Handbook of Plant and Crop Physiology

(Steven Felgate) #1

uptake to the extent that the NO 3 uptake per gram of fresh weight production is also decreased [97]. Dur-
ing the day, roots show higher rates of NO 3 uptake than during the night [128]. Blom-Zandstra et al. [97]
observed that lettuce genotypes (Lactuca sativaL.), differing in NO 3 accumulation, when grown under
light with decreasing intensity, showed decreased NO 3 uptake with a concomitant decrease in growth. In
such plants, NO 3 uptake per plant decreased proportionally even more than fresh weight production with
decline in light intensity.
It has been shown that the uptake of NO 3 by roots is dependent on the continued flux of soluble car-
bohydrates from the shoot [129]. During the day period, because of the metabolic activity of roots as well
as greater demand for carbohydrates from the shoot pool, translocation of carbohydrates from shoot to
root is greater, which parallels the higher uptake of NO 3 by roots under daylight conditions [130]. In-
creased rates of NO 3 uptake by roots are observed due to diurnal variations associated with changes in
day-night or seasonal conditions [131]. Interruption of the dark period for 3 hr using light of low inten-
sity from an incandescent lamp, resulted in a two fold increase in NO 3 uptake in soybean (Glycine max
L.) plants compared with the day period [130]. Raper et al. [130] suggested that the light-induced increase
in NO 3 uptake by plant roots is phytochrome mediated. This, in turn, alters the permeability of plasma
membranes and enhances starch degradation by increasing the activity of starch-degrading enzymes. This
leads to an increase in the availability of soluble carbohydrates for translocation from shoots to roots.
In plant cells, the bulk of NO 3 is stored in vacuoles in the form of a storage pool [132]. This is a
metabolically inactive pool of NO 3 and is not available for the induction of cytosolic NR; however, it
plays a significant role as osmoticum along with organic acids and sugars that are located in the vacuoles
[97]. The metabolically active pool of NO 3 is present in the cytosol [64]. It is believed that light affects
the movement of NO 3 from the storage to the metabolic pool [106]. Nitrate taken up in the dark accu-
mulates largely in vacuoles, and when such dark-kept plants are illuminated, the proportion of NO 3 in the
metabolic pool increases [63]. In the light, NO 3 taken up by plants enters the metabolic pool, where it is
available for NR induction. Thus, the processes of NO 3 uptake and NR induction are interrelated and both
are dependent on light. It was suggested by Aslam et al. [133] that the transfer of NO 3 from the storage
to the metabolic pool is mediated by phytochrome. Light thus regulates the availability of NO 3 in the
metabolic pool.
Plants grown at low light intensities accumulate NO 3 largely in vacuoles, where it serves as an os-
moticum [97]. Accumulation of NO 3 is inversely related to the accumulation of organic compounds, and
in this way accumulating NO 3 may compensate for the shortage of photosynthates as a result of a de-
creased rate of photosynthesis under shade conditions [97]. Plants growing in insufficient light conditions
thus show a twofold demand for N, one for the metabolic pool, which after reduction can be used for pro-
tein synthesis, and the other for the storage pool, which acts as an osmoticum [97]. The distribution of N
between organic-N and nitrate-N changes in plants grown in the shade. Decreasing light intensity de-
creases the organic-N level in vacuoles and increases the nitrate-N level. Lettuce genotypes differing in
the extent of NO 3 accumulation, when grown under shade conditions, show increased NO 3 concentra-
tion in the cell sap in both sets of cultivars accompanied by a decreased concentration of organic-N [97].
Light has a marked stimulatory effect on the reduction of NO 3 by regulating the synthesis as well as
the functioning of NR. Leaves of shade-grown plants show a very low level of NR activity, but when such
plants are transferred to light the NR activity increases severalfold [63]. As with NR, in photosynthetic
tissues light plays a significant role in regulating the activity of NIR [63].
Several regulatory mechanisms for light-mediated enhancement of NR activity have been postulated.
Based on the inhibitor studies and the labeling experiments, it has been suggested that light promotes de
novo synthesis of both NR and NIR. Illumination of leaves leads to increased protein synthesis, indicat-
ing that light enhances the production of NR in leaves [64]. Certain investigators suggest that light-me-
diated enhancement of NR activity is due to enhanced uptake of NO 3 by plants in light [134]. Light en-
hances the movement of NO 3 from the storage pool to the metabolic pool [133], where NO 3 becomes
available for the induction of NR activity. Sharma and Sopory [135] observed that in maize seedlings the
NR activity increased by more than 300% on treatment with red light and kinetin. These investigators sug-
gested that the light-induced increase in NR activity is mediated via phytochrome. Phytochrome action
does not appear to be mediated by hormones; however, there appears to be an overlap in the signal trans-
duction chains of phytochrome and plant hormones [135]. According to Sawhney and Nalik [136], some
early events of photosynthesis, such as the Hill reaction, cause redox changes in green tissues and create
favorable intracellular conditions for the synthesis of NR. These findings indicate that light influences the


NITROGEN ABSORPTION UNDER STRESS 645

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