Limitations to net photosynthesis as affected by nitrogen status in jack pine (Pinus banksiana Lamb.) seedlings
1995
Tan, W. | Hogan, G.D.
Relative limitations of nitrogen (N) status on the processes contributing to photosynthetic rate (A) were investigated. Jack pine (Pinus banksiana Lamb.) seedlings from seeds grown in sand culture were supplied with four different N treatments for 6 weeks, which resulted in a needle N content ranging from 50-85 mmol m-2 (14-32 mg g-1 dry weight). Leaf gas exchange at varying CO2 levels was measured and limitations on A350 (A at ambient CO2 level) caused by finite, limiting carboxylation efficiency (c.e.), maximum A (Amax), and stomatal conductance were estimated from an analysis of the response of A to internal CO2 concentration. Although c.e. and Amax decreased linearly with the decline in needle N, the magnitudes of their changes relative to A350 differed. Amax varied with A350 and always exceeded A350 by 37-38%; c.e., however, declined faster than A350 as needle N level decreased. Consequently, relative limitation on A350 caused by inefficient Amax remained constant, but limitations caused by c.e. increased by 10-15% at low N levels. In contrast, the limitation by stomatal conductance declined initially, but remained stable when N content dropped below 75 mmol m-2. The results suggest: (1) a decrease in biochemical capacity, but not stomatal conductance, contributed to the reduction of A350 induced by N-deficiency in jack pine seedlings; and (2) the capacity of carboxylation appeared to be impaired more than that of electron transport and/or photophosphorylation and its reduction may be the major reason for the reduction in A350.
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