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 pedunc le to translocate assimilate to the ear. Treatments consisted of two l evels of reduced vascular connections to the ear (50 and 75% of pedunc le 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 va scular occlusion. One treatment with peduncles left intact was include d to measure the impact of dehusking. All treatments decreased grain y ield an average of 19.7% from control levels, due to effects on both k ernel number and weight per kernel, but there was no general effect of severing vascular connections to the ear beyond the effect of dehuski ng and removing ear leaves 15 d post silking. There was also no differ ence in grain yield due to the two dates of treatment. An exception oc curred in the second year of the study, when the 75% occlusion treatme nt at either date reduced grain yield through reduction of kernel weig ht. We estimate mass transfer rates of 1.37 g h(-1) cm(-2) phloem in i ntact 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 c ross sectional area. The latter mass transfer rate approaches the maxi mum measured for a number of fruit sinks and may explain the greater i mpact of 75% peduncle occlusion in the second year. The effect may not have been expressed in the first year, due to unfavorable climatic co nditions that shortened the effective grain filling period. Under norm al conditions, assimilate transport through the peduncle seems to func tion at roughly one-fourth maximum capacity and is probably not a majo r grain yield limitation.