RELATIONSHIP BETWEEN LEAF PHOTOSYNTHESIS AND NITROGEN-CONTENT OF FIELD-GROWN RICE IN TROPICS

A field-derived relationship between maximum single leaf net photosynt hetic rate under saturating light (A(max)) and leaf N content per unit leaf area (N-a) is not available for rice (Oryza sativa L.). The rice simulation model ORYZA1 estimates aboveground dry matter production b ased on the relationship between A(max) and N-a determined previously under greenhouse-growth chamber conditions. The objectives of this stu dy were to establish to relationship between A(max) and N-a under fiel d conditions and to determine whether the field-derived relationship c ould improve ORYZA1 estimation of aboveground dry matter production of tropical irrigated rice. Rice plants were grown in the field with dif ferent N rates in the 1993 dry season. The A(max) and N-a were determi ned at 38 and 88 d after transplanting. Aboveground dry matter was det ermined at physiological maturity. Dry matter production data at physi ological maturity from four other field experiments were used for mode l evaluation. There was a close correlation between A(max) and N-a und er field conditions (r(2) = 0.88). Compared with the relationship betw een A(max) and N-a as determined under greenhouse-growth chamber condi tions in previous studies, higher A(max) was observed at low N-a in th is field study. When the held-derived relationship between A(max) and N-a was used, ORYZA1 estimation of dry matter production was improved for rice grown in tropical irrigated environments.