Identifying common crop traits to address the variability in tiller production between sorghum and rice canopies

Developing a common mechanistic model to predict tiller dynamics in sorghum and rice canopies growing in favorable conditions can greatly contribute to the identification of common genetic regions involved in tiller production of both species. Crop growth processes of a high-tillering sorghum hybrid, grown in different plant densities in Australia, and a conventional rice inbred, grown during several crop seasons and under several crop management techniques in the Philippines, were quantified. Total tiller production per plant was up to 6 for sorghum growing at 2 plants m-2, and up to 30 for rice growing at 25 plants m-2. Only primary tillers were observed on sorghum, whereas secondary and tertiary tillers developed on the rice plant. Rate of tiller emergence for sorghum was linearly related to leaf number on the main tiller, with the first tiller emerging at leaf number 6 (when the incomplete leaf was counted) and tiller 5 emerging at leaf number 10. Even if the rate of tiller emergence in rice was exponentially related to leaf number, that of the primary tiller was also linearly related to leaf number, with the same characteristics as that in sorghum. This was also probably the case for emergence of secondary tillers related to leaf number of their mother primary tiller. Dynamics in leaf emergence on primary tillers was identical to that of the main tiller in sorghum and rice. Cessation in tiller emergence in sorghum was correlated to a unique value of leaf area index, 0.6, when plant densities from 2 to 16 plants m-2 were considered. Cessation in tiller emergence in rice occurred, however, with leaf area index between 1.0 and 3.5, with crop stage between 15 and 2 days before panicle initiation or between 9 days and 1 day before start in internode elongation, when considering a large number of field experiments. Delayed cessation in secondary and tertiary tiller emergence compared to that of primary tiller emergence may suggest considering tiller order to predict cessation in tiller emergence. Tiller senescence for both sorghum and rice systematically concerned the youngest tillers of the plant, whatever their order was. The impact of tiller senescence on grain yield appeared, however, different between sorghum and rice. Grain yield of sorghum, when grown at 16 plants m-2, increased from 9.2 to 11.5 t ha-1, and tiller mortality rate decreased from 0.7 to 0, if all individual tillers were systematically removed from the plant as soon as they appeared. However, decrease in rice tiller mortality rate, from 0.33 to 0.24 by increasing water depth at mid-tillering, or from 0.53 to 0.39 by transplanting younger seedlings, did not affect grain yield. Even though tiller production was highly different in rice and sorghum canopies, most of the features describing tiller dynamics were relatively consistent for both species according to the genotypes and growing conditions considered here. It may be possible to develop a common model for both species as long as control of cessation in tiller emergence and tiller senescence impact on grain yield are clearly assessed. (Résumé d'auteur

Performance of contrasted rice genotypes grown under water saving irrigation and trait discovery for genotype improvement : [Abstract, P 3.06]

Water saving technologies without reduction on grain yield, like alternate wetting and drying (AWD), have been implemented successfully in irrigated rice fields. It is now needed to identify key crop traits under more pronounced water constraint for genetic improvement and further increase in water productivity. Five contrasted genotypes were grown under three water treatments in 2006 (AWD30 with irrigation set up whenever soil water potential reached -30 kPa at 15 cm deep, AWD60, and CF as continuous flooding) while nine genotypes were grown under two water treatments in 2008 (AWD30 and CF). While water use decreased significantly for all genotypes by 29% to 37% under AWD30 and by 22% to 34% under AWD60 in 2006DS, and by 17% to 25% in 2008DS, grain yield was maintained only in rare cases which expressed high contrast among genotypes' responses: grain yield of PSBRc80 was maintained in all situations while that of IR64 was significantly and systematically affected. The response of yield components to AWD was, however, not consistent with regard to the performance: cases with stable grain yield included some with unaffected yield components (three genotypes) and others with compensation between panicle number and filled grain number per panicle (PSBRc80). Cases with reduced grain yield included some with reduced grain size (two genotypes including IR64) and others with reduced filled grain number per panicle (two genotypes). Tiller emergence rate and maximum tillering increased under AWD for all genotypes, however, effect on LAI and biomass accumulation was inconsistent. Partitioning to culm was favored under AWD to the detriment of blade during the reproductive phase of the most adapted genotypes in 2008 although this was not reported in 2006. Under CF, the root/shoot ratio, total root dry matter and root dry matter below 25 cm deep were consistently higher, but for the most three adapted genotypes only. As a response to AWD, the root/shoot ratio and total root dry matter of these genotypes were maintained (in 2006) or even increased (in 2008) while those of a genotype bred for aerobic conditions were rather low under CF, but its partitioning to root dry matter at deep layers increased strongly under AWD. Although the plant type and traits responsible to AWD adaptation were not clearly consistent among the promising genotypes under study, the size, distribution and adaptation of the rooting system were clearly identified as key crop traits for adaptation to alternate wetting and drying. (Texte intégral

Expanding alternate wetting and drying and improving its productivity in irrigated rice: Identification of required plant traits and suitable soil types

Irrigated rice consumes two to three times more water than other cereals. The availability of water is, however, decreasing and this prompted researchers to find ways in saving water in irrigated rice fields where high yield is critical to ensure food security. The alternate wetting and drying (AWD) technology has been implemented successfully in farmer's fields. What is now needed is to fine-tune this technology in a site-specific manner with regard to genotype characteristics and soil type suitability. Nine genotypes were evaluated in similar growing conditions under AWD30 (irrigation whenever soil water potential reaches -30 kPa). Water productivity increased for all genotypes and a few were identified as adapted through their efficient sink regulation and deep rooting system. Two of the nine classified as promising genotypes were grown in contrasting soil types from sandy loam to clay soil under AWD30 and continuous flooding. Grain yield reduction was higher with the hybrid (37-57% in light soil and 0-7% in heavy soil) than with the inbred (25-45% in light soil and no reduction in heavy soil). Water input under AWD30 was reduced by 29-55% in both genotypes in light soil and by 6-26% in clay soil. Water productivity was higher in heavy soil and reduction in shoot biomass at physiological maturity was stronger in light soil. Stronger reduction in harvest index and sink size was observed with the hybrid. Selecting adapted genotypes and adjusting water management with respect to soil type will further improve the AWD irrigation technology. (Résumé d'auteur