4.1. Plant Materials

Two cucumber inbred lines WI7167 and WI7237, were used in the present study, which were the donor and the recipient (recurrent parent) in the development of near-isogenic lines (NILs) for the fruit size QTL FS5.2, respectively. WI7167, a selection from PI 618931, is a semi-wild XIS cucumber collected from Yunnan, China that sets nearly round fruits (length to diameter ratio or LD ≈ 1.0) [25]. WI7237 bears very long and slim fruit (>50 cm, LD ≈ 15.0) and is a landrace originally collected from China (Figure 1A and Figure 2A). The NIL pair, WI7237 and fs5.2-WI7237, was used for comparative morphological and histological observations, endogenous hormones measurement, transcriptome analysis, and fine genetic mapping in the present study.

All plant materials were grown in the Walnut Street Greenhouse (WSGH) or the field of the University of Wisconsin-Madison Hancock Agriculture Research Station (HARS), as well as the plastic tunnels in the Horticulture Farm of Northwest A&F University (HF-NWAFU) with standard cultivation managements.

Measurement of fruit size at immature and mature fruit stage of the two NILs was conducted in replicated trials in the greenhouses. For both WI7237 and fs5.2-WI7237, FL and FD were collected at multiple timepoints (i.e., −6, −3, 0, 3, 6, 9, 12, and 30 dpp) with three replications. At least three ovaries or fruits per replication were measured at each timepoint.