Response of Tissues and Organs

At the individual, macroscopic level, increased leaf growth, production, shoot height, blade length and width, and biomass are the most commonly reported responses to increased nutrients (Orth, 1977; Bulthuis and Woelkerling, 1981; Perez et al., 1991; Tomasko and Lapointe, 1991; Murray et al., 1992; Agawin et al., 1996; Alcoverro et al., 1997; Udy and Dennison, 1997a; Terrados et al., 1999; Lee and Dunton, 2000). Another individual response is the change in the pattern of within-plant resource allocation. In effect, shoot: root ratio increases with fertility, both following nutrient gradients (Perez et al., 1994; Lee and Dunton, 2000) and in nutrient additions experiments (Powell et al., 1989; Short et al., 1990; Perez et al., 1991). This behavior implies that seagrasses allocate more biomass in leaf tissues under high-nutrient availability, but more below-ground biomass under low-nutrient conditions. Plants under sediment nutrient deficient conditions increase biomass allocation to below-ground tissues to expand surface area for nutrient uptake (Gleeson, 1993; Vogt et al., 1993), while they increase carbon allocation to the above-ground tissues as a result of nitrogen addition into sediment (Lee and Dunton, 1999a). Changes in seagrass biomass allocation thus reflect a kind of strategy: when nutrients are abundant, leaves seem to be the 'preferred' site for uptake; in contrast, when nutrients are scarce, root uptake is maximized.

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