Photosynthetic responses of loblolly pine (Pinus taeda) needles to experimental reduction in sink demand

Sink strength in loblolly pine (Pinus taeda L.) was experimentally manipula ted on two sun-exposed branches on each of two neighboring trees by excisin g the emerging terminal cohort (second flush of 1996) during a period of ra pid needle expansion. In addition, export of photosynthate was restricted o n one of these branches from each tree by removal of bark and phloem just b elow the second flush of 1995. Treatment-induced changes in needle biochemi stry were measured in 3-month-old (first flush of 1996) and 1-year-old (fin al flush of 1995) needles collected 1, 5 and 8 days after treatment. In 3-m onth-old needles, sugar concentration increased by 24% one day after leader excision, and increased by 86% on Day 8 after leader excision and girdling . Starch concentration increased by 33% in S-month-old needles on Day 1 aft er leader excision, and by 400% in 1-year-old needles on Day 8 after leader excision and girdling. Physiological changes in 3-month-old and 1-year-old needles were measured by open-flow gas exchange and chlorophyll fluorescen ce on Day 8 after leader excision and girdling. Light- and CO2-saturated ne t photosynthesis decreased following treatment in both 3-month-old and 1-ye ar-old needles (23 and 17%, respectively). Maximum rate of carboxylation (V -cmax) decreased by 25% in 3-month-old needles and by 31% in 1-year-old nee dles in response to leader excision and girdling. The combined treat ment r esulted in a 38% decrease in maximum rate of electron transport (J(max)) in S-month-old needles and a 37% decrease in J(max) in 1-year-old needles. Be fore leader excision and girdling, 2% oxygen in air stimulated photosynthes is by 17 to 19%, but this stimulation was only 3 to 4% at 9 days after trea tment. These physiological responses indicate that experimentally lowered s ink strength resulted in rapid feedback inhibition of leaf-level photosynth etic capacity in loblolly pine.