Temperature Influence on Potato Leaf and Branch Distribution and on Canopy Photosynthetic Rate

Mature potato (Solanum tuberosum L. cv. Kennebec) canopies are composed of leaves originating frommainand axillary-stem branches. Canopy leaf distribution and its corresponding contribution to wholecanopy photosynthetic rates have not been quantified. An experiment using SPAR (Soil–Plant–Atmosphere–Research) chambers maintained at 16-h day/night thermoperiods of 14/10, 17/12, 20/15, 23/18, 28/23, and 34/29 C was conducted. Mature canopies were divided into three horizontal layers of equal depth. Canopies were defoliated at each layer, from the ground upward, on successive days. Response curves for photosynthetic rate vs. irradiance were obtained after each defoliation. Leaf area within each layer followed a quadratic relationship with temperature. The largest areas were between 16.6 and 22.1 C. Main-stem leaves accounted for .50% of the total leaf area at temperatures,22 C, while the proportion of axillary-stem leaf area in each layer increased with temperature. Canopymaximum gross photosynthetic rates, AMAX, before harvest ranged from 9.5 to 34.8 mmol CO2 m 22 s (production-area basis) and were higher at 14/10, 17/12, and 20/15 C temperatures than at 23/18, 28/23, and 34/29 C. These values were largely related to the quantity of leaf area in each chamber. The value of AMAX and canopy light use efficiency declined as successive canopy layers were removed, primarily due to decreases in canopy light interception. These results indicate that the relative proportion of mainor axillary-stem leaves are not as important for potato canopy modeling considerations as is the need to simulate the correct quantity of leaf area. TEMPERATURE profoundly influences the growth and development of the potato canopy. Leaf appearance, expansion, and senescence (Kirk and Marshall, 1992; van Delden et al., 2001; Vos, 1995), leaf orientation and physiological age (Ng and Loomis, 1984; Steward et al., 1981), and stem elongation and branching (Allen and Scott, 1980; Marinus and Bodlaender, 1975; Struik et al., 1989) are significantly correlated with temperature. The leaf-level photosynthetic rate also varies with temperature; however, few whole-canopy gas exchange studies have been conducted (Hammes and De Jager, 1990; Ku et al., 1977; Prange et al., 1990; Thornton et al., 1996). Most potato models (e.g., SUBSTOR and LINTULPOTATO) represent the canopy as a single large stem and homogenous leaf layer (e.g., IBSNAT, 1993; Kooman and Haverkort, 1995; Shaykewich et al., 1998). Increases in canopy leaf area are simulated as an exponential or nonlinear function of temperature. Canopy leaf area is used to estimate the interception of PAR (photosynthetically active radiation). Increases in plant mass (grams per plant) are computed by multiplying light interception by a constant value for radiation use efficiency (grams of biomass per megajoule of intercepted PAR; e.g., IBSNAT, 1993; Kooman and Haverkort, 1995; Shaykewich et al., 1998). Potato models can be improved by including more detailed canopy responses to temperature (Vos, 1995). More sophisticated modeling approaches that estimate canopy photosynthetic rate by integrating gas exchange from different leaf layers in the canopy have been developed to improve accuracy in other crop models (e.g., Boote and Pickering, 1994). Knowledge of potato leaf and branch distribution at different canopy depths and their contribution to plant growth rate is needed to adopt these approaches for potato. Potato is an indeterminate crop with regard to its growth habit (Allen and Scott, 1980; Ewing, 1997; Vos, 1995); vegetative growth can continue well after floral and tuber initiation. Potato main stems terminate in an inflorescence, at which point typically two apical, or upper, axillary stems develop from the axils of the second and third leaf below the inflorescence. Basal axillary stems can also emerge between the axils of lower leaves on the main stem. Basal and apical axillary stems also terminate in an inflorescence and may give rise to additional lateral branches, depending on cultivar, planting density, plant assimilate supply, soil nutrition, and environmental conditions (Steward et al., 1981; Vos, 1995; Vos and Biemond, 1992). Thus, mature potato canopies are composed of leaves and branches that can be classified as main, basal, or apical (e.g., main-stem branch and main-stem leaves, basal-stem branch and basal-stem leaves, apical-stem branch and apical-stem leaves). The manner in which potato branches and leaves are distributed throughout the canopy and their corresponding contribution to photosynthetic rate has not been quantified. Air temperatures at 238C and above increase the number of axillary branches and the leaf appearance and senescence rates (Manrique et al., 1989; Marinus and Bodlaender, 1975). Cooler temperatures promote lower total leaf and branch numbers, but produce larger leaves that remain photosynthetically active for longer periods of time (Benoit et al., 1986; Manrique et al., 1989; Marinus and Bodlaender, 1975; Wolf et al., 1990). Growth temperatures.258C produce plants with elongated stems, smaller leaves, increased internode number, and inhibited tuber development (Borah and Milthorpe, 1962; Steward et al., 1981; Struik et al., 1989). Optimum temUSDA-ARS Crop Systems and Global Change Lab., 10300 Baltimore Ave., Beltsville, MD 20705. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply the exclusion of other available products. Received 1 Dec. 2006. *Corresponding author (dfleishe@ asrr.arsusda.gov). Published in Agron. J. 98:1442–1452 (2006). Potato doi:10.2134/agronj2005.0322 a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: IPAR, intercepted photosynthetically active radiation; PAR, photosynthetically active radiation; PPF, photosynthetic photon flux; SPAR, Soil–Plant–Atmosphere–Research. R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d . 1442 Published online October 3, 2006