The impact of drought stress on photosynthetic quantum yield in Haloxylon aphyllum and Haloxylon persicum

The impact of periodic drought stress on photosynthesis changes was examined in Haloxylon aphyllum and Haloxylon persicum. Parameters F0, Fv/Fm, the chlorophyll content and chlorophyll a/b ratio were measured. Drought stress factor was considered as lack of irrigation of potted-seedlings of two Haloxylon species in periods 0, 2, 4, 6... 26 and 28 days, and each drought period were repeated six times. Species factor included H. aphyllum and H. persicum. Drought stress significantly (p=0.01) reduced chlorophyll content and maximum quantum yield of photosynthesis (Fv/Fm ratio) but increased the Chl a/b ratio. Both the species are susceptible to drought stress.


INTRODUCTION
Drought stress is one of the most important abiotic stresses in the arid and semi-arid regions that cause a wide variety of physiological and biochemical changes that inhibit plant growth and development from germination to productivity and disturb photosynthesis (Mekhaldi et al., 2008;Misra et al., 2002).Photosynthesis is the most important process in biological system defining the limits of biomass production (Sofi et al., 2006), and is the main physiological parameter in drought stress process that is affected by drought (Arabzadeh, 2009;Satisha et al., 2007;Misra et al., 2002).Drought stress (Arabzadeh, 2009) and light are important factors in photosynthesis function of plants (Iqbal, 2003b).
There is a considerable difference between C 3 and C 4 plants in terms of drought resistance.C 4 plants have high photosynthetic capacity so that photosynthesis of the plants are dominant compared with their respiration in poor light or in low pressure of CO 2 .The plants are also able to consume enough water for photosynthesis in dry habitats (Cornic and Miginiac, 1983).
Drought stress with creation of structural changes in tilacoide membranes (Soares et al., 2008) has an indirect effect on photosynthetic electron transport chain (Kana et al., 2008).Other mechanisms that are involved on the photosynthetic processes reduction in conditions of water stress are ionic disorders of stroma (Kana et al., 2008).
The studies show that a significant strong relationship was between relative water content (RWC) and photosynthesis rate, so that RWC reduction from 95 to 90% reduced the rate of photosynthesis up to 50% (Kramer, 1983).Chlorophyll is the basic catalyst of photosynthesis that as green pigments exists in all plant tissues that do photosynthesis (Masinovsky et al., 1992).Composition of chlorophyll is relatively unstable that at the time of extraction should be prevented from its decomposition (Masinovsky et al., 1992).
It is well established that PS II play a key role in photosynthetic response to drought and unfavorable environmental conditions (Misra et al., 2011(Misra et al., , 2012)).Photochemical efficiency of PS II (Fv/Fm) was reduced in stipa (Balaguer et al., 2002), in pigweed (Osmond and Grace,  Mattos et al., 1999), in banana (Thomas and Turner, 2001) and many plants under drought stress (Misra et al., 2002(Misra et al., , 2012)).The maximum quantum yield of photosynthesis (Fv/Fm ratio) ratio is taken as a parameter for the study of chloroplast development under normal and stress conditions (Misra et al., 2001a(Misra et al., , b, 2006(Misra et al., , 2012)).
In the present report, the changes in the maximum quantum yield of photosynthesis were studied (Fv/Fm ratio) in two species of Haloxylon to assess their susceptibility to water stress.

Induction of drought stress to seedlings of two Haloxylon species
After taking a month to comply with greenhouse conditions, annual seedlings from seeds of two Haloxylon species were treated with periodic drought stress at 0, 2, 4, 6...26 and 28 days of nonirrigation.It means that no water was given in these days.After 28 days without irrigation, the remaining plants were watered to saturation.This experiment was repeated 11 cycles (periodic stresses).In experiment, five plants in each treatment were stratified.

Experimental design type
The experiment was in a randomized complete design, and data was analyzed using two-way analysis of variance; and comparison of averages was made by using Duncan's test.

Measurement of chlorophyll fluorescence
Chlorophyll fluorescence measurements were performed using Fluorimeter (PAM: H. Walz GmbH, Effeltrich, Germany); and then Fv/Fm parameter was calculated as follows (Van Kooten and Snel, 1990):

Effect of drought stress on chlorophyll content
The total chlorophyll content of both Haloxylon species was affected by water deficit (drought stress).So that it declined from 2.041 mg/g (ww) in control to 0.96 mg/g (ww) in 28 days of non-irrigation treatment in H. aphyllum.The total chlorophyll content in H. persicum was reduced from 1.96 mg/g (ww) in control to 0.92 mg/g (ww) in 28 non-irrigation treatment too (Figure 1).Reduction speed of chlorophyll rate in both Haloxylon species was the function of stress intensity.So that reduction velocity of total chlorophyll rate increased with increasing stress.Chlorophyll a/b ratio in both Haloxylon species increased proportional to the degree of water deficit (stress intensity) to 26 days of non-irrigation treatment.The ratio increased from 1.59 to 1.84 in H. aphyllum and from 1.68 to 1.98 in H. persicum (Figure 2).The changes trend was reversed from the 28 days of non-irrigation treatment so that the ratio decreased from 1.84 to 1.76 in H. aphyllum, and from 1.98 to 1.95 in H. persicum.

Effect of drought stress on the quantum efficiency of PS II (Fv/Fm)
Effects of drought stress on Fv/Fm had same trend in both Haloxylon species (Figure 3).So those with increasing stress levels were reduced.The rate decreased from 0.734 at zero stress to 0.681 at 28 days of non-irrigation stress in Haloxylon aphyllum, and from 0.715 to 0.685 in Haloxylon persicum.Reducing effect of drought stress on the Fv/Fm was intensified from 8 days of non-irrigation treatment in both Haloxylon species.

DISCUSSION
Drought stress in plants occurs when the plant's water intake is less than its loss.This may be due to excessive water loss or loss absorption or both (Erwin et al., 2007).Water potential and osmotic potential with the loss of turgidity, stomata closure and reduction of growth are specific symptoms of water stress.The high intensity of water stress severely reduces photosynthesis, disrupts physiological processes, stops the plant growth, and eventually the plant dies (Singh and Patel, 1996;Misra et al., 2002).
This study shows that the chlorophyll content of assimilating shoots of both species of H. aphyllum and H. persicum decreased under water stress conditions (Figures 1 and 2).The phenomena of chlorophyll loss in drought conditions have been reported in many plants (Balaguer et al., 2002;Steinberg et al., 1990).Reduction of leaf chlorophyll content which is under drought stress reduces photosynthetic efficiency in the plants.The plants that are able to maintain their chlorophylls can also have higher photosynthesis (Arabzadeh, 2009).
According to survey results (this study) and studies of other researchers (Balaguer et al., 2002;Steinberg et al., 1990), it can be admitted that increase drought stress is a major factor in the breakdown of chlorophyll, and is an essential obstacle in the process of making new chlorophylls.In this study, chlorophyll content under drought stress decreased in all treatments in both Haloxylon species (Table 1), but the ratio of chlorophyll a/b increased.
Chlorophyll fluorescence is considered as a strong indicator of photosynthesis and as a decisive factor in stress physiology (Misra et al., 2001(Misra et al., , 2006(Misra et al., , 2012)).
Photochemical reaction of PS II to drought was different in drought stress conditions and in the various species.The parameters of photosynthesis in both Haloxylon species, the control treatment (no stress) and 28 days of  non-irrigation treatment (the most severe stress treatment) are shown in Table 1.This table shows that the difference between these two treatments was significant at the 99% level in all studied parameters in both Haloxylon species.

Figure 1 -
Figure 1-The impact of drought stress on changes mean of total chlorophyll (mg/g ww) in branchlets of Haloxylon aphyllum and Haloxylon persicum

Figure 2 -
Figure 2-The impact of drought stress on changes mean of a/b ratio in branchlets of Haloxylon aphyllum and Haloxylon persicum.

Figure 3 .
Figure 3.The impact of drought stress on the maximum quantum efficiency (Fv/Fm) in branchlets of Haloxylon aphyllum and Haloxylon persicum.

Table 1 .
Comparison of photosynthetic parameters in both control and 28 days of non-irrigation in two species of Haloxylon aphyllum and Haloxylon persicum.