Net photosynthesis, dark respiration and the response to photon flux density were measured on cotton leaves grown in a glasshouse. Leaves at four positions on the plant were examined from their unfolding until 70 days later. Photosynthesis and transpiration per unit of leaf area were unaffected by leaf position and, in all leaves, peak photosynthesis of about 110 ng CO, cm-2 s-' was attained 13-15 days after leaf unfolding, when the leaf was 75-90% of maximum area. Photosynthesis was maintained at this rate for only 12 days before declining linearly to values 20% of the maximum when leaves were 70 days old. Transpiration followed a similar pattern reaching a maximum of about 13 pg HZO cm-2 s-' at 2 kPa vapour pressure deficit (VPD) at 13 days. Stomata1 and internal conductances changed in parallel as leaves aged, with the consequence that internal COP concentration and water use efficiency remainedessentially constant at 220pll- ' and 16.8 ng COz (pg HzO kPa VPD-')-I respectively. Youngest and oldest leaves saturated at lowest light levels (400-800 pE m-2 s-') while 16-18- day-old leaves had light saturation at 1100 pE m-'s-'. The initial slope of the light response curves increased as leaves expanded up to 10 days age then remained constant at 0.25 ng CO, cm-2 (pE m-')- '. Dark respiration reached a maximum of 1 .5 ng C02 mg- ' s- ' 5 days after leaf unfolding, when leaf dry weight was increasing most rapidly. The relationship between the consistent pattern of gas exchange with age and the pattern of morphological development is discussed, along with internal factors associated with age-dependent photosynthesis.
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