01608naa a2200181 a 450000100080000000500110000800800410001910000200006024500840008026000060016452010970017065000180126765000240128565300080130965300170131770000240133477300680135816463891994-01-03               bl ---       0--  u    #d1 aSINCLAIR, T. R.  aCarbon dioxide ant water vapour exchange of leaves on field-grown citrus trees.  c0  aCarbon dioxide and water vapour exchange rates were measured on attached leaves of field-grown citrus trees. The exchange rates were measured continuously during several weeks in the spring of two suddessive years. These data confimed the reather low of maximum CO2 excange (6-ll umol m-2s-1) by citrus leaves. However, the maximum rate was maintained through the midday period on only about half the days.On the other days, characterized by high temperatures and high atmospheric water vapour pressure deficits, pronounced midday depressions in CO2 exchange rates were observed. Since midday transpiration remained stable at a constant rate even with increasing vapour pressure deficit, these results indicate that stomatal closure was occurring. In fact, the data suggest that specific, maximum transpiration rates were asscociated with differing rootstocks. Thus, the rate of water supply to the leaves may be an important factor in determining the maximum transpiration rate, and thereby mediating control os stomatal conductance and the resultant midday depression in CO2 exchange rates.  aFotossíntese  aPoncirus Trifoliata  aDPV  aTranspiracao1 aALLEN JUNIOR, L. H.  tJournal of experimental botanygv.33, n. 137, p.1166-1175 l982.