Seasonal dynamics of the rhizosphere phosphorus and citrate exudation by grapevine roots in a low P soil: a field experiment

Abstract

Low phosphorus (P) availability is a common constraint for crop productivity in both acid and calcareous soils due to formation of insoluble P complexes. As far as we are aware, grapevine is not considered a P-efficient species, whereas P deficiency is rarely reported under the field conditions. We performed a field experiment under extremely low P conditions (Olsen P < 2 mg kg-1) using own-designed rhizotron (80 cm depth), installed in 8-y-old vineyard with cultivar Chardonnay, grafted on 5BB rootstock. The device enables easy access to the new intact roots while preventing root drying. We collected the exudates from root tips every 2 h (during daytime) at the different growth stages (i.e. shoots 10 cm, flowering, berries pea-size and veraison). At the same time, both rhizosphere and bulk soil samples were collected and subjected to the sequential P analyses. The results of HPLC analyses of the root exudates showed that citrate can be considered as the main P-mobilizing compound, with clear seasonal and diurnal patterns: the highest anion exudation rate was recorded at flowering (lower pick) and veraison (higher pick), and at each growth phase in the morning (10 a.m.) and in the afternoon (4 p.m.), with a rapid depression at 1-3 p.m. Consequently, the relative share of soluble P fractions (H2O- and NaHCO3-extractable) in the rhizosphere was very high (20% of the total P fractions), leading to the leaf P concentration of 0.2% DW which is above the critical P-deficient level (0.15% DW) for grapevine leaves at flowering.