Shoot Resistance to Water Flow in Cotton

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Abstract
Studies using excised cotton (Gossypium hirsutum L.) plants, attached to a free water source and undergoing transpiration cycles, were conducted at intervals over a 2 year period in order to quantify shoot resistance components of cotton canopies. Leaf water potential was found to be a linear function of transpiration rate at rates above 0.1 mm h−1, so shoot resistance was evaluated as the slope of this function. The value of 4.8 104 h (0.48 MPa h mm−1) total shoot resistance was consistent for 1.10 m tall, well irrigated, fruit-bearing cotton plants. Further tests, with pre-wrapped and exposed leaves, revealed that total shoot resistance was comprised of an axial component (40%) and a leaf component (60%). The total shoot resistance of 0.48 MPa h mm−1 is likely to be relevant for modelling cotton water relations when LWP is evaluated on exposed, top of the canopy leaves, such as in the ‘big leaf’ type models.