We have set up a system for mixing produce in transport. The best description of it is found in Table 1 in our publication Refrigerated Trailer Transport of Perishable Products. Ordering information is on our web site. The table divides fruits and vegetables into four temperature ranges and three categories of ethylene sensitivity and ethylene production. Humidity is usually not an important issue in transportation because the time in transit is often short and humidity is not easily controlled in highway transport vehicles. If you are handling a product that loses too much moisture in transit, then it needs to be packaged in a liner or with ice to prevent excessive moisture loss. The table indicates that watermelon is ethylene sensitive and should be shipped at 45 - 50°F and grapefruit produces little or no ethylene and should be shipped at 55 - 60°F. This means that the products are ethylene compatible and should be shipped at a temperature between these two ranges. I would recommend 55°F.

-James F. Thompson