Abstract:

A potted plant experiment was conducted under an auto-rain-shelter to investigate C consumption of roots and its relationship to yield formation of spring wheat (Triticum aestivum) as affected by soil moisture. Two cultivars of plants, `Longchun 8139-2' (L) and `Dingxi 24' (D), were grown in a split PVC pot 100 cm long and 10 cm in diameter in a mixture of deep loessial soil and vermiculite (1∶5.2 g/g). Three soil water treatments were employed: wet (W, 75%-85% of field water capacity); medium drought (M, 45%-55% of field water capacity); and serious drought (S, 25%-35% of field water capacity). Water regimes were controlled by watering every 5 days and destructive sampling was performed at jointing, anthesis and mature stage to measure plant biomass in shoots and roots. Respiration in roots and rhizosphere was measured by infrared gas analysis system at these stages. Root exudation was calculated from the rhizosphere respiration. Of the total carbon consumed by roots during jointing and anthesis, 26% of daily C consumption was allocated to root biomass, 58% to root respiration and 16% to root exudation, when averaged across all treatments. During the maturing period, no significant differences in the daily decrease of C allocated to root biomass in the W and M treatments were found between the two cultivars, but, in the S treatment, the daily decrease of C allocated to root biomass was significantly more in D cultivars than in L, and the lowest daily C consumption from root respiration and exudation also occurred in D cultivars in the S treatments. This resulted in a greater than 100 times reduction in the ratio of root biomass C to total C invested in roots, and a 7.89 times and 3.37 times decrease in the ratio of root respiration C and exudation C to total C consumption, which was very different from other treatments/cultivars. The ratio of the sum of root respiration C and exudation C to total C consumption was 53%, 52% and 83% respectively for W, M and S treatments in L cultivars, and 58%, 49% and 55% in D cultivars. The lowest ratio occurred in the M treatment for both cultivars, and this ratio was far lower in D cultivars than in L for the S treatment. Grain yields were similar for both cultivars, but a little higher in L than in D under wet conditions. The ratio of grain yields of individual treatments to the mean yield of the three treatments was 1.34, 1.14 and 0.53 for W, M and S treatments in L cultivars, and 1.04, 1.06 and 0.90 in the three treatments of D cultivars. This suggests a higher stability of grain yields in D than in L cultivars. The decrease in daily C consumption of roots and the relative stabilization of daily photosynthetic rates accounted for the higher water use efficiency and relatively high grain yields under conditions of medium soil moisture.