V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
237
TABLE IV
CHANGES IN THE D I U R N A L VARIATIONS OF ψ8 IN Pirus
elaeagrifolia
(CENTRAL ANATOLIA, T U R K E Y ) DURING SUMMER D R O U G H T "
Month
SepParameter
April
M a y
June
July
August tember
ψ 8 in a t m (measured in
- 1 7 . 6 - 1 9 . 9 - 2 4 . 2 - 2 9 . 9 - 3 4 . 0 - 3 4 . 9
the morning)
Diurnal variations (%)
8 . 4
10.0
8 . 1
2 . 7
0 . 1
0 . 0
° After Birand (1938, p . 139).
face area, and stomatal control. These differences are particularly striking
in desert plants where the water content of the soil also varies over short
distances.
When water supply to plants is initially high, e.g., before the beginning
of a long drought period, and then decreases gradually during drought, the
diurnal variations of φ 8 at first are small but increase gradually. Later in
the drought period, however, the diurnal variations decrease in amplitude
and finally disappear because the stomata close earlier and earlier in the
morning, until they finally may not open at all. The morning ψ 8 values
decrease continuously during the drought period since the leaves no longer
become water saturated during the night (Table IV).
IX. Annual Variations of the Osmotic Potential (φ 8 )
The annual variations of ψ 8 are more pronounced than diurnal fluctuations especially in areas where rain and drought periods alternate during
the year as in Arizona (see Fig. 12). The annual fluctuations are smaller
on habitats with a good water supply. Succulents with their large water
storage tissue have a very constant osmotic potential ψ 8 .
Annual variations of ψ 8 were often investigated in species of the Mediterranean region which have a winter rainy period and a summer drought
(see Fig. 13). Deep-rooting, sclerophyllous plants show very little annual
fluctuations (e.g., Quercus ilex, Q. coccifera, and Pistacia lentiscus). On
the other hand, shallow rooting species with soft leaves (malacophyllous
plants) show pronounced fluctuations (e.g., Viburnum tinus, Cistus albida,
and Thymus vulgaris). Great variations are also observed in herbaceous
237
TABLE IV
CHANGES IN THE D I U R N A L VARIATIONS OF ψ8 IN Pirus
elaeagrifolia
(CENTRAL ANATOLIA, T U R K E Y ) DURING SUMMER D R O U G H T "
Month
SepParameter
April
M a y
June
July
August tember
ψ 8 in a t m (measured in
- 1 7 . 6 - 1 9 . 9 - 2 4 . 2 - 2 9 . 9 - 3 4 . 0 - 3 4 . 9
the morning)
Diurnal variations (%)
8 . 4
10.0
8 . 1
2 . 7
0 . 1
0 . 0
° After Birand (1938, p . 139).
face area, and stomatal control. These differences are particularly striking
in desert plants where the water content of the soil also varies over short
distances.
When water supply to plants is initially high, e.g., before the beginning
of a long drought period, and then decreases gradually during drought, the
diurnal variations of φ 8 at first are small but increase gradually. Later in
the drought period, however, the diurnal variations decrease in amplitude
and finally disappear because the stomata close earlier and earlier in the
morning, until they finally may not open at all. The morning ψ 8 values
decrease continuously during the drought period since the leaves no longer
become water saturated during the night (Table IV).
IX. Annual Variations of the Osmotic Potential (φ 8 )
The annual variations of ψ 8 are more pronounced than diurnal fluctuations especially in areas where rain and drought periods alternate during
the year as in Arizona (see Fig. 12). The annual fluctuations are smaller
on habitats with a good water supply. Succulents with their large water
storage tissue have a very constant osmotic potential ψ 8 .
Annual variations of ψ 8 were often investigated in species of the Mediterranean region which have a winter rainy period and a summer drought
(see Fig. 13). Deep-rooting, sclerophyllous plants show very little annual
fluctuations (e.g., Quercus ilex, Q. coccifera, and Pistacia lentiscus). On
the other hand, shallow rooting species with soft leaves (malacophyllous
plants) show pronounced fluctuations (e.g., Viburnum tinus, Cistus albida,
and Thymus vulgaris). Great variations are also observed in herbaceous