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Tadashi Hirasawa
Faculty of Agriculture, Tokyo University of Agriculture and
Technology,
Fuchu, Tokyo 183, Japan
Introduction
Root functions, such as anchorage, the absorption of water and mineral
nutrients and synthesis of plant hormones, are supposed to be important
for successful growth of plants and for attaining high yield of crop plants
(Honya,1966, Kramer and Boyer,1995). However, quantitative relationships
between each of the functions and physiological processes affecting dry
matter production and yield are not known well. In this subject, our research
has been focused on investigating the root characteristics in the view
of leaf photosynthesis. In this paper, the relationship between water uptake
capacity and photosynthetic rate will be presented and root properties
for high water uptake capacity will be discussed.
Stomatal conductance and photosynthetic rate of rice plants decrease in
the midday and in the afternoon under intense transpiration on a clear
day due to water stress even though they are growing under submerged soil
conditions (Ishihara and Saito,1987). In the plants with higher water uptake
capacity, the reduction in leaf water potential would be smaller when transpiration
increases, and stomatal conductance and photosynthetic rate would be kept
higher during daytime (Ishihara and Kuroda,1986). However, there is no
useful indicator for estimating water uptake capacity quantitatively. First
of all, we investigated whether resistance to water transport calculated
by using the Ohm's law analogy (van den Honert,1948) can be adopted in
estimating water uptake and transport capacity of transpiring plants.
Whole Plant Resistance to Water Transport
Since rice plants were growing under submerged soil conditions, soil
water potential could be regarded as 0 MPa. The resistance to water transport
from soil through root to leaf (R, whole plant resistance) was calculated
as follows:
R = -yx
/ T
where yx
and T was leaf xylem water potential and transpiration rate, respectively.
A whole plant resistance decreased markedly as the transpiration rate increased
at low transpiration rates. But it became constant at high transpiration
rates where there was a close linear correlation between transpiration
rate and leaf xylem water potential and the linear extrapolation of the
regression line down to zero transpiration gave the water potential quite
near to the origin (Hirasawa and Ishihara,1991). This means that the resistance
can be adopted in estimating water uptake and transport capacity under
intense transpiration.
Root Resistance in the Whole Plant Resistance
Although there were large differences in the whole plant resistance, no significant differences in the resistance from the base of a stem to the leaf were observed between the plants grown under different conditions (Hirasawa et al.,1992a). The whole resistance to a leaf at a given position on a stem increased considerably after the leaf had fully expanded. However, the increase in the resistance from the base of a stem to the leaf was far smaller than that in the whole resistance (Hirasawa et al.,1992b). Root resistance is the most variable in a plant resistance except for the case of the rapid marked increase in the stem resistance due to cavitation.
Relationship between the Resistance to Water Transport and the Midday and Afternoon Depression of Stomatal Aperture and Photosynthetic Rate
The degree of the midday and afternoon depression in the stomatal aperture was larger in the plants of which whole resistance was larger and there was a close correlation between the resistance and the degree of the midday depression (Hirasawa and Ishihara,1992, Jiang et al.,1988). As there is a close correlation between stomatal aperture and photosynthetic rate, and as the increase in the whole resistance is mainly due to the increase in the root resistance, there should be a close correlation between the root resistance and the degree of the midday depression in photosynthetic rate.
Root Properties for High Water Uptake Capacity
Root resistance was large in the plants with poor developed root system,
in the plants with roots in which physiological activity decreased and
in the plants with root system consisting of old roots (Hirasawa et al.,1992a,b,
Jiang et al.,1988). This means that both total root length (absorbing surface)
and water uptake capacity per unit root length (surface) contribute to
root water uptake capacity.
The radial hydraulic resistance in a root is larger than the axial resistance
except for the apical portion (Frensch and Steudle,1989). Therefore, the
results mentioned above indicate that radial resistance may increase with
the decrease in physiological activity and with senescence. The pathway
of radial water transport and characteristics facilitating membrane water
transport such as water channel (Chrispeels and Maurel, 1994) would be
required to be investigated for clarifying the nature of the radial hydraulic
resistance in a root.
References