EFFECT OF RESTRICTED CROSS-SECTIONAL PEDUNCLE AREA ON GRAIN-YIELD OF MAIZE

Field experiments were conducted over 2 yr to determine if maize (Zea mays L.) grain yield is limited by the physical capacity of the peduncle to translocate assimilate to the ear. Treatments consisted of two levels of reduced vascular connections to the ear (50 and 75% of peduncle cross sectional area) and two dates of treatment (15 and 30 d post silking). Treatment involved first removing ear leaf and husks at 15 d post silking, to gain complete access to the peduncle, followed by vascular occlusion. One treatment with peduncles left intact was included to measure the impact of dehusking. All treatments decreased grain yield an average of 19.7% from control levels, due to effects on both kernel number and weight per kernel, but there was no general effect of severing vascular connections to the ear beyond the effect of dehusking and removing ear leaves 15 d post silking. There was also no difference in grain yield due to the two dates of treatment. An exception occurred in the second year of the study, when the 75% occlusion treatment at either date reduced grain yield through reduction of kernel weight. We estimate mass transfer rates of 1.37 g h(-1) cm(-2) phloem in intact peduncles of dehusked plants and infer that rates may have risen to 5.49 g h(-1) cm(-2) phloem in peduncles with only 25% functional cross sectional area. The latter mass transfer rate approaches the maximum measured for a number of fruit sinks and may explain the greater impact of 75% peduncle occlusion in the second year. The effect may not have been expressed in the first year, due to unfavorable climatic conditions that shortened the effective grain filling period. Under normal conditions, assimilate transport through the peduncle seems to function at roughly one-fourth maximum capacity and is probably not a major grain yield limitation.