Architectural root responses of rice to reduced water availability can overcome phosphorus stress.

Drought and low phosphorus (P) availability are major limitations for rainfed rice production. Crop roots are important for soil resource acquisition and tolerance to P and water limitations. Two pot and two field trials were conducted to evaluate architectural root responses of contrasting rice varieties to combinations of different levels of P (deficient to non-limiting) and water availability (water stressed to submergence) and to identify the interactions with different varieties. Root development was then related to drought and/or low P tolerance. Although shoot and root growth responded more to P than to water availability, architectural root responses to water were much more prominent than responses to P availability. Reduced water availability decreased nodal thickness and increased secondary root branching, both factors partially enhancing P uptake efficiency and even overcoming a decreased root:shoot ratio under reduced water availability. In contrast to root thickness and secondary branching, basal lateral root density was strongly determined by variety and was related to enhanced P uptake. Reduced water availability induces root modifications whichapart from enhancing drought resiliencealso affect P uptake efficiency. Future research on rice roots and nutrient uptake may hence take into account the large effects of water on root development

Anatomical root responses of rice to combined phosphorus and water stress - relations to tolerance and breeding opportunities.

Drought and low P availability are major limitations for rainfed rice (Oryza spp.) production. Root anatomy plays a key role in resource acquisition and tolerance to P and water limitations. Root anatomical responses of three contrasting rice varieties to combinations of different levels of P (deficient to non-limiting) and water availability (water stress to submergence) were evaluated in two pot trials. P availability was the dominant growth-limiting factor, but anatomical root responses to water availability were more prominent than responses to P availability. Cortical cell file number and number of xylem vessels decreased as a response to water stress, but stele and xylem diameter increased. Low P availability induced thinner xylem vessels and a thinner stele. Drought tolerance related to an overall thicker root stele, thicker xylem vessels and a larger water conductance. Some root traits were observed to be more responsive to water and P availability, whereas other traits were more robust to these environmental factors but highly determined by variety. The observed genotypic variation in root anatomy provides opportunities for trait-based breeding. The plasticity of several traits to multiple environmental factors highlights the need for strategic trait selection or breeding adapted to specific target environments

Phosphorus micro-dosing as an entry point to sustainable intensification of rice systems in sub-Saharan Africa

Phosphorus (P) deficiency is a major biophysical limitation to rice grain yields in sub-Saharan Africa, and low-cost P management strategies are needed by smallholder farmers to reverse soil fertility decline. On-station field and pot experiments were combined with on-farm testing to evaluate the effect of P micro-dose placement in dry-seeded, dibbled rice on emergence, early vigour and grain yield. Placing a micro-dose of 3–6 kg P ha−1 in the planting hole consistently increased early vigour and grain yield of both a P-efficient and less P-efficient rice genotype. An agronomic efficiency of P fertilizer (AEP) of 356–817 kg grains kg−1 P was achieved with a micro-dose of 3 kg P ha−1. A negative effect on emergence by placing a P micro-dose in the planting hole observed in one experiment was compensated by a higher grain weight per hill. In other experiments, no negative effect on plant emergence under both well-watered and water-stressed conditions and with different P sources was observed even when fertilizers were mixed with seeds. A micro-dose of 20–30 kg of DAP ha−1 placed in the planting hole resulted in an average net increase in profit of $91 to$136 ha−1 and benefit:cost ratio of 3–12 in on-farm experiments. Farmers' appreciation of the technology was positive but lack of credit, availability of appropriate fertilizers in local agrodealers and increased labour requirements were identified as potential constraints for adoption. P micro-dose placement in the planting hole can be used as an entry point towards sustainable intensification in dry-seeded, dibbled rice systems in SSA, provided it is accompanied by institutional support and mechanization options to increase its adoption potential