1163 J. AMER. SOC. HORT. SCI. 119(6):1163–1168. 1994. J. AMER. SOC. HORT. SCI. 119(6):1163–1168. 1994. Growth, Carbon Acquisition, and Source–Sink Relationships in ‘Titan’ Red Raspberry Gina E. Fernandez and Marvin P. Pritts 1 Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853 Additional index words. Rubus, growth analysis, photosynthesis Abstract. Seasonal changes in growth, mean maximal photosynthetic rates, and the temperature and light response curves of ‘Titan’ red raspberry (Rubus idaeus L.) were obtained from potted plants grown under field conditions. Primocane dry weight accumulation increased steadily at the beginning and the end of the season, but growth slowed midseason during fruiting. The slower midseason dry-weight accumulation rate coincided with an increase in root dry weight. Primocane net assimilation rate (NAR) was highest early in the season. Floricane photosynthetic rates (A) were highest during the fruiting period, while primocane A remained steady throughout the season. Primocane and floricane leaflets displayed a midday depression in A under field conditions, with a partial recovery in the late afternoon. Photosynthetic rates of primocane and floricane leaves were very sensitive to temperature, exhibiting a decline from 15 to 40C. Light-response curves differed depending on cane type and time of year. A temporal convergence of sink demand from fruit, primocanes, and roots occurs when plants experience high temperatures. These factors may account for low red raspberry yield. trellising recommendations have been developed by trial and error, without understanding the response that raspberry plants show to altered light and temperature or understanding which components of the plant function as C sources or sinks. Quantifying seasonal changes in C acquisition and allocation and plant growth should enhance our understanding of why certain cultural practices are successful and suggest other approaches that may improve long-term productivity. The objectives of this re- search were to 1) form a detailed growth analysis of the seasonal changes of dry-matter growth and partitioning between primocanes, floricanes, and roots; 2) document seasonal changes in C assimi- lation rates for both cane types; and 3) quantify seasonal changes in net photosynthetic rates in response to changing light and temperature using field-grown plants. Materials and Methods Growth analysis. ‘Titan’ red raspberry is a crown-forming, high-yielding, widely planted cultivar adapted to New York con- ditions (Sanford et al., 1985). On 17 May 1991, tissue-cultured plugs of ‘Titan’ were planted in 25-liter (0.40-m-i.d.) pots. The potting medium consisted of 2 montmorillomite–illite baked clay : 1 no. 4 coarse sand (by volume). The plants were grown outdoors for two growing seasons at the Cornell Univ. Orchards, Ithaca, N.Y., in 10-m rows. Irrigation and fertilization were supplied using a trickle system with a constant feed of 75 to 100 ppm N with Peters 20–20–20 soluble fertilizer (W.R. Grace & Co., Fogelsville, Pa.) alternated with water as needed. The potted plants were overwin- tered in a soil trench and covered with straw mulch. In 1992, the plants were pruned to two floricanes per pot. Floricanes were pruned to a height of 1.0 m and tied to a V-trellis with a maximum spread of 70 cm at 1.0 m from the soil level. Eight randomly selected plants were harvested on each of nine dates from late April to late September 1992. The plants spanned the growth stages from dormancy to senescence of most floricane leaves. At each harvest date, plants were divided into primocanes, floricanes, and roots. Dry weight of all leaves, canes, and roots and number of nodes were recorded. Leaf area for the first three primocanes to emerge and three randomly selected floricane laterals from each of the two floricanes were collected and used to estimate leaf areas for entire canes based on the dry weight and leaf area relationship. Leaf areas were measured using a leaf area meter Quantifying plant growth is an essential first step in developing cultural practices that increase yield in crop plants (Gutschick, 1987). Furthermore, quantifying photosynthetic responses, C allo- cation patterns, and source–sink relationships contributes to un- derstanding plant growth and yield potential. Considerable effort has been expended to obtain these data for most agronomic and horticultural crops. Such data, although important, have not been obtained for raspberries, despite their prominence as a high-value fruit crop. Interpreting plant growth data can be difficult because various plant parts have multiple functions that change over time. For example, leaves support vegetative growth early in the season and fruit growth later in the season. Clear differences in physiology and C allocation have been observed for vegetative and reproductive structures in cultivated (Forshey and Elfving, 1989; Fujii and Kennedy, 1985; Wright, 1989) and wild plants (Dawson and Bliss, 1993; Watson, 1984). Rubus sp., including raspberry, provide a unique model system for studying relationships among vegetative and fruiting structures in perennial crops. A raspberry plant pro- duces canes that grow vegetatively for 1 year and flower, fruit, and senesce the following year. While the second year canes are fruiting (floricanes), new vegetative canes (primocanes) are pro- duced from the same underground crown and root system. This distinct separation of vegetative and reproductive function allows one to study source–sink relationships without the level of con- founding often encountered with other crops in which shoots have dual functions. Many have tried to improve raspberry yield without knowledge of the plant’s basic physiology. For example, short-term yield has been increased by removing emerging primocanes (Freeman et al., 1989; Lawson and Wiseman, 1983; Nehrbas and Pritts, 1988). Whether this is caused by eliminating competing sinks or im- proved light interception of floricanes is unknown. Pruning and Received for publication 24 Jan. 1994. Accepted for publication 6 June 1994. We gratefully acknowledge the technical assistance of Caitlan Morse, Todd Dawson, MaryJo Kelly, and Liora Kahn. We thank Scott Cameron, Todd Dawson, and Tom Whitlow for their critical review of the manuscript. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regula- tions, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1 Dept. of Fruit and Vegetable Science Paper no. 47. Supported by Hatch Project NY 142-402.