Effects of Selected Plant Growth Regulators on Bread Wheat Spike Development

Although the grain yield of wheat is finally determined after anthesis, the yield potential is largely dependent on early growth and development. At the specific stage from double ridge to terminal spikelet, spikelet initiation occurs and can affect the number of grains per spike and the grain yield. A factorial experiment using a randomized complete blocks design with six replicates was used to study the effect of three growth regulators (3‑indoleacetic acid [IAA], gibberellic acid [GA3], and 6‑benzylaminopurine [6‑BAP]) on two bread wheat (Triticum aestivum L.) cultivars (Rijaw and Azar‑2), at the Campus of Agriculture and Natural Resources of Razi University, in Kermanshah, Iran, during the 2013–2014 and 2014–2015 cropping seasons. The effect of the hormones was not significant for spikelet initiation number or spikelet initiation rate based on days and growing degree days (GDDs), but apical meristem length and rate of elongation of the apical meristem were affected by exogenous application of hormones in both years. The Rijaw genotype was better than Azar‑2 with respect to apical meristem traits. As well, biplot diagrams showed that the treatment combination 6‑BAP × Rijaw was the best in terms of shoot apex length and rate of shoot apex elongation and that the treatment combination GA3×Rijaw was the best in terms of spikelet number and rate of spikelet initiation. It is concluded that each hormone can improve specific apical meristem characteristics and that the rate of each hormone’s effect depends on the plant’s genetic feature and on the environmental conditions.