Phosphorus utilisation efficiency and depletion of phosphate fractions in the rhizosphere of three tea (Camellia sinensis L.) clones

Tea (Camellia sinensis L.) is mostly grown on highly weathered acidic Ultis ols in the humid and sub-humid tropics. Phosphorus (P) availability in Ulti sols is naturally low due to it's low diffusivity caused by high P-fixation of Fe and Al oxides. Tea is generally fertilised with low-cost reactive ph osphate rocks (RPR) because of enhanced solubility of RPR under acidic cond itions. In many countries, new tea clones have been developed to improve yi eld, drought tolerance and resistance to pest and diseases, but the effecti veness of these clones in utilising P from RPR and native soil P forms has not been studied. A study was conducted to investigate the effects of tripl e superphosphate (TSP) and a sparingly soluble phosphate rock (Eppawala pho sphate rock, EPR) on plant P uptake and soil P fractions in the rhizosphere of three tea clones developed in Sri Lanka (S 106, TRI 2023 and TRI 2025). Phosphate uptake by TRI 2023 (3.3 +/- 0.02 mg P plant(-1)) and TRI 2025 (2 .6 +/- 0.08 mg P plant-l) was significantly greater than S 106 (1.1 +/- 0.0 1 mg P plant(-1)) for both P treatments. However the type of P fertiliser d id not show any significant difference in P uptake by any of the clones. In all tea clones, the rhizosphere soil pH decreased significantly compared t o that of the bulk soil in both P fertiliser treatments. The decrease of pH near the rhizoplane (0-0.5 mm) for TRI 2023, TRI 2025 and S 106 were 0.30 +/- 0.02, 0.19 +/- 0.03 and 0.17 +/- 0.05 respectively. Dissolution of EPR in the rhizosphere of TRI 2023 and TRI 2025 was greater than S 106, which c ould be attributed to enhanced H+ efflux. TRI 2023 and TRI 2025 depleted mo re resin-P, NaOH-Piand H2SO4-P-i in the rhizosphere compared to S 106, whic h is consistent with the P uptake results. NaOH-P-o accumulated in the rhiz osphere of all clones suggesting that soluble inorganic P was transformed i nto organic P forms possibly as a result of increased microbial activity in the rhizosphere. TRI 2023 and TRI 2025 showed a higher external P efficien cy (total P uptake) compared to S 106 due to higher root surface area and h igher P uptake per unit root surface area. The higher P uptake per unit roo t surface area in TRI 2023 and TRI 2025 compared to S 106 may be due to hig her rhizosphere acidification, root exudation and mycorrhizal associations. Between TRI 2023 and TRI 2025, the former had higher external P efficiency due to greater root surface area. S 106 had higher internal P efficiency ( shoot dry matter production per unit plant P) compared to the other two clo nes. In future tea breeding programmes, attempts should be made to combine these two traits to maximise P utilisation efficiency from tea clones.