ROOT BIOLOGY AND PHYSIOLOGY OF APPLE TREES AS AFFECTED BY FRUIT REMOVAL
Open Access
- Author:
- Lavely, Emily Kay
- Graduate Program:
- Horticulture
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 27, 2019
- Committee Members:
- Richard P Marini, Dissertation Advisor/Co-Advisor
David Eissenstat, Committee Chair/Co-Chair
Robert Michael Crassweller, Committee Member
David Eissenstat, Committee Member
Maria Del Mar Jimenez Gasco, Outside Member - Keywords:
- Malus x domestica
chemical thinning
carbohydrate reserves
starch
fine roots
arbuscular mycorrhizal fungi (AMF)
non-mycorrhizal fungi (NMF)
nutrient foraging
root diameter - Abstract:
- Abstract. In perennial horticultural crops, the balance between carbohydrate supply and demand is notable because of the high carbohydrate demand required for fruit development. Common management practices that manipulate aboveground growth such as fruit removal, are used to enhance fruit quality, return bloom, and ease of harvest. However, many of the underlying mechanisms behind above- and belowground physiological responses and effects on carbohydrate partitioning remain unknown. Moreover, although the fine root system is critical to water and nutrient acquisition, characterization of absorptive fine roots in mature woody plants is challenging. For apple (Malus x domestica Borkh.), crop load management with chemical thinners continues to be a challenge. Early season carbohydrate supply affects fruitlet competition and is assumed to influence efficacy of chemical thinners. The effect of carbohydrate reserves on early season glucose and starch concentrations and the response to chemical thinning were evaluated in mature ‘Golden Delicious’ apple trees on M.26, M.9, and G.16 rootstocks. In 2013, fruit removal at 29, 125, 154, and 182 days after full bloom was used to alter tree carbohydrate reserves (starch). In 2015, crop density of 2014 was used to alter tree carbohydrate reserves. In 2014, a water control and a chemical thinner of 0.95 L of Carbaryl 4L, 0.95 L Ultrafine oil, and Fruitone N (1-Napthaleneacetic acid) at 15 mg·L-1 in 379 L of water were applied to apple trees when fruitlets were 10 to 12 mm in diameter. In 2015, a water control and a chemical thinner of 0.95 L of Carbaryl 4L and Fruitone N (1-Napthaleneacetic acid) at 15 mg·L-1 in 379 L of water were applied to apple trees when fruitlets were 10 to 12 mm in diameter. In 2014 and 2015, shoots containing 1- and 2-year-old wood were sampled at different phenological stages from early spring dormancy to 10 days after thinner application to measure total nonstructural carbohydrate concentrations. Glucose and starch concentrations in December 2013 were not affected by fruit removal date, but total nonstructural carbohydrates declined from early spring dormancy to thinning time in 2014 and 2015. Fruit removal treatments in 2013 and crop density in 2014 had little effect on early season glucose and starch concentrations in 1- and 2-year-old wood in 2014 and 2015. In 2014, fruit set (fruit per 100 flower clusters) on non-thinned trees was positively and linearly related to glucose concentration in 2-year-old wood the day before thinning; however, fruit set on thinned trees was not related to early season glucose concentration. Fruit set was not related to early season starch concentration of 2-year-old wood across all sampling dates regardless of thinning. In 2015, fruit set was not related to glucose and starch concentrations at early season dormancy or thinning time regardless of thinning. Fruit set in 2015 was negatively and linearly related to crop density in 2014 for non-thinned and thinned trees. In addition to the high carbohydrate demand required for fruit development, apple trees allocate available carbohydrates for belowground processes such as nutrient foraging. Belowground processes such as root production and nutrient foraging require energy to take up nutrients and to support interactions with beneficial microbes such as mycorrhizal fungi. To investigate how carbohydrate availability affected nutrient foraging of ‘Golden Delicious’ trees with or without fruit, fine root and arbuscular mycorrhizal fungal growth was compared in unfertilized soil and localized nitrogen (N)-rich patches (containing inorganic or organic sources of N). Fruit removal enhanced root production compared to fruiting trees across all N treatments. For fruiting trees, roots proliferated more in the inorganic-N patch than in unfertilized soil or the organic-N patch. For non-fruiting trees, root proliferation was similar regardless of N addition. Arbuscular mycorrhizal extramatrical-hyphal biomass was not affected by fruit removal but was greater in the organic-N patch than in the inorganic-N patch or unfertilized soil. Arbuscular mycorrhizal fungal colonization of apple roots was modestly affected by fruit removal and N treatments, and non-mycorrhizal fungal colonization was unaffected by treatments. Apple trees may manipulate root foraging strategies more than mycorrhizal foraging if carbohydrate availability is limited by fruiting. Fine roots of woody plants have traditionally been classified using an often arbitrary diameter cutoff approach, such as less than 2 mm. This approach, however, commonly includes both non-woody and woody roots. Alternatively, a functionally based root-order approach can be used to identify absorptive fine roots. The utility of this approach was examined for the horticultural fruit and nut crops: apple (Malus x domestica Borkh), peach (Prunus persica), grape (Vitus vinifera), almond (Prunus dulcis), and citrus (Citrus x clementina). In addition, variation in first- and second-order roots (most distal) was characterized for a wide range of woody horticultural species (33 in total), as diameter variation among species could influence the utility of a diameter cutoff approach, and because diameter has been strongly linked to root function. First-order roots of grape and first- and second-order roots of apple and peach were consistently thin, non-woody, mycorrhizal, and had high N:C ratios. In contrast, fourth and fifth order roots of grape and fifth order roots of apple and peach were woody, non-mycorrhizal, had low N:C ratios, and were thicker than lower order roots. Among the 33 horticultural species, diameter of first- and second-order roots varied about 15-fold, ranging from 0.04 to 0.60 mm and 0.05 to 0.89 mm respectively. The weakness of an arbitrary diameter approach is reflected in comparing, for example, first-order roots of date palm (Phoenix dactylifera L.) and lemon (Citrus x limon L.), which had diameters as large as fourth-order roots of apple and peach. This research shows that root order characterization has considerably more utility than an arbitrary diameter approach in the identification of roots with different functions in perennial horticultural crops.