Rice crop duration and leaf appearance rate in a variable thermal environment : I. Development of an empirically based model

Variable crop duration is a major constraint to rice double cropping in and irrigated environments, such as the Sabel. 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 (Tw) varied from 13 to 35°C. After seed soaking, the rate at which the first leaf (L,) appeared was linearly related with Tw, with a base temperature (T base) of about 10°C. Appearance rates of the subsequent three leaves (L2-L4) had a similar Tbase, 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 L5 to the flag leaf (L12 to L20) 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. (Résumé d'auteur

Rice crop duration and leaf appearance rate in a variable thermal environment : III. Heretability of photothermal traits

In arid, irrigated, rice environments, crop duration is highly variable, resulting in uncertain crop calendars for double cropping. The main causes of this variability are varying temperature and daylength. Breeding for stable crop duration in such environments might make a major contribution to rice production. A previous study established genetic differences in phenological responses to temperature and photoperiod, based on a field study with sequential planting dates in Senegal. The present study applied this experimental approach to a five-parent complete dialled scheme. The five parents and 20 F1 progeny populations were sown on 14 dates at 2-week intervals at Ndiaye, Senegal (16°14' N) during the cold-dry, hot-dry and wet seasons in 1996. The model LAP (Leaf Appearance) described previously was used to disaggregate photoperiodic and thermal traits, which gave reproducible results for the thermal component, but not for the photoperiodic responses. Responses to photoperiod were therefore measured using a subset of environments in the Wet season (stable temperatures and 12.6-13.1 h daylength). Cross involving IR29725-40-3-2-3 (IR29725; short duration) and IR4630-22-2 (IR4630; medium duration), in combination with any of the other lines in the diallel, gave consistent patterns of heritability for earliness of flowering in the different seasons, the sensitivity of panicle induction to low temperatures, and photoperiod-sensitivity in the range of 12.613.1 h daylength. The results permitted three conclusions: (1) short duration in the wet season was a dominant trait, and in the cold-dry season, a super-dominant trait. IR29725 is a promising donor for this trait. (2) Despite its moderate to high phenological sensitivity to low temperatures, IR29725 acted as donor for low sensitivity of panicle initiation to temperature. This trait was dominant. (3) At least under the thermally stable conditions of the wet season, IR4630 acted as donor of a dominant trait for photopetiod-sensitivity, whereas IR29725 acted as donor for photoperiod-insensitivity. In summary, IR29725 is a promising donor of traits, such as short duration, low photoperiod-sensitivity and low sensitivity of phenological processes to temperature. (Résumé d'auteur

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

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 I I 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 (Topt) between 22°C and 25°C, with significant differences between lines. Lines with low Topt 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 Topt 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. (Résumé d'auteur