Effects of Drought on Primary Photosynthetic Processes of Cotton Leaves

Abstract

The effects of drought on Photosystem II (PSII) fluorescence and photosynthetic electron transport activities were analyzed in cotton. Water stress did not modify the amplitude of leaf variable fluorescence at room temperature in the presence of 343,4-dichlorophenyl)-1,1-dimethylurea (DCMU) nor at 77 K. It is therefore concluded that photon collection, their distribution between the two photosystems, and PSII photochemistry are unaffected by the stress. In droughted leaves at room temperature under low exciting light, the transitory maximum (F,) PFD, 300 ,mol quanta m-2s-' in case of cotton, 100 in case of Nerium oleander provided by a bank of metal halide lamps; in some experiments (Table IV) a higher PFD, 1200 ,umol quanta m-2s , was used for both species. Plants were watered twice daily and fertilized once weekly. They were submitted to water stress by withholding watering 6 weeks after germination in case of cotton and 4 months after propagation by cuttings in case of N. oleander. Drought was achieved in 8 d. Plants were rehydrated by rewatering the pots to field capacity. Measurements were performed on fully expanded leaves (the fourth to fifth leaves from the top).Leaf Water Potential Measurements. Leafwater potential was determined using Wescor C52 psychrometers (Wescor Inc., Logan, UT) by the dew point method or using a pressure chamber (PMS, Corvallis, OR).Fluorescence Measurements. Room temperature fluorescence kinetics at 685 nm were measured from attached and detached leaves using a bifurcated light pipe together with a quartz light mixing rod to excite and collect the fluorescent light from the upper leaf surface. The exciting blue light was produced by a DC stabilized projector tungsten lamp passing through a combination of filters (Schott BG18+BG38+KG3) via a photographic shutter (opening time: 2 ms). The fluorescence was detected through a 685 nm interference filter (Schott PIL 0.1) and a red colored filter (Coming 2-64) using a photomultiplier tube (RTC, XP 1017). The signal was simultaneously displayed on a storage oscilloscope for initial rapid phases of the induction and on a chart recorded for slower phases. Fluorescence measurements were made on attached leaves which have been dark adapted during all the night period. Leaf discs of 1.5 cm diameter, were used for experiments with DCMU. They were infiltrated in the dark with a 20 ,uM DCMU solution.

Diurnal variations of photosynthesis and dew absorption by leaves in two evergreen shrubs growing in Mediterranean field conditions

“…Although the F v : F m values obtained at midday were slightly smaller than those obtained at predawn (0.75 vs 0.80), the maintenance of constant F v : F m values at midday throughout the experiment demonstrates the lack of drought-induced damage to PSII photochemistry, as already reported for many species (Genty et al, 1987 ;Cornic, 1994). This was accompanied by a decrease in chlorophyll, but also by an increase in the ratio Car : Chl, which partly explains the absence of drought-induced damage to PSII (Demmig-Adams & Adams III, 1996).…”

The Synergistic Effect of Drought and Light Stresses in Sorghum and Pearl Millet

“…3,24). In contradiction, Genty et al (12) concluded that PSI-mediated electron transport was inhibited by drought, whereas PSII electron transport remained the same. They also reported that drought does not induce sensitization to photoinhibition.…”

“…Several mechanisms of protection against high irradiance occur at different levels (18 The mechanism by which drought stress inhibits the photosynthetic electron transport system is still unclear, although a number of authors have attempted an explanation (3,12,15,17,26). Ogren and Oquist (24) concluded from their results that the drought-induced decline in CO2 uptake was initially equally attributable to stomatal and nonstomatal factors, but the further decline as drought continued was solely due to nonstomatal factors.…”

“…However, from recent results, the response to drought seems to be a more complex event. Genty et al (12) concluded that drought mainly slowed down the rate of plastoquinone reoxidation and that it inhibited PSI-mediated transport more than PSII photochemistry. Thus, it is still a matter of controversy as to which chloroplast processes are most affected by drought.…”

Growth Characteristics, Physiological and Metabolic Responses of Teak (Tectona Grandis Linn. f.) Clones Differing in Rejuvenation Capacity Subjected to Drought Stress

“…After 5 days of re-watering, gas exchange parameters did not show a complete recovery may be due to limitation of stomatal and mesophyll conductance and biochemical limitations . In many species, PSII is quite resistant to drought (GENTY et al, 1987;CORNIC et al, 1992). Maximum quantum yield of PSII (Fv/Fm) did not change significantly under moderate drought stress in both clones, indicating no photodamage to PSII reaction centers (FOYER et al, 1994).…”