Evaluation of Low Phosphorus Tolerance of Rice Varieties in Northern Ghana

Phosphorus (P) deficiency is a major constraint to upland rice production on highly weathered, low activity clay soils in the humid zones of West Africa. There is a paucity of information on the short-term fertilizer P effects on rice on these soils. A field experiment was conducted in 2011 to determine the response of twenty-four (24) upland rice cultivars to fertilizer Phosphorus (P) applied at 0 and 60 kg P ha. An uncultivated field at SARI research area with available P (Bray 1) P content of 3.0 mg/kg was used for the experiment. The ploughed area was divided into two plots with one plot for +P and the other for -P treatment. Each of the 24 varieties was allocated three rows and spaced at 20 x 10 cm in four replications. The varieties were randomized for each replicate and planted on 2 July 2011 at one seed per hill. Pre-emergence herbicide Pendimethaline 400 g/l (Alligator) was applied at 3.2 L/ha two days after planting followed by one hand weeding. For the (-) P plot, the entire field received N at 60 kg/ha as Sulphate of ammonia and K2O at 60 kg/ha as Muriate of Potash. For the (+) P plot, fertilizer was applied at NPK 60-60-60 kg/ha from Sulphate of ammonia, Triple superphosphate and Muriate of potash, respectively. For each plot, the N was split applied. Results showed that the plants that received P were more vigorous and healthier. There was significant variety effect on number of tillers per plant (NOTPP), days to 50% flowering (DFF), days to maturity (DTM), dry weight of biomass (DWOB), number of panicles per plant (NOPPP), and grain yield per plant. Similarly, there was significant phosphorus effect on number of tillers per plant (NOTPP), days to 50% flowering (DFF), days to maturity (DTM), dry weight of biomass (DWOB), number of panicles per plant (NOPPP), and grain yield per plant. There was generally no interactive effect of variety by fertilizer except for DFF and DTM. The number of tillers per plant ranges from 3 – 16 with the overall mean without P application being 4 while overall mean with P application was 9. When P was applied, the number of days to 50 % flowering reduced from 86 days to 79 days. Days to maturity (DTM) was also reduced from 118 to 111 on average by P application. The varieties that were most tolerant to low P were ITA 257, Nerica 3 and TOX 1011-4-A2. The grain yield of ITA 257 remained the same whether P was applied or not. This is the variety that is best adapted to low P. We concluded that rice growth and yield components were affected by Phosphorus application. Plants that received phosphorus flowered and matured earlier. They also accumulated higher biomass and grain yield. ITA 257, Nerica 3 and TOX 1011-4-A2 were most tolerant to P deficiency.


Introduction
Phosphorus is one of the major nutrient elements out of 16 essential elements required in rice nutrition.Its beneficial effects on plant growth are stimulation of root development, early flowering, and ripening and promotion of good grain development, and quality (Datta, 1965(Datta, , 1981)).Phosphorus deficiency in soils is an important nutritional problem in tropical soils.
However, all the soils in the tropics are not P deficient and those that are deficient do not always require large amounts of P fertilizers (Fox & Searley, 1978).In deficient soils application of large amounts of phosphatic fertilizer may pose financial problems to the farmers.An alternative method to overcome the P. deficiency .inwetland rice soils is to breed varieties tolerant to P deficiency even though it may not be effective under extreme conditions.Tadano and Tanaka (1980) concluded that rice compared to other plant species has a strong tolerance to low phosphate conditions.They also indicated that differences in tolerance to low phosphate conditions were controlled (1) mainly by tolerance to low P content in the plant, (2) phosphorus absorbing power per unit root weight and (3) distribution of absorbed phosphorus between root and' shoot.
Phosphorus (P) is one of the most important but also most limiting plant nutrients, especially in Africa where an estimated 50 % of cropland suffers some degree of P deficiency.On these soils, upland rice is often cultivated without or only with little fertilizer input by smallholder farmers that depend on the crop for food security and their livelihoods.For these farmers varieties are required that thrive on little available indigenous soil P especially during early growth stages to better compete with weeds, the main constraint apart from low soil fertility.As a first step towards developing cultivars with higher P efficiency, a screening experiment was conducted to identify the level of tolerance to P deficiency in 24 rice cultivars that are widely grown in the African uplands in a collaborative research with JIRCAS under the project "Development of rice-production technology in Africa".The experiment was conducted at the experimental field of SARI from July to November 2011 to determine the mechanism of varietal tolerance to P deficiency in upland rice.

Materials and Methods
In order to site the experiment on a P deficient soil, an uncultivated upland field at the SARI research field near the water treatment site was selected.Composite soil samples were taken from 0-20 cm depth over 0.1 ha area and analyzed for Bray-1 P, which was found to be 3.04 mg/kg.The site was considered appropriate for the experiment.The land was ploughed with disc plough and harrowed with a disc harrow.Thereafter, leveling was done with hand hoe.
The ploughed area was divided into two plots with one plot for P application and the other without P application.The separate fields were lined and pegged for the experiment.Each variety was allocated three rows and spaced at 20 x 10 (20) cm.The distance between varieties was 30 cm.There were four replications.Planting (Kindly check the spacing).
The varieties were randomized for each replicate and planted on 2 nd July at one seed per hill when the soil was moist sufficiently for seed germination.The entire field was fenced to keep away animals.Pre-emergence herbicide Pendimethaline 400 g/l (Alligator) was applied at 3.2 L/ha 2 days after planting.Thereafter weeds were hand-picked once when it became necessary.
For the (-) P plot, the entire field received N at 60 kg/ha as Sulphate of ammonia and K 2 O at 60 kg/ha as Muriate of Potash.For the (+)P plot, Fertilizer was applied at NPK 60-60-60 kg/ha from Sulphate of ammonia, Triple superphosphate and Muriate of potash, respectively.For each plot, the N was split applied.The basal N (30 kg/ha) was applied with all the P and K at 14 days after planting.The remaining N was applied 42 days after planting.Germination count was done 14 days after planting.Other data collected were, plant vigour, Days to 50% flowering, tiller count/plant for 10 plants, biomass at vegetative and harvest stage and grain yield.

Results
Germination of the rice seeds was generally low.Germination percentage ranged from 12 to 52%.The Nericas were more vigorous at vegetative stage and maintained fairly high biomass throughout the growth period.Generally, the plants that received P were more vigorous and healthier.There was significant variety effect on number of plants germinated (NOPG) and germination percent (GP), number of plants established per plot (NOPEPP), number of tillers per plant (NOTPP), days to 50% flowering (DFF), days to maturity (DTM), dry weight of biomass (DWOB), number of panicles per plant (NOPPP), and grain yield per plant.Similarly, there was significant phosphorus effect on number of tillers per plant (NOTPP), days to 50% flowering (DFF), days to maturity (DTM), dry weight of biomass (DWOB), number of panicles per plant (NOPPP), and grain yield per plant (Table 1).There was generally no interactive effect of variety by fertilizer except for DFF and DTM (Table 1 & 2).The number of tillers per plant ranges from 3 -16 with the overall mean without P application being 4 while overall mean with P application was 9 (Table 2).When P was applied, the number of days to 50% flowering reduced from 86 days to 79 days.Days to maturity (DTM) was also reduced from 118 to 111 on average by P application.The varieties that were least affected by low P were ITA 257, Nerica 3 and TOX 1011-4-A2 (Table 5).The grain yield of ITA 257 remained the same whether P was applied or not.This is the variety that is best adapted to low P. Moroberecan did not have any grain yield due to abrupt end of the rains in early October.

Discussion
The significant effect recorded with addition of P fertilizer on number of tillers per plant, days to maturity, dry weight of biomass and grain yield showed that P fertilizer is very critical in rice production in the northern region of Ghana.This result is consistent with findings by other research in the region that suggest that P is second most limiting nutrient in soils in the Guinea Savanna agro-ecological zone of the region.More so, there results further stressed the importance of P in rice cultivation.Varieties that received P fertilizer resulted in higher and significant grain yield as a result of improved photosynthetic activity leading to higher tillering by the rice crop.Furthermore, phosphorus is also a component of other compounds necessary for protein synthesis and transfer of genetic material DNA, RNA (Wilson et al., 2006).
Additionally, results of this experiment showed increased grain yield by varieties which was significantly different from varieties that received no P because water availability was controlled to avoid flooding.Other studies have showed that phosphorus application to rice increased P accumulation but did not consistently increase rice yields because flooding which decreased soil P sorption and increased P diffusion.It is however clear from the results of this experiment that, addition of P to upland rice will not only improve P accumulation but will increase grain yield significantly.The higher tolerance of ITA 257, Nerica 3 and TOX 1011-4-A2 of plants germinated; GP = Germination Percentage; NOPEPP = Number of plants established per plot; NOTPP = Number of tillers per plant; DFF = Days to 50% flowering; DTM = Days to Maturity; FWOB = Fresh weight of biomass (g, 3 plants); DWOB = Dry weight of biomass (g, 3 plants); NOPPP = Number of panicles per plant; GYPP = Grain yield per plant (g).

Table 1 .
Mean sums of squares for the various parameters considered

Table 2 .
Effect of variety, P application and their interaction on rice growth parameters

Table 3 .
Main effects of genotypes and fertilizer on the parameters

Table 4 .
Crop establishment and phenological responses to interactive effects of genotype and phosphorus treatment

Table 5 .
Maturity and yield component responses to the interactive effects of genotype and phosphorus