 Conceptually an upper cut-off would be applied if the plant system stopped being photosynthetically active at some point due to heat stress from high temperatures.While this may be proven for some crops, there is not a universal number for an upper threshold for grapes so the majority of the published data for comparison purposes in viticulture and wine production does not limit maximum temperatures.This issue is problematic because many weather stations today have integrated the corn growing degree-day method in their software.