Root-zone temperature effects on the nutrient uptake of horticultural crops
Economically important temperate horticulture crops often are subject to root-zone temperatures (RZT) that exceed 30C, but little is known about the effects of supraoptimal RZT on nutrient absorption. In my first study, I determined the amount of phosphorus (P), zinc (Zn), and manganese (Mn) in tomato (Lycopersicon esculentum Mill. 'Jet Star'), muskmelon (Cucumis melo L. 'Gold Star'), and thornless honey locust (Gleditsia triacanthos L. var. inermis Willd.) plants grown in Hoagland's No. 1 solution that was held at 24, 27, 30, 33, and 36C RZT. In a second study, I monitored the changes in root respiration, root surface phosphatase activity, and P uptake of tomato plants grown in Hoagland's No. 1 solution held at 25 and 36C RZT for 19 days. Tomato dry mass and P, Zn, and Mn content were greatest within the RZT range of 24 to 30C RZT, whereas muskmelon dry mass and P, Zn, and Mn content were greatest at 36C RZT. Honey locust dry mass and P and Zn content did not vary with RZT, but honey locust Mn content decreased linearly with increasing RZT. Root respiration of tomato plants grown at 25C increased linearly over time, and respiration in tomato roots at 36C increased from RZT initiation to day 8 and then decreased. Tomato shoot P concentration and root phosphatase activity in plants at 25C did not change during the experiment. However, shoot P concentration in plants at 36C decreased linearly over time, whereas root phosphatase activity at 36C increased linearly for the duration of the experiment. In conclusion, growth and P, Zn, and Mn acquisition of 'Gold Star' muskmelon were increased in response to continuous exposure to RZT > 30C, whereas honey locust growth and nutrient acquisition were not affected by RZT > 30C. Growth and P, Zn, and Mn acquisition of 'Jet Star' tomato were decreased from continuous exposure to RZT > 30C. Increased tomato root respiration in response to 36C RZT appeared to deplete available metabolic energy for growth and nutrient acquisition. The decrease in respiration after 8 days also was probably in response to depletion of carbohydrates and/or inactivation of respiratory enzymes.