Drought effects on photosynthesis and fluorescence in hard wheat cultivars grown in the field.

Net photosynthesis of the flag leaf of hard wheat (Triticum durum L. cvs Vatforte, Produra, Adamello, Karel, Appulo and El Amel from the collection of the Instituto di Cerealicultura, Foggia, Italy) of different water potential has been studied on three consecutive years. Net photosynthesis was measured in natural conditions with a LI-COR portable instrument and in saturating CO2 with an oxygen electrode. Net photosynthesis and stomatal conductance were significantly lower in the unirrigated leaves. However, the ratio of intercellular CO2 concentration (C) to ambient CO2, concentration (Ci) around the stressed plants was similar to the irrigated control. The maximal rate of photosynthesis in saturating CO2 (Pnmax), measured in the second year of the experiment, was quite close to photosynthesis under natural conditions, indicating that CO2 supply was not limiting. These results suggest that altered mesophyll photosynthetic capacity, rather than stomatal closure, causes the observed reduction in photosynthesis in the unirrigated plants. The variable fluorescence yield (Fv/Fm) in predarkened leaves measured for two consecutive years, did not show differences between treatments or between cultivars. However, the analysis of the slow transients, measured the last year of the experiment, showed a linear relation between the fluorescence decline from the maximum initial level (P) and maximum photosynthesis (Pnmax).