Dynamics of rooting and root-length: leaf-area relationships as affected by plant population in sunflower crops
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AbstractCrops of sunflower (Helianthus annuus L.) were grown in a coarse loam Typic Hapludoll to characterize the spatial and temporal development of the root systems, the relationships between root systems and leaf area, and their responses to plant population. Two experiments were carried out, one involving single plant population, the other (using a different cultivar) covering the range of population from 2.04 to 5.10 plants m−2. The trench profile method was used to estimate root density in the row and in the inter-row space. These data were used to estimate root depth and root length on a ground-area basis. The former was defined as the depth which contained 90% of the total root system below the 0-0.2-m layer (D90).Rooting depth (D90) and total root length increased up to anthesis and root length decreased thereafter. Plant population had negligible effects on (D90). The inter-row space was colonized by roots at a slower rate than the within-row space, and after anthesis loss of roots from the former space took place more rapidly. A single allometric function fitted the seasonal changes in the relationship between D90 and leaf area per plant for all crops.The dynamics of roots in the plough and below-plough layers showed different responses to population. Low-population crops had the highest and lowest peak values of root length in the plough and below-plough layers, respectively. In both layers, roots of low-population crops continued to grow until later in the season. Root length and leaf area per plant decreased markedly with population but on a ground-area basis these effects were small. These morphological responses to population are analyzed in relation to their possible effects on crop water use.

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