Nitrogen uptake, leaf nitrogen, chlorophyll content and leaf color of rice (Oryza sativa L.) as affected by drought stress

Nitrogen and water play very important roles in rice production. Nitrogen fertilizer application is essential for attaining high yields of rice, and water is a major factor limiting rice yields in rainfed lowlands. This study investigated how water stress modifies N-uptake, leaf-N, chlorophyll content, leaf color chart (LCC), chlorophyll meter reading or SPAD (Soil-Plant Analysis Department, SPAD-502 Minolta Camera Co., Osaka, Japan), and the relationships among these parameters. Also, it aimed to determine how many days after re-irrigation these relationships will return to normal condition. The study was conducted in a greenhouse at the International Rice Research Institute (Philippines) following a randomized complete block design (RCBD), replicated three times. The treatments were two nitrogen levels: no nitrogen (N sub o) and 150 kg N/ha (N sub 150), and 3 water regimes: well-watered (Wsub o)mild water stress (Wsub-20), and severe water st ress (W sub-80). Stress was imposed for 12 days which started at 30 days after seeding (DHS). Leaf relative water content (RWC) of the uppermost fully expanded leaf decreased with intensity of stress. During the stress period, specific leaf weight (SLW), leaf-N per unit dry weight (Nsubdw), leaf-N per unit leaf area (Nsuba), LCC and SPAD reading increased with levels of stress but declined rapidly within 3 days after re-irrigation. Chlorophyll content (Chl sub dw) of the uppermost fully expanded leaf, however, decreased under water stress. Mild water stress treatment (W sub-20) did not alter the relationships among N sub dw, Chl sub ddw, and SPAD reading that existed in well-watered condition but broke down when plants were subjected to severe water stress (W sub-80). Within three days after re-irrigation the relationships among the above parameters were restored up to the level of well-watered condition. The linear relationship between SPAD and leaf-N content (N sub a) held true irrespective of water stress level. Severe water stress was highly detrimental to rice crops as shown be very low grain yield compared with those subjected to mild water stress. The findings suggests, that LCC/SPAD reading can be used as basis for timing of N application in rainfed lowland rice exposed to mild water stress. Under severe water stress condition, however, LCC/SPAD readings may cause inaccurate prediction of leaf nitrogen status as basis for N-management.