Arbuscule frequency in grapevine roots is more responsive to reduction in photosynthetic capacity than to increased levels of shoot phosphorus

Authors: Schreiner, Roger - Paul; Scagel, Carolyn

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2016
Publication Date: 3/1/2016
Citation: Schreiner, R.P., Scagel, C.F. 2016. Arbuscule frequency in grapevine roots is more responsive to reduction in photosynthetic capacity than to increased levels of shoot phosphorus. Journal of the American Society for Horticultural Science. 141(2):151-161.

Interpretive Summary: The symbiosis between plants and mycorrhizal fungi can improve yield and quality of numerous agronomic and horticultural crops. Optimizing the benefits from these fungi requires knowledge of how the environment can impact the efficiency of the symbiosis. The impact of defoliation, shading, and application of phosphorus fertilizer to the foliage of grapevines on the extent of fine roots that contained specialized symbiotic structures known as arbuscules was studied to better understand how plant carbon and plant phosphorus status regulates the activity of mycorrhizal fungi. Results from two experiments showed that reducing plant carbon fixation had an immediate and reversible impact on arbuscules in roots, whereas boosting shoot phosphorus only altered arbuscules after a prolonged period of time. In addition, fertilizing grapevines with foliar phosphorus fertilizer reduced subsequent uptake of copper. These findings indicate that carbon flow to roots is a more significant regulator of arbuscules in roots than is high plant phosphorus, and suggests that grapevines continue to support nutrient uptake from mycorrhizal fungi even when phosphorus is high, but lose this capacity when light levels are too low.

Technical Abstract: We evaluated whether altering photosynthetic capacity or shoot P plays bigger role in regulating arbuscule abundance in fine roots of grapevine. Pinot noir grapevines were grown in an unsterilized vineyard soil and colonized by indigenous arbuscular mycorrhizal fungi (AMF) in two experiments where photosynthetic capacity (defoliation or shading) and shoot nutrition (foliar fertilizer) were manipulated. Changes in root colonization by AMF and plant growth and nutrition were determined. Decreased photosynthetic capacity of shoots due to defoliation or shading rapidly reduced arbuscules in fine roots, while a ~ 3 fold boost in shoot P status from foliar fertilizer only reduced arbuscules after a longer time. The combination of shading and foliar P reduced arbuscules more so than shading alone, while in full sun foliar P did not reduce arbuscules. Returning plants to full sun after 28 d in shade resulted in a resurgence of arbuscules in roots regardless of plant P-status. Arbuscules in grapevine roots are regulated by the interaction between plant C and P status, such that high shoot P is less inhibitory to arbuscule formation or maintenance when plant photosynthesis is not limited. This indicates that grapevines do not shut down AMF nutrient transfer even when shoot P is high unless the relative cost of doing so becomes a more significant proportion of the plant’s carbon economy.