Two wheat cultivars (Triticum aestivum L.) were used to evaluate the effects of post-anthesis severe water deficit (SD) on starch content and granule size distribution and their relations with ethylene and spermidine (Spd). Comparison to the well-watered (WW) treatment, SD led to lower Spd and higher 1-aminocylopropane-1-carboxylic acid (ACC) concentrations and ethylene evolution rate (EER) in grains at the critical stage of forming starch granules. Application of Spd or aminoethoxyvinylglycine (AVG) significantly reduced ACC concentration and EER and increased Spd concentration, while ethephon or methylglyoxal-bis (MGBG) had an opposite impact. The volume and surface area distribution of starch granules showed a bimodal curve, while the number distribution exhibited a unimodal curve. SD caused a marked drop in grain weight, grain number and starch content, also led to a significant reduction in the proportion (both by volume and by surface area) of B-type starch granules (<10 μm), with an increase in those of A-type starch granules (>10 μm). Application of Spd or AVG increased the proportion (both by volume and by surface area) of B-type starch granules under SD. Correlation analysis suggested that ethylene and Spd showed an antagonism relation in the formation of B-type granules. These results suggested that it would be good for the formation of B-type starch granules to have the physiological traits of higher Spd and lower ACC concentrations and ethylene emission under SD.

Granule size distribution of wheat starch is an important characteristic that can affect its chemical composition and the functionality of wheat products. Two high-yield winter wheat cultivars were used to evaluate the effects of the application of exogenous ABA or GA during the reproductive phase of the initial grain filling on starch granule size distribution and starch components in grains at maturity. The results indicated that a bimodal curve was found in the volume and surface area distribution of grain starch granules, and a unimodal curve was observed for the number distribution under all treatments. The exogenous ABA resulted in a significant increase in the proportions (both by volume and by surface area) of B-type (<9.9 μm in diameter) starch granules, with a reduction in those of A-type (>9.9 μm) starch granules, while, the exogenous GA3 led to converse effects on size distribution of those starch granules. The exogenous ABA also increased starch, amylose and amylopectin contents at maturity but significantly reduced the ratio of amylose to amylopectin. Application of GA3 significantly reduced starch content, amylopectin content but increased the ratio of amylose to amylopectin. The ratio of amylose to amylopectin showed a significant and negative relationship with the volume proportion of granules <9.9 μm, but was positively related to the volume proportion of granules 22.8-42.8 μm.