Influence of Partial Defoliation on Photosynthesis, Photorespiration and Transpiration by Lucerne Leaves of Different Ages

Abstract

A study was made of the short- and long-term effects of partial defoliation (cutting at 15 cm above the crown) of lucerne plants (Medicago sativa L. cv. Hunter River) on the net photosynthesis, transpiration, photorespiration and CO₂ transfer resistances of remaining leaves. The response in gas-exchange properties of leaves of different ages to partial defoliation of the plant was also investigated.

Partial defoliation always induced rejuvenation in photosynthetic rate of remaining leaves. Young and middle-aged leaves rejuvenated to rates comparable to those of recently expanded leaves but old leaves only partially rejuvenated. Time after defoliation to attain peak rates increased as leaves aged; values were 5, 9 and 12 days for plants partially defoliated on days 16, 30 and 65 of regrowth respectively. Peak rates were maintained for only 3 or 4 days before declining. Rates of photorespiration and photosynthesis were closely coupled. Transpiration rates varied over time in a similar but more erratic pattern to net photosynthetic rates.

Changes in net photosynthetic rates associated with senescence, defoliation treatments and irradiance levels were largely attributable to changes in intracellular resistance to CO₂ transfer. Intracellular resistances ranged from 2.6 to 30 s cm^-1, constituting 67-95 % of the total resistance to photosynthesis. Stomatal resistance to CO₂ diffusion remained low, 0.2 - 1.0 s cm^-1, for all but very old leaves.

Partial defoliation followed by continual removal of new crown and stubble shoots induced very high net photosynthetic rates, c. 15 days later. Highest net photosynthetic rate was 238 ng CO₂ cm^-2 s^-1.

Possible mechanisms responsible for photosynthetic rejuvenation following partial defoliation are discussed, together with ecological implications of this phenomenon.