Metabolite changes in cucumber xylem sap under rhizosphere aeration

The impact of rhizosphere aeration on the composition of xylem sap in cucumber grown in soil was studied to determine its effects on (i) water and nutrient uptake, (ii) xylem transport, and (iii) amino acid synthesis in the roots. Plants grown under three different aeration conditions were subjected to progressive gas stress throughout the whole growth period. The shoot and root growth, xylem nitrate (NO3-) concentration, potassium (K+) concentration, xylem sap flow rate, and amino acid concentrations were higher in the Z1 treatment plants than in the control (CK) plants, and the former also showed a lower CO2 (0.25-0.84%) and a higher O2 (18.22-20.02%). However, the Z3 treatment plants exhibited decreased xylem loading of amino acids, NO3- concentrations, and K+ concentrations and a lower flow rate of xylem sap, which restricted shoot and root growth due to poor aeration caused by a higher CO2 (0.75-3.68%) and a lower O2 (11.51-18.6%). Furthermore, the xylem pH in these plants was lower by approximately 0.17 units, and the conductivity was decreased by approximately 0.31 mS/cm. Based on the observed results, the CO2 and O2 in the soil can potentially regulate root and shoot growth and the xylem sap composition in cucumber. Therefore, rhizosphere aeration is important for the vigorous growth of plants.