PHYSIOLOGICAL-RESPONSE OF RICE AND WEEDS TO LOW LIGHT-INTENSITY AT DIFFERENT GROWTH-STAGES

The response of upland rice (Oryza sativa L.) cultivars Kinandang Patong and UPLRi-7, and the C4 weeds Echinochloa colonum (L.) Link, Eleusine indica (L.) Gaertn., and Rottboellia cochinchinensis (Lour.) W.D. Clayton to low light intensity of 150, 250, and 400 mumol m-2 s-1 photosynthetically active radiation (PAR) at different development stages was studied in a combined growth chamber and glasshouse experiment. The weed species had higher net CO2 exchange rates (CER) than rice at all light intensities and growth phases. The response of CER to light intensity was greatest at the early vegetative stage and became less pronounced at later stages in all species. Low light intensity reduced CER and growth rates at an early vegetative phase, particularly in rice, while Rottboellia maintained the highest CER. Differences in low-light-induced growth reduction between the species became less marked with plant age. Although prolonged exposure to low light intensity increased CER for all species, it was least for Eleusine, indicating acclimation of the photosynthetic system. All three weed species had a lower leaf conductance, low transpiration rate, and higher water use efficiency than rice. Both rice and weeds recovered from low-light treatment, particularly if the latter was imposed early. Continuous exposure to 150 mumol m-2 s-1 PAR strongly inhibited growth of Eleusine and Echinochloa, had an intermediate effect on rice, and affected Rottboellia least. It is concluded that Rottboellia has superior growth and assimilation compared to rice under both low and high intensity light, whereas Echinochloa and, to an even greater extent, Eleusine, are more susceptible to shading. Possible physiological causes of these differences are discussed.