Sample size for measurement of root traits on common bean by image analysis

A avaliação de caracteres radiculares pode ser facilitada se realizada em amostras do sistema radicular. O objetivo deste trabalho foi delimitar o tamanho de amostras para determinação de caracteres radiculares no feijoeiro (Phaseolus vulgaris L.) pela análise digital de imagens. Cultivou-se uma planta por vaso, que foi colhida na emissão de vagens, em 64 e 16 vasos correspondentes a duas e quatro cultivares no primeiro e segundo experimentos, respectivamente. Amostras das raízes foram digitalizadas até completar o sistema radicular, estimando-se a área e comprimento radiculares. A digitalização de uma amostra demandou 21 minutos, e de todo o sistema radicular, 4 horas e 53 minutos. No primeiro experimento, a área e comprimento radicular estimados com duas amostras apresentaram, respectivamente, correlação de 0,977 e 0,860 com estes caracteres mensurados na raiz inteira; no segundo experimento, a correlação foi 0,889 e 0,915. O aumento da correlação com mais de duas amostras foi desprezível. As duas amostras corresponderam a 13,4% e 16,9% da massa radicular total (excluindo nódulos e pivotante) no primeiro e segundo experimentos. A raiz pivotante constitui elevada proporção da massa radicular e deve ser descontada na estimativa de caracteres radiculares por amostragem. Amostras com cerca de 15% da massa radicular total fornecem estimativas confiáveis de caracteres radiculares.Evaluation of root traits may be facilitated if they are assessed on samples of the root system. The objective of this work was to determine the sample size of the root system in order to estimate root traits of common bean (Phaseolus vulgaris L.) cultivars by digital image analysis. One plant was grown per pot and harvested at pod setting, with 64 and 16 pots corresponding to two and four cultivars in the first and second experiments, respectively. Root samples were scanned up to the completeness of the root system and the root area and length were estimated. Scanning a root sample demanded 21 minutes, and scanning the entire root system demanded 4 hours and 53 minutes. In the first experiment, root area and length estimated with two samples showed, respectively, a correlation of 0.977 and 0.860, with these traits measured in the entire root. In the second experiment, the correlation was 0.889 and 0.915. The increase in the correlation with more than two samples was negligible. The two samples corresponded to 13.4% and 16.9% of total root mass (excluding taproot and nodules) in the first and second experiments. Taproot stands for a high proportion of root mass and must be deducted on root trait estimations. Samples with nearly 15% of total root mass produce reliable root trait estimates

Leaf senescence of common bean plants as affected by soil phosphorus supply

Responses of leaf senescence to P supply could constitute adaptive mechanisms for plant growth under P-limiting conditions. The aim of this study was to evaluate the effects of soil P supply on leaf senescence of common bean (Phaseolus vulgaris L.). Eight P levels, ranging from 5 to 640 mg kg-1 P, were applied to pots containing four bean plants of cultivar Carioca in 10 kg of an Oxic Haplustult soil. Attached leaves were counted weekly, abscised leaves were collected every other day, and seeds were harvested at maturity. The number of live leaves increased until 48 days after emergence (DAE) and decreased afterwards, irrespective of applied P levels. At lower applied P levels, the initial increase and the final decrease of leaf number was weak, whereas at higher applied P levels the leaf number increased intensively at the beginning of the growth cycle and decreased strongly after 48 DAE. Dry matter and P accumulated in senesced leaves increased as soil P levels increased until 61 DAE, but differences between P treatments narrowed thereafter. The greatest amounts of dry mass and P deposited by senesced leaves were observed at 48-54 DAE for high P levels, at 62-68 DAE for intermediate P levels and at 69-76 DAE for low P levels. These results indicate that soil P supply did not affect the stage of maximal leaf number and the beginning of leaf senescence of common bean plants, but the stage of greatest deposition of senesced leaves occurred earlier in the growth cycle as the soil P supply was raised