Supplemental irrigation at jointing improves spike formation of wheat tillers by regulating sugar distribution in ear and stem

Irrigation at jointing increases spike numbers at wheat harvest, but the mechanisms underlying increased tiller spike formation rate (SR) during wheat production caused by irrigation have remained unclear. We therefore designed the supplemental irrigation (SI) treatments at jointing in which SI was received (SIJ) or did not receive (NSI) by wheat, and analyzed the morphological differentiation of young ears and physiological responses of young ears and stems to SI. Results shown that SIJ relative to NSI mainly improved the SR of the first primary tiller (T1) and second primary tiller (T2), which significantly increased the grain yield. The tillers SR positively correlated to the soluble sugar and sucrose contents in young ears, but negatively correlated to that in stems. SIJ significantly increased the soluble sugar and sucrose contents in young ears and reduced that in stems in both the main stem (MS) and tillers. Compared with MS, the young ear differentiation of T1 and T2 tillers that can form spikes was always behind by 0–2 developmental periods between jointing and flowering. Although SIJ decelerated the young ears differentiation of the MS and the T1 tiller, the increased sugar content in young ears causing a significant increase in grain numbers. Moreover, SIJ positively regulated the young ear differentiation of T2 tiller with more sucrose contained in young ears and less in stems to narrow gaps in young ear differentiation between T2 tiller and MS. Some T1 and T2 as well as other high-position tillers composed invalid tillers, where young ears mostly ceased differentiating at the stages of glume primordium present to carpel primordium present, and contained significantly less sugar than effective tillers. Significant morphological differences in young ears and nutritional physiological differences in tillers led to high-position tillers that were difficult to convert into effective tillers even under favorable conditions by SIJ. Overall, tillers can be converted into effective spikes by increasing sugar metabolism in young ears via SIJ when the discrepancy in young ear differentiation does not exceed two developmental stages compared to MS. •Tiller spike formation rate positively correlated to sugar content in young ears.•SIJ positively regulated tiller spike formation via increase sugar in young ears.•Spike can be formatted by SIJ when its differentiation is not slower than MS two periods.•High-position tillers were difficultly converted into spi