INFLUENCE OF LEAF XYLEM WATER POTENTIAL AND LEAF POSITION ON RELATIONSHIP BETWEEN NITROGEN-CONTENT AND PHOTOSYNTHETIC RATE OF RICE PLANT

Even though the nitrogen content (mgNdm(-2)) of rice leaves is the same, a large difference in the net photosynthetic rate (Pn) has been recognized. To determine the reasons for the difference, leaf xylem water potential (Wp), leaf position on stem, days after fully expanded and root respiration of plant having sample leaves were studied. Throughout the period from the booting stage to the middle ripening stage, Wp was measured by the pressure chamber method in the leaf of potted rice plant, cv. Yamabiko and Ochikara immediately after measuring Pn under the conditions 25 degrees C air temperature and 45-50 Klx artificial light. The correlation between mgNdm(-2) and Pn in both cultivars is shown for each group of first leaves at the top of the stem, and for the second leaves and third leaves, but not the fourth leaves. In the case of young first leaves at the top of the stem and also second leaves, showing the higher Pn (i.e., higher rate of transpiration), a positive correlation was seen between Wp and Pn. Otherwise, old leaves positioned third and fourth, existing a short distance from the roots, had a negative correlation between Wp and Pn. It seemed that the relationship between mgNdm(-2) and Pn in the lower leaves was altered by Wp. The lower leaves of the plants which had a high respiratory rate in the roots were maintained at a comparatively high Pn until Wp decreased below - 1.4 MPa. The lower leaves of a plant having poor root activity were specialized as lower Pn and higher Wp;possibly because of the lower degree of water transport resistance from the root system to the leaf.