## 3 Results and Discussion  
### Accuracy of Gas Exchange Measurements on Excised Leaves  
Comparing accuracy of gas exchange measurements on field-grown excised and _in situ_ leaves indicated a potential 'window' in time where gas exchange in both the treatments remained statistically identical (Figure 1). At the point of excision (120 s), \(A_{\mathrm{n}}\) ranged from 14.8 \(\pm\) 1.1 \(\mathrm{\SIUnitSymbolMicro m}^{-2}\) s\({}^{-1}\) in Tempranillo to 18.3 \(\pm\) 0.6 \(\mathrm{\SIUnitSymbolMicro m}^{-2}\) s\({}^{-1}\) in Cabernet franc, respectively, while \(g_{\mathrm{s}}\) ranged from 0.34 \(\pm\) 0.03 \(\mathrm{m}\mathrm{s}\mathrm{m}^{-2}\) s\({}^{-1}\) in Grenache to 0.22 \(\pm\) 0.02 \(\mathrm{m}\mathrm{s}\mathrm{m}^{-2}\) s\({}^{-1}\) in Tempranillo, respectively (Figure 1). Under _in situ_ treatment, each cultivar maintained stable \(A_{\mathrm{n}}\) and \(g_{\mathrm{s}}\) throughout the measurement period (Figure 1). Cabernet franc maintained the greatest \(A_{\mathrm{n}}\) rates (17.9 \(\pm\) 0.6-18.7 \(\pm\) 0.6 \(\mathrm{\SIUnitSymbolMicro m}^{-2}\) s\({}^{-1}\)) under the _in situ_ treatment, while Tempranillo had the lowest \(A_{\mathrm{n}}\) rates (14.6 \(\pm\) 1.1-14.9 \(\pm\) 1.1 \(\mathrm{\SIUnitSymbolMicro m}^{-2}\) s\({}^{-1}\)) (Figure 1). Continued gas exchange measurements after leaf excision revealed a decrease in \(A_{\mathrm{n}}\) and \(g_{\mathrm{s}}\) in all cultivars following a standard sigmoid curve (Figure 1). However, across each cultivar, \(A_{\mathrm{n}}\) data suggest even after a minute post excision gas exchange can be recorded at a 96.4 to 99.5% accuracy compared to pre-excision. Cultivars showed differing lengths of time within which gas exchange could be accurately measured (Figure 1). While Grenache data indicate a wide window of 330 s post excision, Tempranillo data reveal a much narrower window of 180 s post excision (Figure 1). Remaining cultivars showed an intermediate time window between 210 and 240 s. A quadratic equation fit (R\({}^{2}\) > 0.99) between \(A_{\mathrm{n}}\) and \(g_{\mathrm{s}}\) post excision across cultivars indicates that closure of stomata directly results in reduction in photosynthetic CO\({}_{2}\) assimilation when leaves were detached from the plant (Figure 2). Although data suggest no significant drop in gas exchange within eachtime window, from the point of excision to the end of the window, there is a very apparent numerical decrease in leaf gas exchange rates. Hence, we suggest gas exchange on excised grape leaves be recorded within a maximum of 50% of the length of the time window. The duration of \(A_{\mathrm{n}}\) remaining stable post leaf excision has been observed in several ornamental tree species such as Shantung map (_Acer truncatum_), Mexican redbud (_Cercis canadensis_ var. _mexicana_), Texas redbud (_C. canadensis_ var. _texensis_), white Texas redbud (_C. canadensis_ var. _texensis_ cv Oklahoma), Chinquapin oak (_Quercus muehlenbergii_), and English oak (_Q. robur_) [3]. Relatively stable post excision \(A_{\mathrm{n}}\) values observed in every cultivar were likely due to maintenance of water status in leaf tissue [3; 12; 13; 14]. The rapid drop in \(A_{\mathrm{n}}\) past the post excision 'window' as observed in our study, which is consistent with previous findings [3; 14], indicates rapid stomata closure to prevent water loss from leaf tissue [13]. Stomatal conductance decreases quickly under rapidly decreasing leaf water status, thus restricting flow of CO\({}_{2}\) into leaf tissues, and resulting in a decrease in \(A_{\mathrm{n}}\)[31]. However, CO\({}_{2}\) assimilation may still continue as long as sufficient water is retained by leaf tissue after excision occurs [14]. Other related gas exchange parameters such as transpiration, leaf to air vapor pressure deficit, and leaf internal CO\({}_{2}\) partial pressure, showed similar trends to \(A_{\mathrm{n}}\) and \(g_{\mathrm{s}}\) (data not shown).