Ecologia Mediterranea

CHIRAZ CHAFFEI HAOUARI, AFEF HAJJAJI NASRAOUI, HOUDA GOUIA AND ALI HAOUARI
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germinate and the embryonic tissues development
towards a seedling.
One of the most important abiotic factors limiting
plant germination and early seedling
stages is water stress brought about by
drought and salinity (Mantovani & Iglesias
2010). However, the presence of salt in the
medium culture cause inhibitory effects on the
seed’s water uptake and retard and/or suppress
germination (Alatar 2011). Salinity and
drought affect the plants in a similar way
reducing water potential (Legocka & Kluk
2005). Water stress acts by decreasing the percentage
and rate of germination and seedling
growth. The detrimental effects of NaCl on
seeds germination are a consequence of both
a water deficit resulting in osmotic stress and
the effects of excess sodium ions on key biochemical
processes. Seed germination and
early seedling growth are critical stages for
the establishment of plant populations under
saline conditions (Srivastava et al. 2000).
Plants differs in their upper limit of salinity
tolerance as the increase in salinity concentration
usually delays and reduces seed germination
(Srivastava et al. 2000). The degree
to which salinity affects germination by an
osmotic effect or specific ion toxicity and
whether salt tolerance varies in different
species is still a subject of studies (Pilar et al.
2011).
The temperature variations have a major
impact on a number of processes which regulate
seed germination, including membrane
permeability and the activity of membranebound
as well as cytosolic enzymes (Ajmal
Khan & Gulzar 2003) and its interaction with
the variable soil water content in the surface
layers of the soil. By consequence salinitytemperature
interaction, determines seed
germination pattern in many salt affected
environments. Despite the importance of Pennisetum
dichotomum, there is no available
information on its autecology, and most
drought resistant grasses of the Tunisia desert.
The aim of this study is to investigate the
effects of osmotic stress generated by NaCl
and depth with temperature on germination
characteristics (germination period and germination
percentage) and seedling growth of
Pennisetum dichotomum.
Materials and methods
Pennisetum dichotomum is a graminous
(Mian 2003) grasses which inflorescence is a
dense, spicate, terminal panicle. This is a useful
grazing plant, much browsed by camels
(Mandaville 1990).
Seeds of P. dichotomum were collected during
April, 2009, from naturally growing
stands in the sandy soil of Mednine- South of
Tunisia. Seeds were separated from inflorescences
and stored dry at 4oC. During September
2009, stored seeds were sterilized with
0.5% sodium hypochlorite solution for 1 min
and washed twice with distilled water.
Effect of temperature on germination
Seeds were germinated on double-layered
with Whatman No. 1 filter papers placed in
10cm diameter plastic dishes. Seeds were
placed (30 seeds per Petri dishes) in a growth
chamber (26 o C/70% relative humidity during
the day, 20 o C/90% relative humidity during
the night). The photoperiod was 16 h daily
with a light irradiance of 150 µmol m -2 . s -1 at
the canopy level. All of the Petri dishes were
kept in dark incubators maintained at room
temperature at four alternating night and day
temperature regime (10, 25 and 35 o C) representing
the seasonal common temperatures
prevailing in some selected meteorological
stations in south Tunisia (Mednine: 33 o 20’
56.27” N, 10 o 29’ 41.26” E).
Effect of depth
In the nature, every sort is characterized by an
optimal depth of sowing. This optimal depth
depends on intrinsic characteristics of the
semen. The basic rule is to sow a seed in
a depth equal to 2-5 times the diameter
(Seabloom 2007). In the present experiment
six values of depth were tested: 1, 2, 3, 4, 5
and 6 cm.
Effect of water potential
and NaCl doses
Seeds were submitted to germination using
osmotic potentials (0, – 0.2, – 0.4, – 0.6 and
- 0.8 MPa) of sodium chloride solutions
according to Murillo-Amador et al. (2002).
Twenty-five seeds were germinated on double-layered
with Whatman No. 1 filter papers
ecologia mediterranea – Vol. 38 (2) – 2012