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Sunflower ( Helianthus annuus  L.) hybrid NK-212 was planted at National Agricultural Research Center, Islamabad, during 1992 and 1993 to assess the impact of various levels of simulated leaf damage, commonly caused by insects, hail or foliar diseases, on source and sink relationship. Insect attack, hail and foliar diseases often partially defoliate sunflowers and may become important factors limiting the production. Reduction in leaf area may also occur in the field as a consequence of pathogens or atmospheric conditions. Artificial defoliation was chosen as an experimental means to assess its effect on seed yield and plant behavior. It had also made possible to evaluate the effect of such a reduction at different phenological stages and to identify the leaves that contribute greatly to the seed yield. The leaves were clipped in various proportions from different parts of the plant,  i.e ., lower, middle and upper. The removal of upper 2/3 and 1/2 leaves caused a yield reduction of 29 and 55.8%, in 1992 and 37 and 44.8%, in 1993, respectively. In 1992, about 1% yield decline was recorded when the lower 1/3 leaves were removed and 6% yield decline with the removal of the lower 1/2 leaves. In 1993, the yield reduction was 26.7 and 39.2% due to removal of lower 1/3 and 1/2 leaves, respectively. This indicated that the upper leaves (source) contribute more towards seed yield (sink) than the lower ones. Correspondingly, the results indicated that insects and pests feeding on the upper portion of the sunflower plant can cause larger reduction in seed yield than those feeding on the lower leaves.

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SIMULATION OF LEAF DAMAGE BY ARTIFICIAL DEFOLIATION AND ITS EFFECT ON SUNFLOWER (Helianthus annuus L.) PERFORMANCE / SIMULACION DEL DAÑO DE HOJA POR LA ELIMINACION ARTIFICIAL DE HOJAS Y SU EFECTO SOBRE LAS PERFORMANCES DEL GIRASOL / SIMULATION DE DÉTÉRIORIATION DES FEUILLES CAUSÉE PAR DEFOLIATION ARTIFICIELLE ET SON EFFET SUR LE TOURNESOL (Helianthus annuus L.)