Dynamic relationships between field temperatures and romaine lettuce yield and head quality

Abstract

Abstract Romaine lettuce (Lactuca sativa L. var. longifolia Lam.) has great potential as a winter alternative crop in coastal South Carolina. Apache (A), Darkland (D), Green Tower (GT), Green Forest (GF), Ideal Cos (IC), and Tall Guzmaine (TG) romaine lettuce cultivar transplants were planted on 9 different planting dates in Charleston, South Carolina (SC) on monthly intervals from September to April from 1998 to 2001. The objective was to determine romaine's yield and quality responses to different growing season climates. Growing season mean (GSM) minimum or maximum temperatures (temps) were determined by averaging daily min and max temps from planting date to harvest date. The growing season for the September planting date (PD) was the warmest, followed by the April PD. The coldest growing season was the December PD, followed by January and November PDs. Collectively, GSM max temps ranged from 17 to 27 °C and GSM min temps ranged from 5 to 15 °C with these planting dates. The total accumulated heat units (Base 4.5 °C) were similar for all 8 PD growing seasons. The harvest maturities were similar between: September versus April PDs; October versus March PDs; and November versus February PDs. The December PD growing season required the greatest number of days to reach harvest from transplanting followed by the January PD growing season, with every 1 °C decrease in GSM min or max temps, days to harvest increased by 5 days. GSM min temperature affected number of marketable heads produced per plot more than GSM max temps for A, D, GT and TG but not GF and IC. Individual marketable head weight, however, was unaffected by GSM min and max temps. When all cultivar data were pooled and regressed, we found that from about 3 to 6 °C GSM min temps, the number of culls produced dropped steeply, but then plateaued from about 6 to 11 °C, and then decreased rapidly from 11 to 14 °C. The major cull defect was loss due to field rots. Generally, the pooled cultivar mean indicated that with an increase in GSM min temps from 3 to 6.5 °C, the rate of field rot production accelerated, but from 6.5 to 14 °C, field rot production was much slower in contrast.