The Effects of Simazine and Temperature on Photosynthesis in Rye

Abstract

Temperature durinig growth lhad a strong influienice on light-saturated rates of pliotosynthesis anid respiration in rye (Secale cereale). Carbon were removed and uniform plants containing about four leaves were selected. On the 7th day the plants in 4-inch pots were placed in the growth chambers, and treatments with nutrient solution, according to Hoagland and Arnon (6), and simazine were started. Three days a week each pot received 70 ml of nutrient solution containing 0.00, 0.03, 0.08, or 0.12 mg/liter simazine. On nontreatment days, 150 ml of distilled water were used to "flush" each pot.The seedlings were grown under a 12-hr photoperiod and a light intensity of 1600 ft-c. Relative humidity was maintained between 35 and 50' c The four temperature regimes coincided with the photoperiods and were about 10 C cooler during the dark period (D) than during the light period (L) (7°L:0°D, 17°L: 70D, 27°L:170D, 35°L :270D). Twenty-four plants were grown under each experimental condition, and the pots were randomly mixed after each treatment.Terminal analyses were made near the end of the light period 30 days after germination was started. Young, fully expanded leaves were used for determinations of chlorophyll (1) and photosynthesis. In the photosynthesis studies, two or three intact leaf segments (about 75 mm long) were enclosed in special copper assimilation chambers which consisted of water jackets for temperature regulation and a variable speed fan to mix the air. Modeling clay was used to form an air-tight seal around the leaves and also between the Plexiglass lid and the lower copper chamber. Air containing 310 mg liter of CO2 was passed through the chamber at measured flow rates from a compressed source. The concentration of CO2 leaving the chamber was monitored by the gas analyzer and never deviated more than 20 mg liter from the source. Chamber temperature was measured by enclosed thermistors which activated the water bath and maintained the air temperature at 20 4 0.5 C. The light source consisted of General Electric Cool Beam incandescent bulbs filtered through 2 inches of water. Intensity was altered by inserting filters in the light path and by making fine voltage adjustments. The relative humidity of the air entering the chamber was around 40%c. Normally, five plants were analyzed at the same time, and less than 45 min were necessary to attain stable readings of dark respiration and photosynthesis at 100 and 2500 ft-c. RESULTSGrowth was markedly influenced by temperature (Table I), with the most leaf production occurring at 27°L:17°D, and the least at the lowest temperature. Leaf length generally increased as temperature increased. Effects of simazine were not evident at the lower temperatures but were quite apparent at the higher temperatures where increasing concentrations resulted in both fewer and shorter leaves. These leaves also appeared generally chlorotic and were quite lax.The effect of simazine on chlorophyll content also varied as a function of temperature (Fig. 1). At the two highe...

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