Genetic variations of Brassica cultivars for P acquisition in a P stress environment and comparison of P sources for sustainable crop management

To compare the growth performance of Brassica in a phosphorus (P) stress environment and response to added P, six Brassica cultivars were grown in pots for 49 days after sowing, using a soil low in P [sodium bicarbonate (NaHCO3)-extractable P = 3.97mgkg^-1, Mehlich III-extractable P = 6.13mgkg-1] with (+P = 60mg P kg-1 soil) or without P addition (0P). Phosphorus-stress markedly reduced biomass accumulation and P uptake by roots and shoots. However, root-shoot ratio remained unaffected, implying that relative partitioning of biomass into roots and shoots had little role to play in shoot dry matter (SDM) production by cultivars. Biomass correlated significantly (P < 0.01) with total P uptake. Under P stress, the cultivars that produced greater root biomass were able to accumulate more total P content (r = 0.95**), which in turn was related positively to SDM and total biomass (r > 0.89**) and negatively to P-stress factor (r = -0.91**). There was no correlation between P efficiency (PE) (relative shoot growth) and plant P, but PE showed a very significant correlation with shoot P content and SDM. Wide differences in growth and better performance of cultivars such as 'Brown Raya' and 'Con-1' under P stress encouraged screening of more germplasm, especially in the field, to identify P-tolerant cultivars. In another study, potential relative agronomic effectiveness (RAE) of sparingly soluble P sources was investigated by growing two contrasting cultivars. The P sources incorporated into soil at 0, 10, 25, 50, and 100mg P Kg-1 were (i) powdered Jordan rock P (RP), (ii) triple superphosphate (TSP), (iii) powdered low-grade TSP [TSP(PLG)], (iv) a mixture of RP + TSP compacted into pellets at 50:50 P ratio [RP + TSP(PelC)], and (v) a mixture of powdered RP + TSP at 50:50 P ratio [RP + TSP(PM)]. The RP was low in RAE and only 5 and 29% as effective as TSP in producing dry matter (DM) of P-sensitive 'B.S.A.' and P-tolerant 'Brown Raya' cultivars, respectively. There were no significant differences between TSP and RP + TSP(PelC) in DM yield of 'Brown Raya,' whereas, in the case of 'B.S.A.' RP + TSP(PM) was significantly less effective than RP + TSP(PelC) compared with TSP. Combined utilization of superior genome and P sources [such as TSP(PLG) and RP + TSP(PelC)] produced from low-grade RP (that cannot be used either for direct application or acidulated P fertilizers) can be used as an alternative strategy for sustainable crop production, especially in resource-poor environments. Further field trials at the level of cropping systems are needed.