Water Stress in Sunflower (Helianthus annuus L.): I. Effect on Plant Development

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Abstract
Development of sunflower plants grown in pots under controlled conditions was monitored at frequent intervals. Two separate water stresses, which reached a minimum of –1.7 MPa, were imposed at specified developmental stages on two separate batches of plants. Leaf initiation and leaf unfolding were significantly reduced by water stress. Inflorescence primordium initiation, involucre bract formation, disc and disc floret development were not affected. The mitotic index was reduced by water stress in unfolding leaves from 5 to 0.5 per cent. In other leaves, the mitotic index was reduced to a lesser extent. This reduced mitotic index helps explain reduction in leaf development. The time between seed germination and inflorescence primordium initiation remained relatively constant despite the reduced leaf number associated with water stress. The discussion evaluates the complications that this may cause in the comparison of leaves in the upper half of the leaf profile.