Water Relations and Net Carbon Assimilation Rate Response In Two Commonly Used Pecan Rootstocks

Pecan is an important nut crop grown throughout much of the southern U.S, with New Mexico, Georgia, and Texas ranked as the leading producers. The commercial pecan nursery industry currently relies on improved cultivars grafted on rootstocks derived from open-pollinated seeds. Seedstocks are chosen in relation to their geographical origin and general historical performance as rootstocks. Despite the fact that certain seedstocks are recommended more than others for particular soil or climate conditions, little is known about the physiology of known rootstock cultivars of pecan. An experiment was conducted to investigate the physiological differences between two open-pollinated pecan seedlings (Apache and Elliott) grown in field conditions.  Traditionally, ‘Apache' has been recommended for the dryer regions of the pecan producing area, whereas ‘Elliott' is more frequently used in the more humid southeast.  The study was conducted from May to September 2007 on rain-fed, six-year-old, open-pollinated ‘Elliott' and ‘Apache' seedlings. Leaf relative water content (RWC) and leaf water potential were measured throughout the season at predawn and midday. Single leaf net CO₂ assimilation rate (A) was measured with a portable infrared gas analyzer in May, June, August, and September at 1000 and 1500 HR. At 1000 HR the two rootstocks had similar A values, but at 1500 HR ‘Apache' A values were greater in May and lower in August and September than ‘Elliott'. Pressure-volume (P-V) curves were also performed four times during the season. Except for isolated exceptions, all parameters derived from the analysis of the P-V curves (modulus of elasticity, RWC at zero turgor, and osmotic potential at full turgor) were similar between the two rootstocks. A/C_i curves [net CO₂ assimilation rate (A) versus calculated internal CO₂ concentration, C_i] were performed once in September. The maximum velocity of RuBisCO for carboxilation, the rate of photosynthetic electron transport, the rate of use of triose phosphates, respiration, and mesophyll conductance were calculated and there were no significant differences between the two rootstocks in any of these parameters. Overall the results from the present investigation revealed that the known performance differences in phenotype performance of the two rootstocks did not translate into differences in gas exchange or water relations parameters. Further evaluation of pecan germplasm for useful sources as rootstock in diverse environmental conditions is warranted.