Response of Soybean Fruit Respiration to Changes in Whole Plant Light and CO2 Environment1

Abstract

A nondestructive indicator of instantaneous seed growth rate in soybean [Glycine max (L.) Merr.] and other legumes would useful in studies of the physiology of seed growth. The responsiveness of soybean fruit respiration rate to whole plant environmental treatments known to affect photosynthate supply and seed growth was evaluated. Dark respiration (CO2 efflux) rates of individual, intact soybean fruits at the mid‐pod‐filling stage were measured while manipulating photosynthetically active radiation (PAR) or ambient CO2 concentration in growth chambers. The PAR levels employed were 450, 110, or 30 μmol m−2 s−1 and CO2 concentrations were 300 or 150 μL L−1. Fruit respiration rate.began to decline within 24 h after plants were subjected to either decreased PAR or CO2 concentration. After 2 to 3 days the response was complete and fruit respiration rate remained constant at a level determined by the treatment for an additional 4 days, the longest period evaluated. The respiratory response was rapidly reversible when either PAR or CO2 concentration were returned to control levels. As PAR was lowered from 450 to 110 or 30 μmol m−2 s−1, seed growth rate declined from 20.3 to 11.0 or 8.7 mg day−1 fruit−1, respectively. Fruit respiration and seed growth were apparently responding to changes in photosynthate supply. The results indicate that in situ fruit respiration rate is responsive to changes in the physiological status of the plant and may be useful as an indicator of instantaneous seed growth rate in some situations.