Hamido Said A, Morgan Kelly T
Southwest Florida Research and Education Center, University of Florida, Immokalee, FL 34142, USA.
Plants (Basel). 2020 Oct 29;9(11):1462. doi: 10.3390/plants9111462.
Citrus yields have declined by almost 56% since Huanglongbing (HLB) was first found in Florida (2005). That reduction forced citrus growers to replant trees at much higher densities to counter-balance tree loss. The current project aims to determine how much water is required to grow citrus trees at higher planting densities without reducing their productivity. The study was initiated in November 2017 on eight-month-old sweet orange () trees grafted on the 'US-897' (Cleopatra mandarin × Flying Dragon trifoliate orange) citrus rootstock planted in the University of Florida, Southwest Florida Research and Education Center (SWFREC) demonstration grove, in Immokalee, FL (lat. 26.42° N, long. 81.42° W). The soil in the grove is Immokalee fine sand (Sandy, siliceous, hyperthermic Arenic Alaquods). The demonstration grove included three densities on two rows of beds (447, 598, and 745 trees per ha) replicated four times each and three densities of three rows of beds (512, 717, 897 trees per ha) replicated six times. Each density treatment was irrigated at one of two irrigation rates (62% or 100%) during the first 15 months (2017-2019) then adjusted (2019-2020) to represent 26.5, 40.5, 53, and 81% based on recommended young citrus trees evapotranspiration (ETc). Tree growth measurements including trunk diameter, height, canopy volume, leaf area, and root development were evaluated. During the first year, reducing the irrigation rate from 100% to 62% ETc did not significantly reduce the young citrus tree growth. Conversely, the lower irrigation rate (62% ETc) had increased citrus tree's leaf area, canopy volume and tree heights, root lifespan, and root length by 4, 9, 1, 2, and 24% compared with the higher irrigation rate (100%), respectively. Furthermore, the root lifespan was promoted by increasing planting density. For instance, the average root lifespan increased by 12% when planting density increased from 447 to 897 trees per ha, indicating that planting young trees much closer to each other enhanced the root's longevity. However, when treatments were adjusted from April 2019 through June 2020, results changed. Increasing the irrigation rate from 26.5% to 81% ETc significantly enhanced the young citrus tree growth by increasing citrus tree's canopy volume (four fold), tree heights (29%), root lifespan (86%), and root length (two fold), respectively. Thus, the application of 81% ET irrigation rate in commercial citrus groves is more efficient for trees from two to four years of age.
自2005年佛罗里达州首次发现黄龙病（HLB）以来，柑橘产量下降了近56%。产量的下降迫使柑橘种植者以更高的密度重新种植树木，以抵消树木损失。当前项目旨在确定在不降低生产力的情况下，以更高种植密度种植柑橘树需要多少水分。该研究于2017年11月在佛罗里达大学西南佛罗里达研究与教育中心（SWFREC）位于伊莫卡利（佛罗里达州，北纬26.42°，西经81.42°）的示范果园中，对嫁接在“US - 897”（埃及酸橙×飞龙枳橙）柑橘砧木上的8个月大的甜橙树展开。果园土壤为伊莫卡利细沙（沙质、硅质、高温干旱老成土）。示范果园包括两行苗床的三种密度（每公顷447、598和745棵树），每种密度重复4次，以及三行苗床的三种密度（每公顷512、717、897棵树），每种密度重复6次。在最初的15个月（2017 - 2019年），每个密度处理以两种灌溉速率之一（62%或100%）进行灌溉，然后在2019 - 2020年根据推荐的幼龄柑橘树蒸散量（ETc）进行调整，分别代表26.5%、40.5%、53%和81%。对包括树干直径、高度、树冠体积、叶面积和根系发育在内的树木生长指标进行了评估。在第一年，将灌溉速率从100%降至62% ETc并未显著降低幼龄柑橘树的生长。相反，较低的灌溉速率（62% ETc）使柑橘树的叶面积、树冠体积和树高、根系寿命以及根长分别比高灌溉速率（100%）增加了4%、9%、1%、2%和24%。此外，增加种植密度可促进根系寿命。例如，当种植密度从每公顷447棵增加到897棵时，平均根系寿命增加了12%，这表明将幼树种植得彼此更靠近可延长根系寿命。然而，当在2019年4月至2020年6月期间对处理进行调整时，结果发生了变化。将灌溉速率从26.5%提高到81% ETc，通过分别增加柑橘树的树冠体积（四倍）、树高（29%）、根系寿命（86%）和根长（两倍），显著促进了幼龄柑橘树的生长。因此，在商业柑橘果园中，对两到四年树龄的树木采用81% ET的灌溉速率效率更高。
