Source‐Sink Ratio Alteration and Its Effect on Physiological Parameters in Maize1

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Abstract
Improving grain yield in maize(Zea mays L.) can be facilitated by determining whether the source (leaf photosynthesis) or the economic sink (grain development)is more limiting. The long‐term effects defoliation and/or ear removal were measured on five maize genotypes (three inbreds and two hybrids) differing in relative source‐sink ratios. Plants were sown in the field on Clarion (fine‐loamy, mixed, mesic Typic Hapludoll) soils during the 1978 and 1979 growing seasons. Leaf carbon dioxide exchange rates (CER), stalk total nonstructural carbohydrates (TNC) and the dry weights of the leaves, sheaths, stalk, and ears were measuread at 10‐day intervals after source and/or sink manipulation (defoliation and/or ear removal).Grain yield, 100‐kernel weight, and the days to physiological maturity also were measured. The relative source‐sink ratio of the genotypes was the most important factor measured in determining how the imposed treatments affected leaf CER. Leaf CER increased after defoliation only in genotypes with large source‐slnk ratios in untreated plants and decreased following ear removal only in genotypes with small source‐sinkr atios in untreated plants. None of the treatments delayed leaf senescence. Defoliation caused a decrease in the weights of stalks, leaves, sheaths, ears, and stalk TNC of most genotypes. Ear removal increased these parameters, with the exception of leaf weight. The cob and grain weights, 100‐kernel weight, and the days to physiological maturity were decreased by defoliation. Both stalk TNC and stalk dry weight were reduced when plants were defoliated indicating increased mobilization of stored assimilates to other plant parts (i.e., the ear). These experiments showed that manipulation of source‐sink ratios affected leaf CER and utilization of stored assimilates according to the source‐sink ratio in untreated plants of that genotype.