Global warming will likely exacerbate the negative effects of limited water availability in the Mediterranean area. The Italian Aleppo pine (Pinus halepensis Mill.) provenances are distributed along the coasts except Otricoli provenance growing in an unusual location between 300 and 1,000 m a. s. l., in Umbria (central Italy). The aim of the present study was to investigate the photosynthetic response to a 28-day-long drought and to a subsequent reestablishment of water availability in Otricoli and North Euboea (Greece) provenances, representing different locations along a rainfall gradient in the natural range of this species. Six-month-old seedlings were used in this experiment since at this age Aleppo pine plants in Mediterranean climate face their first water stress potentially affecting plant survival. Water potential (ψw), net photosynthesis and stomatal conductance decreased during drought in both provenances and showed minimal values 28 days after beginning the treatment (DAT). Otricoli seedlings adjusted ψw gradually as the stress level increased and 21 DAT showed a lower ψw than North Euboea. In contrast, in North Euboea seedlings ψw that was not affected until 21 DAT rapidly dropped to a minimum of -3.81 MPa 28 DAT. At the onset of the stress the intercellular CO2 concentration (Ci) was reduced, and the "instrinsic" water-use efficiency (WUEi) was enhanced in both provenances, as stomatal conductance decreased more rapidly than photosynthesis. However, 28 DAT, Ci increased and WUEi decreased as stomatal conductance and photosynthesis declined to minimum levels, revealing nonstomatal limitations of photosynthesis. A rapid decrease in PSII maximal photochemical efficiency estimated by chlorophyll (Chl) fluorescence (Fv/Fm) was also observed when the stress became severe. At the final stage of water stress, North Euboea seedlings maintained significantly higher values of Fv/Fm than Otricoli seedlings. Upon rewatering, photosynthesis did not fully recover in Otricoli seedlings (41 DAT), while all other parameters recovered to control levels in both provenances. No drought-induced physiological differences were consistent with the regional climatic features of these two provenances. Our results suggest that phenotypic plasticity in drought response may help Otricoli provenance cope with global warming, but that recurrent drought episode may slow down the primary productivity of this provenance. and M. Michelozzi ... [et al.].
We analyzed the effect of NaCl stress on photorespiration of spinach leaves by calculating the rate of carboxylation/oxygenation of ribulose-1,5-bisphosphate carboxylase/oxygenase, and by measuring the content of amino acids produced through photorespiration. After 20 d of NaCl stress the carboxylation rate was reduced while the oxygenation rate was not affected. The contents of serine, glycine, and alanine increased relevantly. The amount of glutamine also increased after 20 d but the amount of glutamate did not. Hence photorespiration may be stimulated under moderate NaCl stress. A relevant electron transport rate was observed under CO2-free air, which may indicate refixation of photorespiratory CO2. When NaCl accumulation proceeded for more than 20 d, photosynthesis was reduced and the content of photo-respiratory amino acids started to decrease, but the oxygenation rate did not change. and C. di Martino ... [et al.].
Simultaneous measurements of net photosynthetic rate {P^ and fluorescence were taken on flag leaves of fíeld-grown wheat {Triíicum aestivum L., Thlicum durum L.) from anthesis to senescence. By using leaf discs inaintained in saturating CO2 in tlie O2 electrode we found that the electron transport measured by fluorescence, and that calculated from O2 evolution rate were similar througliout the experimental period, which indicated that fluorescence might be ušed to measure the linear electron transport rate. In field measurements on intact attached leaves, the electron transport rate declined less than during leaf senescence, Measurements taken in the aftemoon indicated that the electron transport remained constant during the day while slightly decreased. Thus, in fíeld-grown wheat leaves photoinhibition was not a relevant phenomenon. When the alternativě electron routes were negligible, the increasing discrepancy between Pn and the electron transport during leaf senescence could be explained by an increment in photorespiration rate (Pp), The change of oxygenation to carboxylation ratio (Vq/Vj,) might be caused by increasing resistances to CO2 diffusion in the leaf CO2 lost through photorespiration was about half of that fixed with Pn in the sun-exposed leaves. Yet Pp was lower in the basal part of leaves which mostly grew in shade. Fluorescence coupled with gas exchange proved to be a useful method for evaluation of the photorespiratory losses in field conditions.