Vergara - 1976 - Physiological and morphological adaptability of ri

(TROWTH AND NTJTRIENT UPTAKE OF RITE ROOTS 269
EFFECT OF LIGHT INTENSITY AND TEMPERATURE ON GROWTH
0F ROOTS OF LOWLAND RICE
The growth of a rice plant advances in a chain of shoot units and roots emerge
as a unit. This shoots elongation syrnchronized with the upper third node leaf
blade and may complicate the study’ of rice root growth as affected by climatic
factors such as light intensity" and temperature. The regularity and i11terdependence
in emergence and elongation of roots and leaves should be closely
related to the transloeation and circulation of metabolites in rice plants. 111
studying the translocation of photosynthetic products from leaves to roots of
rice at yjoung panicle initiation, a stage of the most vigorous emergence and
elongation of roots, Yoshida (1968) ShOWCd that the rate of sucrose translocation
per day amounted to 5 to 10% of the dry weight of roots (Table 1). But
more than 90% of the nitrogen absorbed by roots was supplied to the aerial
parts at that growth stage (Yoshida and Takahashi. 1958; Inada, 1967).
The interrelation between aerial and underground organs in the mutual
lranslocation of metabolites would result in the change in aerial parts, affecting
the roots in a relatively short time, and vice versa. A conspicuous example of
such interrelation is the effect of light intensity’ on the growth of roots. Decrease
in light intensity’ by shading almost completely’ interrupted dry matter increase
in both aerial and underground parts (Oshima and Murayama. 1960). Moreover,
shading more markedly affected roots than shoots, decreasing the R/T
ratio (Yazaiva, 1955, unpublished), and disturbing the synchronized emergence
of roots and leaves; that is, root emergence was started midway in the formation
ofthe related leaf. which remained unaltered even under 50% reduction in light
intensity (Komatsu, 1961). Since light does not directly‘ influence rice roots that
grow in paddy soil, the foregoing facts are attributed with certainty to the decrease
in photosynthate supply’ to the roots which was more rapid than the
reduction in photosynthesis in green organs of rice. Actually, Yazawa (1960,
unpublished) observed that when light intensity was reduced by 50% through
shading. the total sugar content in roots after 12.5 hours fell to 50% of the eontrol,
whereas that in leaves fell 79%. indicating that more acute shortage of
sugar took place in roots (Table 2).
The effect of temperature on root growth has been studied by many workers,
but the results are to some extent contradictory’. Nagai and Matsushita (1963)
Table 1. Rate oil‘ sucrose translocation from leaves to roots in lowland rice at young
panlcle initiation tYoshtda, 196B).
Specific activity of
Sucrose transloeation
Nitrogen source in sucrose translocated (mgtg dw of root)
culture solution (cprnfmg C) per hour per day
No nitrogen 4,326 5.08 121.8
Ammonium-N 18,368 1.91 455
Nitrate-N 18,359 1.92 46 D