More than half of the world’s population rely on rice (Oryza sativa L.) as the major principal staple cereal. Rice production is increasing in order to meet the current consumption needs of the expanding world population. Scarcity of agricultural land is exacerbated by abiotic stresses such as P (phosphorus) deficient soils, especially in areas where most subsistence farmers are affected by droughts. Strategies to increase stress tolerance for improved growing of rice under stress are needed in order to maximize crop production in upland areas with poor soil nutrients especially in drought prone areas. Adaptive stress tolerance traits in rice for increased root length and nutrient uptake and use are key criteria for genetic analysis.
In the main pot experiment with three different P treatment levels (T) and three upland varieties (Ashoka 200F, Ashoka 228 and Kalinga III) there were significant differences between genotypes for root length and chlorophyll content on day 45 (booting) and for plant height, tiller number and chlorophyll content on day 75 (flowering). While at day 120 (harvesting), there were significant differences between genotypes for plant height, tiller number, chlorophyll contents, plant biomass and P concentration in roots as well as the number of grains, total grain weight and single grain weight.
Meanwhile, the three levels of P nutrient treatments did not significantly influence the plant height and root length at day 45 and day 75 but there were significant differences for P concentration in shoots, P uptake in shoots and total Plant P uptake. Significant differences were found on day 120 with P treatment having significant effects on root length, dry shoot weight and P uptake in root. However, there were no significant interaction (GxT) effects for any of the measured traits at all three harvesting days (d45, d75 and d120). The main experiment does support the hypothesis that Ashoka 200F outperforms Kalinga III in tillering (day 75 and above), root length (day 45) and counted grain, but not for grain weight when grown under low P soil.
A separate experiment carried out using an aerated hydroponic system (AHS) found a significant genotypic effect on root length where Ashoka 200F had longer roots (9.53 ± 0.29 cm) compared to Kalinga III (7.70 ± 0.56 cm), PY 84 (7.13 ± 0.39 cm) and Ashoka 228 (6.88 ± 0.24 cm) when grown under 0 P for 7 days after germination. Under 0 P, genotypes PY 84 (10.13 ± 0.77 cm) and Ashoka 228 (9.95 ± 0.21 cm) grew taller shoots than Ashoka 200F (8.63 ± 0.17 cm) and Kalinga III (8.75 ± 0.25 cm). Gene expression of the inorganic pyrophosphatase (IPP) gene (LOC_Os05g02310) showed no significant difference in roots between genotypes across all treatments, but its expression was down regulated in shoots of Ashoka 228 under half P treatment. Gene expression of the ß-Glucosidase gene (09g31430_41), a candidate gene for a root length QTL in PY84, was upregulated in PY84 under half P treatment compared to
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Kalinga III. On the other hand, no significant up regulation of this gene in PY 84 shoots was found compared to Kalinga III across different P treatments. This study depicts the complexity of P uptake, P use efficiency (PUE) and root traits associated with low P environments in rice genotypes. Further research is necessary to better understand the genetic and possible epigenetic variations acting in upland rice genotypes during their responses to low P environments.