## 2 Materials and Methods  
### Field Experiment Testing Accuracy of Gas Exchange Measurement on Excised Grape Leaves  
Research comparing gas exchange of excised and _in situ_ grape leaves was conducted in 2013 and 2014 at Texas A&M AgriLife Research and Extension Center vineyard in Lubbock, TX (33\({}^{\circ}\)41\({}^{\prime}\)33\({}^{\prime}\) N; 101\({}^{\circ}\)49\({}^{\prime}\)17\({}^{\prime\prime}\) W). The vineyard was established in 2004 and has a north-south row orientation with 1.8 m \(\times\) 3.0 m (vine \(\times\) row) spacing. Vines were spravel trained with bilateral cordons (height of 90 cm), and spur pruned to four spurs per cordon and two buds per spur [24]. Soil at the vineyard site was a deep, well drained Olton series fine sandy loam, which has been described previously [25]. Five cultivars were included (Cabernet franc, Cabernet Sauvignon, Chardonnay, Grenache, and Tempranillo), which were bench grafted onto 110R rootstocks. All vines used for excised and non-excised gas exchange measurements were well watered, and managed according to viticultural practices common to the region [26].  
Leaf gas exchange measurements were made on 1 and 8 August 2013 and 30 May and 3 June 2014. Data collection was conducted on cloudless days. Mid-day (\(\pm\)1 h of solar noon) leaf stomatal conductance (\(g_{\text{s}}\)), and net photosynthetic rate (\(A_{\text{n}}\)) were measured using two LI-6400 XT systems. Each machine was equipped with a 6400-02B red/blue external LED light source, a CO\({}_{2}\) mixer, and a leaf cuvette attached to a tripod. To simulate environmental growing conditions during measurements, light intensity within each LI-6400 XT chamber was maintained at ambient levels, which ranged between 1500 and 2000 \(\upmu\)mol/m\({}^{2}\)/s over the measurement period, representative of a light-saturated photosynthetic state [27]. Chamber CO\({}_{2}\) was sustained at 400 \(\upmu\)L/L. Flow rate was set at 500 \(\upmu\)mol/s. To measure leaf temperature and ambient relative humidity, the leaf cuvette was clamped to a nearby non-sample leaf prior to and several times during daily measurement periods. Conditionswithin each cuvette were then set to closely represent these conditions [28]. In all cases leaf temperature was recorded between 33 and 35 \({}^{\circ}\)C. All leaves measured were fully exposed, mature, and selected from positions seven to nine from the shoot tip [29].  
Experimental design for each cultivar was a randomized complete block/paired with the two treatments, excised and _in situ_, and 16 blocks. The block factor of the design was represented by the measurement time, with 10 replicate times in 2013 and 6 replicate times in 2014. At each measurement time (replicate), gas exchange of two leaves exposed to full sunlight in close proximity on the same shoot of an individual vine were measured simultaneously. One of the leaves, selected randomly, was excised during the measurement period. At the initiation of the measurement, leaf chamber conditions were set to identical settings. Cuvettes were clamped onto leaves at the same time, and gas exchange parameters were allowed to reach a steady state. Once equilibrium was achieved in each cuvette, an auto program recorded gas exchange measurements every 30 s, and ran for a total of 10 min. After 120 s, the petiole of the leaf randomly assigned to the excised treatment was cut just above its subtending node (a minimum 2.5 cm of petiole remained attached to the leaf). On each measurement date, once the auto program was complete for one set of leaves, LI-6400 XT machines were moved to a random vine of another cultivar and the process was replicated. This process was repeated during each measurement date for each cultivar over the course of the two experiment years. Therefore, sixteen simultaneous measurements of leaf gas exchange were recorded for excised and _in situ_ leaves for each of the five cultivars.