Canopy Eco-Physiological Properties And The Influence Factors In Irrigated Rice

Canopy is the location where yield formation takes place and the main target that diseases and insects attack. The characteristics of rice canopy affect disease susceptibility directly and indirectly by changing microclimate inside the canopy. Understanding the eco-physiological properties of rice canopy is a prerequisite for achieving a healthy canopy that leads to maximum grain yield and minimum disease infestation. The crop management strategies that are guided by healthy canopy concept will be crucial for sustaining high grain yield and minimizing the negative impact of rice production on the environment. Field experiments were conducted in 2004 dry season (DS) and wet season (WS) at IRRI farm in Philippines, on the basis of a preliminary experiment conducted in 2003 WS. The objectives were: (1) to determine the differences in canopy characteristics, microclimate inside canopy and sheath blight infestation among the canopies of different rice varieties with different fertilizer nitrogen (N) rates; (2) to investigate the relationship among rice canopy characteristics, microclimate inside canopy and disease infestation; (3) to determine the effect of canopy characteristics, microclimate inside canopy and disease infestation on rice grain yield; and (4) to explore the indexes for quantifying rice healthy canopy that will eventually lead to the development of healthy-canopy crop management strategy. The main results of the study are listed below:1. Rice canopy characteristics of four varieties grown at four N levels in the field were studied by plant sampling at different growth stages. The results showed significant differences in plant height (PH), tiller number (TN), leaf area index (LAI), aboveground total dry weight (TDW), and crop growth rate (CGR) among varieties and N treatments. Higher N gave a larger canopy with higher PH, TN, LAI and TDW. Hybrid varieties showed significantly higher PH and LAI, and lower TN compared to the conventional varieties. There were significant conic correlations between grain yield and LAI, PH, TN, TDW, and CGR. The optimum LAI and TDW for highest yield were 5.1 and 1170 g m^-2, respectively, and were highly consistent in DS and WS. Almost all the canopy parameters under zero N application and under highest N treatment were respectively enhanced and reduced in WS, caused smaller differences in canopy size among N treatments than that in DS.2. The changes in temperature (T) and relative humidity (RH) within the canopy were recorded by HOBO dataloggers. The following results were obtained: (1) T and RH within canopy were affected by the T and RH above the canopy, canopy size, growing season and growth stage. (2) T and RH within canopy, which showed a trend similar as that above the canopy, were stable, respectively lower and higher at night (19:00-07:00), and unstable, respectively higher and lower at daytime (07:00-19:00). The daily extreme T and RH within canopy occurred at about 13:30. (3) Differences in T and RH among different treatments were mainly caused by day T and day RH, and were greater in DS and at FL stage. The daily maximum T (Tmax) and the daily minimum RH (RHmin) varied most among the treatments. The day T and day RH within canopy decreased and increased respectively as N level increased. (4) PH, TN, LAI showed significant negative correlations with Tmax and positive correlations with RHmin.3. Canopy light transmission rate (LTR) of different treatments was studied with sunscan canopy analysis system and by plant sampling at different stages. LTR was significantly reduced as N rate increased and it was greater in DS than in WS. There were significant negative correlations between LTR and PH, TN, and LAI. LTR was affected by TN and PH in DS and by LAI and TN in WS.4. Sheath blight index (SHBI) was established as a simple, quick, accurate, and objective method for disease assessment in the study. SHBI showed distinctly significant correlations with lesion area percentage on leaf (SL), relative lesion height (RLH) and yield loss.5. The degree of sheath blight disease caused by artificial inoculation with Rhizoctonia solan/were assessed with the following results: (1) there was significant difference in SHBI among varieties, but none among varietal types. (2) SHBI increased with higher N level and had significant linear correlations and multiple regressions with LAI, TN, and PH. (3) N mainly affected the leafborn phase of the disease epidemics since leaf N contents showed significant correlation with SL, but not with lesion area percentage on sheath (SS). (4) Leaf contact frequency (LCF) showed significant correlation with SHBI. (5) SHBI had significant negative relationships with day T and LTR and positive relationship with day RH. There were multiple regressions between the disease indexes and the microclimate parameters inside canopy. (6) Comprehensive analysis of canopy characteristics, microclimate insider canopy and plant nutrition status showed that the disease indexes were greatly and directly affected by the canopy characteristics during the early stages after inoculation and by the microclimate insider canopy during the later stages.6. The ultimate goals of healthy-canopy crop management are to maximize grain yield and resource use efficiency, and minimize the negative impact of rice production on the environment. A healthy rice canopy should have optimum LAI and source-sink ratio that are required for maximum grain yield. Other indexes such as canopy structure index (CSI), canopy comprehensive index (CI), ratio of canopy height to plant height, air surge index, and recurrent mean value are also useful for quantifying healthy rice canopy.