Leaf Gas Exchange Characteristics of Red Raspberry Germplasm in a Hot Environment

Net CO2 assimilation (A), evapotranspiration (ET), and stomatal conductance (gs) were determined in two experiments for 14 and 18 raspberry (Rubus sp.) genotypes, respectively, grown in 4-L containers and exposed to 35 °C daytime temperatures 2 weeks and 4 weeks after placement in growth chambers. Measurements were taken on two successive leaves on the same primocane between the third and seventh node (≈75% to 85% of full leaf expansion). In Expt. 1, selections from Louisiana exhibited higher A (3.10– 5.73 μmol·m–2·s–1) than those from Oregon (0.50–2.65 μmol·m–2·s–1). In Expt. 2, the genotype × time interactions were nonsignificant, and time of measurement did not affect A or ET (P ≤ 0.05). Assimilation ranged from 2.08 to 6.84 μmol·m–2·s–1 and varied greatly among genotypes, indicating that diverse A levels exist at high temperatures in raspberry germplasm. NC 296, a selection of R. coreanus Miq. from China, and ‘Dormanred’, a southern-adapted raspberry cultivar with R. parvifolius Hemsl. as a parent, had the highest A rates. Evapotranspiration and gs did not differ among genotypes. Average gs for all genotypes declined from 234 mmol·m·s in week 2 to 157 mmol·m·s in week 4. Our findings, coupled with plant performance under hot conditions, can be used to identify potential parental raspberry germplasm for breeding southern-adapted cultivars. dure exists to identify those selections that have inherited heat-tolerance or to determine which species have the most to offer a breeding program. Gas exchange characteristics of Rubus germplasm may indicate adaptation to hot conditions. Donnelly and Vidaver (1984) reported results on a raspberry selection (BC 72-1-7, a cross of ‘Haida’ x ‘Canby’, later released as ‘Algonquin’) with the maximum rate of A between 6.3 and 9.5 μmol·m·s. No studies have evaluated genotype response at high temperatures in the South. A few studies have focused on established cultivars in general, such as ‘Amity’, ‘Autumn Bliss’, ‘Heritage’, ‘Meeker’, and ‘Titan’ (Cameron and Hartley, 1989; Fernandez and Pritts, 1994; Goulart, 1989; Hunt et al., 1991; Klauer et al., 1992; Percival et al., 1996; Privé et al., 1997). Cameron and Hartley (1989) examined gas exchange characteristics of ‘Amity’ leaves after partial defoliation and discovered that A, gs, and transpiration increased after one-third or two-thirds of the total leaf area on one leaf were removed. Gas exchange rates reported for whole leaves in this study were 8.9 μmol·m ·s (A), 99.3 mmol·m·s (gs), and 2.6 mmol·m·s (transpiration). In ‘Titan’ and ‘Heritage’, Goulart (1989) found that A and gs increased as soil water content increased from deficit to high levels. Hunt et al. (1991) determined that maximum assimilation levels were achieved at 1000 to 1200 μmol·m·s photosynthetically active radiation (PAR) when ‘Titan’ leaflets reached 75% to 85% of full expansion. The highest levels of CO2 assimilation were reached from 0700 to 1000 HR in a study conducted by Klauer et al. (1992) on ‘Meeker’. Fernandez and Pritts (1994), Percival et al. (1996), and Privé et al. (1997) examined gas exchange response to temperature, but response to high temperature was not the major focus of these studies. Fernandez and Pritts (1994) found that gas exchange of primocane leaves on ‘Titan’ declined as temperature increased from 15 to 40 °C. Assimilation rate at 20 °C was ≈8 μmol·m·s, whereas at 35 °C it declined to ≈4 μmol·m·s. Similar results were reported by Percival et al. (1996) for whole-plant A of ‘Heritage’. However, Privé et al. (1997) stated that high A and gs rates were obtained at both warm (≈30 °C) and cool (≈15 °C) temperatures. They also reported a wide range in gs throughout the season; the mean for leaves in the absence of a subtending fruiting or flowering lateral was 159 ± 110 mmol·m·s. The objectives of this study were to examine gas exchange characteristics among a broad range of genotypes that had not been studied previously and to determine if any expressed qualities contributed to heat tolerance. Materials and Methods Fourteen diverse genotypes from varying origins were used in Expt. 1 (Table 1), and 18 genotypes were used in Expt. 2 (Table 2). Many of these genotypes are native to regions where summer conditions are hot and humid, as in the upper South. These were obtained from the U.S. Dept. of Agriculture, Agricultural Research Service (USDA, ARS) National Clonal Germplasm Repository and the USDA, ARS, Northwest Center for Small Fruit Research, both at Corvallis, Ore., from North Carolina State Univ., and from Glen Melcher, a private Rubus breeder in Louisiana. Plants were potted in 4-L pots in a Universal Mix media (Strong-Lite Hort. Prod., Pine Bluff, Ark.) containing a controlled-release fertilizer (Osmocote 18N–2.6P–9.9K, 14 g/ pot) and grown in the greenhouse for several months until the experiment was initiated. Plants were placed in three 2.13-m Conviron 3244 growth chambers (Winnipeg, Man., Canada) on 3 July 1998 for Expt. 1 and 22 May 1998 for Expt. 2. Growth chamber temperatures were 35 °C day (16-h photoperiod) and 25 °C night to approximate conditions for the southern United States. Measurements were taken 2 and 4 weeks after placement in the growth chamber with a CIRAS-1 (PP-Systems, Hitchens, Herts, U.K.) portable infrared gas analyzer (IRGA) and Parkinson leaf cuvette (2.5 cm) on two leaves per primocane. Leaves were at 75% to 85% of full expansion at the time of measurement. The air flow rate within the cuvette was set at 200 mL·min with 350 mL·L CO2 and 50% of ambient water vapor. Leaf and chamber temperature were maintained by a thermoconductive peltier plate, set at 35 °C, and measured using an infrared temperature sensor. PAR was maintained between 1000–1500 μmol·m·s during measurements, the reported light saturation level for Rubus (Hunt et al., 1991; Rom and Clark, 1991). Stability in measurement was reached between 1 and 2 min. Plants were watered as needed, and no water stress was experienced prior to or during Interest in red raspberry (Rubus idaeus L.) in the southern states has increased recently (Clark and Rom, 1997), but the region suffers from a lack of southern-adapted cultivars with commercial quality (Moore, 1997). Currently available cultivars developed outside of the South are not well-adapted for several reasons, including high light intensities and hot summer temperatures (Moore, 1997). Southern Asiatic countries traditionally have been the source of Rubus plant material with tolerance to heat and drought. North American researchers have been the leaders in introducing wild species to the commercial genetic base for incorporation of heat tolerance (Jennings et al., 1991). Unfortunately, commitment to the development of heat-tolerant, commercial-quality cultivars has not been sustained. Considerable efforts (Hull, 1969; Overcash, 1972; Williams, 1950) have proved only mildly successful, but these researchers have generated many selections from their breeding programs. Yet, no efficient proce-