Belowground carbon expenditure has been difficult to measure, and even more so to predict. Root growth is influenced by both direct effects on plant metabolism and indirect effects of soil drying and interactions with soil organisms. In Concord grape, crop load may compete with root growth when soil moisture is not limiting. Root lifespan also is difficult to predict but some success can be achieved by taking a cost-benefit perspective to unraveling some of the processes affecting root lifespan. Root respiration is affected by both short- and long-term temperature conditions. Short-term studies suggested that elevated soil temperature may have direct effects on root respiration that follow a Q10 response of about 1.7 - 2. More recent work indicates that plant respiratory responses to soil temperature may be a function of plant growing temperature and soil moisture. Citrus and grape roots exposed to increasing temperatures above 20 °C for more than three days exhibited homeostatic respiratory responses, presumably to avoid excess carbohydrate metabolism. Moreover, citrus roots in dry soil exhibited no temperature response but maintain respiration at a stable basal rate.
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