TEMPERATURE EFFECTS ON PHOTOSYNTHESIS, GROWTH RESPIRATION, AND MAINTENANCE RESPIRATION OF MARIGOLD

Marigolds (Tagetes patula) were grown from seed to flowering at either 20 or 30 EC, and the CO2 exchange rate of the plants was measured throughout the 60-day growing period. Plants grew faster at 30 than at 20 EC, because of higher whole-plant photosynthesis (Pnet) from 5 to 40 days after seeding. The maintenance respiration coefficient (rm) was lower at 20 than at 30 EC (Q10 = 1.3), while the growth respiration coefficient was not affected by temperature (0.51 g@g). The ratio of maintenance respiration to total respiration increased throughout plant development and reached 43 and 55% at 20 and 30 EC, respectively. Carbon use efficiency (CUE) of the plants increased during germination, and then decreased throughout the remainder of the growing period. This decrease in CUE was the result of a decrease in relative growth rate (RGR). Plants grown at 20 EC had a higher CUE than those at 30 EC from 40 to 51 days after seeding, which was the result of a lower RGR and higher rm at 30 EC. Although plants grown at 20 EC had equal or higher CUE, a lower rm, and less dark respiration (Rdark), their growth was less than that of plants at 30 EC, suggesting that whole-plant Pnet is a more important determinant of growth than CUE, rm, or Rdark. INTRODUCTION Temperature greatly affects the carbon exchange rate and growth of plants. However, the exact method by which it effects growth is not always clear, because temperature affects both photosynthesis and respiration. Respiration, in turn, can be divided into two separate processes, i.e., growth respiration (Rg) and maintenance respiration (Rm). Because differences in plant size and growth rate make it difficult to make direct, meaningful comparisons of Rg and Rm among different treatments, Rg is commonly expressed as the growth respiration coefficient (rg) H growth rate, while Rm is expressed as the maintenance respiration coefficient (rm) H plant size. Maintenance respiration (and rm) is considered to be highly sensitive to temperature, mainly due to increased protein turnover and membrane leakage at higher temperatures (Hesketh et al., 1980). On the other hand, rg will only be affected by temperature, if temperature affects the chemical composition of the plants or the biochemical pathways by which the plant compounds are synthesized (Penning de Vries et al., 1989). Much of the previous research on temperature effects on Rg and Rm has looked at short-term effects of temperature changes. However, long-term effects may be different from short-term effects, since the isozyme composition of plants can change in response to the environmental conditions. If plants adapt to higher temperatures by producing enzymes with a lower turnover rate, long-term effects of temperature on Rm are likely to be less than shortterm effects. The objective of this research project was to quantify the effect of temperature on photosynthesis and respiratory processes of marigold, throughout plant development. We hypothesized that temperature would increase rm (although less than normally seen in shortterm studies), but not rg. Proc. XXVI IHC – Elegant Science in Floriculture Eds. Th. Blom and R. Criley Acta Hort. 624, ISHS 2003 Publication supported by Can. Int. Dev. Agency (CIDA) 550 MATERIALS AND METHODS Growing Conditions and CO2 Exchange Measurements Plants were grown in 50H25H5 cm trays (6.3 L), filled with a 50/50% (v/v) mixture of fine sand and perlite. The growing medium was watered thoroughly with nutrient solution before seeding each flat with 36 seeds of marigold ‘Queen Sophia’. These trays subsequently were enclosed in CO2 exchange chambers (van Iersel and Bugbee, 2000), which were placed inside two growth chambers (four CO2 exchange chambers per growth chamber). The growth chambers were programmed for a 14 hour light period and had a photosynthetic photon flux of 410 μmol@m@s at the surface of the growing medium, which resulted in a total daily photon flux of 20.7 mol@m. The plants were subirrigated as needed with a nutrient solution made from a complete, water-soluble fertilizer (Peters’ 15-5-15 CalMag, The Scotts Co., Marysville, Ohio, USA) dissolved in deionized water with a concentration of 100 mg@L N. The CO2 exchange rate of whole trays of plants was measured every 10 minutes for 60 days, using a multi-chamber, semi-continuous CO2 exchange system (van Iersel and Bugbee, 2000). Treatments Temperature inside the gas exchange chambers was measured every 2 seconds with type-T thermocouples. Based on these temperature measurements, a datalogger controlled a 200-W heater in each chamber. Temperature set points in the gas exchange chambers were 20 or 30 EC and were kept constant throughout the entire experiment. The average temperature was within 0.2 EC of the set point, with short-term fluctuations of approximately 0.5 EC. Calculated Parameters and Statistical Analysis The CO2 exchange measurements resulted in direct measurement of net photosynthesis and dark respiration (Pnet and Rdark, mol@plant@s). These values were used to calculate daily carbon gain (DCG, a measure of growth rate; mol@plant@day), carbon use efficiency (CUE, the amount of carbon incorporated into the plant divided by the amount of carbon fixed in gross photosynthesis; mol@mol), and cumulative carbon gain (CCG, a measure of plant size; mol@plant) according to van Iersel and Bugbee (2000). Relative growth rate (RGR, mol@mol@day) was calculated as: RGR = DCG/CCG [1]. To minimize the effect of the C present in the seeds, which contained approximately 0.063 mmol/plant, data with a CCG < 6.3 mmol/plant were not used for further analyses. Growth and maintenance respiration coefficients at 20 and 30 EC were estimated using linear regression, using data from all eight CO2 exchange chambers: Rd = x0HDCG + x1HDCGHT + x2HCCG + x3HCCGHT [2], where Rd = daily respiration (mol@day), T = temperature, and x0 . x3 are regression coefficients. RESULTS AND DISCUSSION CO2 Exchange Rate and Plant Growth Plants grown at 30 EC germinated faster and had higher photosynthetic rates from 5 until 40 days after seeding than plants grown at 20 EC (Fig.1). During the latter part of the experiment, Pnet remained higher in the 30 than in the 20 EC treatment, but these differences were no longer statistically significant. In the 30 EC treatment, Pnet reached is maximum value after 32 days, while it took 44 days at 20 EC. Presumably, at his stage the crop had reached canopy closure and almost all the available light was intercepted by the foliage. The subsequent small decrease in Pnet may have been caused by the continuing increase in Rdark (Fig. 1), as well as by interception of some of the light by flowers, thus reducing the amount of light available for photosynthesis.