Excessively High Temperatures of Seed Onion Umbels1

Open Access
Publisher Website
Google Scholar
Abstract
High temp may adversely affect onion seed production since the occurrence of physiological damage to the umbel, resembling sunscald, is associated with a reduction in the number of viable seeds produced. Temperature measurements show that the portion of the umbel directly facing the sun is the hottest plant part. Ovary temp as high as 60°C (140°F) have been observed at the “hot spot”, which would denature many plant enzymes. A method was developed from Wisconsin data for estimating the max and min possible temp of the umbel “hot spot” using only air temp and wind speed. This method was successful in predicting max and min umbel temp measured at Parma, Idaho. The temp rise of the hot spot above air temp is inversely proportional to the 2/3 power of wind speed. The cooling efficiency of the wind is not only a function of its speed but also of where it impinges on the umbel relative to the location of the “hot spot”; thus the direction of the sun and wind must be known to compute the actual hot spot temp, rather than the possible max and min extremes. When wind speed and direction, air temp, and hot spot location (or sun angles) are known, umbel temp can be predicted within 2 to 3°C. The above results apply to “tight”, spherical umbels. “Loose” umbels will heat above air temp only about 1/5 as much as “tight” umbels; consequently, they are subject to much less heat stress.

This publication has 0 references indexed in Scilit: