Leaf Gas Exchange Response of ‘Arapaho’ Blackberry and Six Red Raspberry Cultivars to Moderate and High Temperatures

Leaf gas exchange of six red raspberry (Rubus idaeus L.) and one blackberry (Rubus L. subgenus Rubus Watson) genotypes growing in 12-L containers was measured at four temperatures (20, 25, 30, and 35 °C) once a month for 3 months in growth chambers by infrared gas analysis. Measurements were taken on three successive leaves on the same primocane between the third and seventh nodes (≈75% to 85% of full leaf expansion). The plants were grown in ambient (field) conditions except when measurements were taken. Maximum daily ambient temperatures rose as high as ≈37 °C during this period. Net CO2 assimilation (A), evapotranspiration (ET), and stomatal conductance (gs) were measured during June, July, and August. Significant differences (P ≤ 0.01) in A were found among the seven genotypes. ‘Arapaho’ blackberry displayed the highest mean A rate at all temperatures. Only in the raspberry cultivars Nova and Reveille did the rate of A drop significantly when temperature increased from 20 to 30 °C. ‘Reveille’ was also the only cultivar in which A significantly declined between 30 and 35 °C. The ET increased significantly over the four temperatures in four cultivars (‘Arapaho’, ‘Heritage’, ‘Nova’, and ‘Southland’). The ET rate at 35 °C was higher for ‘Arapaho’ than for all other cultivars. ‘Autumn Bliss’, ‘Dormanred’, and ‘Reveille’ did not change significantly as the temperature rose from 20 to 35 °C. Stomatal conductance of ‘Heritage’ and ‘Arapaho’ did not change significantly between 20 and 35 °C, whereas that of ‘Autumn Bliss’ and ‘Reveille’ declined almost 50% when temperature increased to 30 or 35 °C. with primocane leaves exhibiting higher A rates at all temperature levels. At 30 °C, A in primocane leaves was nearly 6 μmol·m·s. As temperature increased to 35 °C, A declined to ≈4 μmol·m·s and at 40 °C it dropped to around 2 μmol·m·s. Percival et al. (1996) found that an air temperature of 35 °C yielded a whole-plant A rate of ≈4 μmol·m·s in ‘Heritage’. They concluded that maximum A was reached between 17 and 20 °C, while temperatures above 20 °C reduced it. Privé et al. (1997) reported that high net carbon exchange rates could be attained at warm temperatures (25 to 30 °C) in potted ‘Autumn Bliss’. The objective of our study was to assess the gas exchange capabilities of a broader range of Rubus germplasm over moderate to high temperatures. Materials and Methods Dormant plants from virus-tested stock of red raspberry cultivars were obtained from two local nurseries: Pense Nurseries and Simmons Berry Farm, both in Mountainburg, Ark. The blackberry plants were obtained from the University of Arkansas Agricultural Research and Extension Center, Fayetteville (Table 1). The first ‘Arapaho’ plants did not produce any growth, and were replaced with 1-month-old plants 1 month later. Thirty-five total plants were potted in 12-L pots with a Universal Mix media (Strong-Lite Hort. Prod., Pine Bluff, Ark.) and a controlledrelease fertilizer (Osmocote 18N–2.6P–9.9K, 51 g/pot) on 17 Apr. 1998 and placed in ambient field conditions on the same day at Fayetteville, Ark. There were five replications per cultivar, with a block consisting of a single plant of each cultivar. They were drip irrigated at the rate of 4-L per hour for 1 h per day from 0800 to 0900 HR. Plants were also watered to saturation the night before measurements were taken. Beginning in June, leaf gas exchange measurements were undertaken. These consisted of CO2 assimilation (A), evapotranspiration (ET), and stomatal conductance (gs). On the morning of the day measurements were to be taken, one block of pots was transported to the Rosen Alternative Pest Control Center (APC) building and placed in a 70 × 105 × 70cm (height × width × depth) growth chamber (Conviron 3244, Winnipeg, Manitoba, Canada) at a randomly selected temperature of 20, 25, 30, or 35 °C. The plants were allowed to acclimate to the temperature for 1 h before any measurements were taken (Fernandez and Pritts, 1994; Moon et al., 1987). Lighting was provided inside the growth chamber, but humidity could not be controlled. A CIRAS-1 portable infrared gas analyzer (PP-Systems; Hitchens, Herts, U.K.) with a Parkinson leaf cuvette (2.5 cm) was used to take gas exchange measurements. The cuvette temperature was set at the same temperature as that of the growth chamber. Leaf and cuvette temperatures were maintained by a thermoconductive peltier plate and measured using an infrared temperature sensor. Photosynthetically active radiation (PAR) was set at 1500 μmol·m·s, considered a saturating light level for raspberries (Fernandez and Pritts, 1994; Hunt et al., 1991; Rom and Clark, 1991). The air flow rate inside the cuvette was set at 200 mL·min with 350 mL·L CO2 and 50% of ambient water vapor. Interest in red raspberry production has grown in the southern states in recent years (Clark and Rom, 1997). However, there are no commercially viable, southern-adapted cultivars available to satisfy the increasing demand (Moore, 1997). Most cultivars have been bred in cooler, more northerly areas and have limited adaptation in the South, where high temperatures pose serious problems. Additional obstacles are loss of hardiness due to fluctuating winter temperatures, disease, and high light intensities (Hull, 1961; Lawrence, 1980; Moore, 1997). Few studies on red raspberry have focused on the leaf gas exchange capabilities at high temperatures. Fernandez and Pritts (1994) observed that CO2 assimilation (A) declined in both floricane and primocane leaves of potted ‘Titan’ red raspberry as temperature increased, Table 1. Origin and parentage of cultivars used in this study. Cultivar Origin Parentage Arapaho Clarksville, Ark. Ark. 631 x Ark. 883 (Moore and Clark, 1993) Autumn Bliss East Malling, England Complex involving R. strigosus Michx., R. arcticus L., R. occidentalis L. and several red raspberry cultivars (Keep et al., 1984) Dormanred State College, Miss. R. parvifolius x Dorsett (Overcash, 1972) Heritage Geneva, N.Y. (Milton x Cuthbert) x Durham (Ourecky, 1969) Nova Kentville, Nova Scotia Southland x Boyne (Brooks and Olmo, 1983) Reveille College Park, Md. [VPI 1 (Indian Summer x Sunrise)] x September (Brooks and Olmo, 1968) Southland Raleigh, N.C. N.C. 237 x Md. S420-5 (Hull, 1969) Measurements were taken on five consecutive days during a 1-week period, with one block measured per day, during 23–27 June, 20–24 July, and 17–21 Aug. For each plant, data for three individual leaves on one primocane were averaged for each temperature. Leaves were ≈75% to 85% of full expansion, from the third to the seventh node. Measurements were taken between 0800 and 1230 HR to avoid any diurnal response, which has been reported in raspberry (Fernandez and Pritts, 1994; Hunt et al., 1991; Klauer et al., 1992). A total of three growth chambers were used: the first to allow the plants to acclimate to the desired temperature (1 h), the second to measure the gas exchange (one pot at a time), and the third to allow the plants to acclimate to the next randomly selected temperature to be measured. Once a plant was measured, it was immediately placed into the next growth chamber for acclimation. Plants were rotated until gas exchange had been measured at all temperatures for each cultivar, then the block of plants was taken back to the field. The design of the study was a randomized complete block with five replications per cultivar. The data were analyzed as a split-plot in time for A, ET, and gs. Fixed effects were cultivar, temperature, month, and all interactions involving these effects. Random effects were block, interactions involving block, and residual error. The data analysis was performed with the Statistical Analysis Systems Program (SAS Institute, 1989). Analysis of variance was calculated by the MIXED procedure and mean separation was by t test at P ≤ 0.01. Trend analysis was done using the contrast statement in the MIXED procedure.