Phenological observations included leaf tip appearance rates on a daily basis, and heading and flowering dates. Water and air temperatures were also monitored. The duration from seed soaking to the appearance of the first leaf depended linearly on water temperature, with a base temperature of about 8°C. No differences were observed among lines. The appearance rate of the first to the fourth leaf had an optimum temperature ( T opt ) between 22°C and 25°C, with significant differences between lines. Lines with low T opt had high maximal leaf appearance rates during seedling stage. The quantitatively most important component of the seasonal variability of crop duration was IP, which depended on temperature and photoperiod. Photoperiodic and temperature effects were disaggregated using an empirically based model leaf appearance (LAP), which, by way of parametrization, helped quantify genotypic differences. The photothermal differences between lines were associated with adaptation to specific seasons. Lines such as IR13240-108-2-2-3, which is adapted to several seasons, were comparatively insensitive to photoperiod and temperature. That line also had a low T opt and a high Leaf Appearance. In conclusion, screening for temperature-insensitive leaf appearance rates may be promising for rice breeding for arid, irrigated environments. Selection for combined low thermal and photoperiod sensitivity, however, would be impractical on an experimental basis, and would therefore require genetic markers.

A conceptual model for sodium uptake and distribution in irrigated rice

Sodium uptake and distribution in rice was investigated in various field and screenhouse trials at WARDA’s research station in Ndiaye, Senegal. A conceptual model for sodium uptake and distribution was developed on the basis of the following results: (i) sodium uptake to the plant was transpiration driven; (ii) varieties differed in the way they regulated their stomata in relation to relative humidity and salt stress; (iii) sodium uptake was modulated at the root level by a ‘root filter’; and (iv) sodium was taken out of the transpiration stream and retained in the sheaths with the daily retention capacity as a varietal constant. The model describes the passive uptake of sodium to the plant and its distribution as a function of several varietal constants and transpiration. The root filter function for sodium, the maximum stem sodium retention and the sodium toxicity threshold in the leaves vary among varieties. Varietal differences in stomatal reactions to relative humidity and salt stress were implemented in the ORYZA_W evapotranspiration routine to simulate these reactions. The interactions between sodium and potassium are discussed in relation to available information from literature and preliminary results obtained from screenhouse studies. A concept of potassium distribution in the plant and its interaction with sodium uptake and distribution was included in the model structure.