Seasonal changes of chlorophyll content in field-grown rice crops and their relationships with growth

Field experiments were conducted on the experimental farm of the Taiwan Agricultural Research Institute (TARI) to examine seasonal changes in the amounts of chlorophylls in leaves and whole plants of rice (Oryza sativa L. cv. Tainung 67) grown in the first and the second crop seasons in 1998. Chlorophyll (Chl) differences between two crops and their relationships with growth were also investigated. The differences in climate conditions between the two crop seasons were quite phenomenal and led to different rates of plant growth and amounts of chlorophyll produced during the growth periods. Seasonal changes in Chla, Chlb, and total Chl were curvilinear in style, with Chla making up the major portion of total chlorophyll. Young plants of rice began with lower levels of chlorophyll, which increased as the plants developed, and decreased as the plants aged. A significant increase in chlorophyll occurred in the early stage of the second crop compared to that of the first crop, and the difference was maintained throughout the growing season. When the unit time transformation technique was used to exclude the difference in growth rate, the curves of the chlorophylls (Chla, Chlb, and total ChI) were similar in the two crops, but plants grown in the second crop season had higher levels of chlorophylls at the same time unit scale. Generally, the ratio of Chla to Chlb (Chla/Chlb) decreased after transplanting, indicating a significant reduction (p < 0.01) in the green-to-yellow pigment relationship. Plants grown in the first crop had higher Chla/Chlb ratios than those grown in the second crop, especially in leaf blades. Good agreement was found between growth traits and the respective total Chl cotent from regression analyses, and these growth traits were highly correlated with total Chl (R2 > 0.8). However, rice plants grown in the first crop showed a higher levels of plant growth and leaf area at the same chlorophyll level than those grown in the second crop. These results suggest that rice plants grown in the first crop may exhibit better growth and production than those grown in the second crop under normal growing conditions.