The effects of soil type, sulfur fertility, and maturity on the pungency of 'Texas Grano 1015Y' onions

Abstract

A field study was conducted on 'Texas Grano 1015Y' onions (Allium cepa L.) grown in the Lower Rio Grande Valley of Texas. Treatments included two soil types (clay and loam), five maturities from beginning of bulbing throughout the bulbing process, and two S rates (O kg S/ha and 22.4 kg S/ha). Laboratory analysis included pyruvic acid concentration for pungency measurement, percent dry matter, and sucrose, glucose, and fructose concentrations. Maturity influenced all variables tested. Percent dry matter (DM) generally decreased as bulbs m,7taFed (9.1 to 697o DM) with a slight increase at maturity (7 4110 DM). Enzymatically developed pyruvic acid concentrations ranged from 3.13 to 4.03 limol-g-I fresh wt. However, the first harvest had a pyruvic acid content of 5.54 AMOI-G-I fresh wt. There was an increasing trend of pyruvic acid over the bulbing process. Total sugars tended to increase beginning with the second maturity (39.3 to 46.5 mg-g-I fresh wt.). However, sucrose decreased during the last two sampling dates resulting in a corresponding increase in glucose and fructose. The S fertility treatment had no effect on any of the factors measured. The only influence by soil type was sugar concentration, with the loam field producing onions higher in glucose. A greenhouse study was also conducted to test the effects of S fertility on onion pungency. Cloned plants were used to avoid genetic variation between treatments. The low S treatment (2 ppm) produced onions with a pyruvic acid concentration of 1.9 gmol.g-1 fresh wt. and the high S treatment (123 ppm) onions contained 5.5 timol-g-I fresh wt. of pyruvic acid. However, bulb weight was greatly reduced by the low S treatment. Sugars were also slightly higher in onion bulbs from the low S treatment. Percent S in leaf tissue also showed a deficiency in the low S grown plants.