The influence of air humidity on leaf-air gas exchange and leaf water potential (4^) was investigated during daily courses in control and water stressed potted young plants of Copaifera langsdorffii. When leaf-air water vapour concentration difference (AW) increased during the day, stomatal conductance (g^) and net photosynthetic rate (P]vj) decreased under both soil moistures. Moderate AW induced lower values of g^ and Pn unwatered than control plants, High AW in atmosphere produced strong depression in g^ (from 0.22 to 0,01 mol m'^ s’’) and (from 6.5 to 0.7 pmol m'2 s'*) in control plants around midday, with recuperation of T'. Expected conductance tese) was calculated as fimction of AW, which was useful for discriminating soil to atmosphere water stress. In špite of momentary T or soil water stress, P^, g^, and water use efficiency decreased when AW increased during day course.
Four grapevine cultivars, i.e. Cabernet Sauvignon (a member of the Western Europe cultivar group), Rizamat (a member of the East cultivar group), Red Double Taste (a hybridized cultivar from Vitis vinifera L. and V. labrusca L.), and 1103Paulsen (a hybridized rootstock), were treated by three severity orders of drought stress for 25 d. Then net photosynthetic rate (PN), maximal photochemical efficiency (Fv/Fm), actual photochemical efficiency (ΦPS2) of photosystem 2, total electron transport rate (JT), and electron transport flows used in carboxylation (JC) and in oxygenation (JO) reactions catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase were determined. PN was determined again after re-watering for 2 d by gas exchange measurement. Along with the increase in severity of drought stress, PN, Fv/Fm, ΦPS2, JT, and JC in all four cultivars decreased. The range of decrease differed among cultivars. JO expressed various trends from cultivar to cultivar. In Rizamat that received slight and moderate drought stress, PN evidently decreased, but JO markedly increased, thus maintaining high values of JT and ΦPS2. Prior to the moderate drought stress, the Fv/Fm was high in Rizamat, indicating that the photodamage had not happened ahead of the moderate drought stress given. Under the severe drought stress, the photorespiration rate in Rizamat decreased by 70 %, and JT, ΦPS2, and Fv/Fm also dropped to very low values, i.e. the photodamage of photosynthetic apparatus has taken place. This suggested that the photorespiration has consumed the excessive assimilatory power and the photo-protective function of photorespiration is very important for Rizamat. When Cabernet Sauvignon grew under drought stress, its JO decreased in a small range, thus maintaining higher values of JC, JT, ΦPS2, and Fv/Fm; hence no serious photodamage occurred. Despite of the fact that PN of cv. Red Double Taste decreased markedly under the slight drought stress, JO still increased under the severe drought stress. This suggests that photorespiration is important in photoprotection under drought stress. JO in cv. 1103Paulsen markedly decreased under slight stress. Accordingly, PN, Fv/Fm, ΦPS2, JT, and JC decreased to extremely low values. Thus photorespiration effectively protects the photosynthetic apparatus from photo-damage under drought, assists in maintaining a relatively high ΦPS2, and helps PN to be rapidly recovered after re-watering. and X. Q. Guan ... [et al.].