Water and irrigation
It’s about time: Using phosphorus efficiently
By Dr. Barry Shelp, William Sutton and Edward Flaherty
University of Guelph researchers reduced phosphorus use by 75 per cent in greenhouse chrysanthemum production, with no change to market quality.
By Dr. Barry Shelp, William Sutton and Edward Flaherty
For plants, phosphorus (P) is a crucial component of nucleic acids, phospholipids and intermediates in many metabolic pathways, and is involved in the control of cell signaling pathways in response to developmental and environmental cues. Inorganic P (Pi), taken up by plants in a form known as orthophosphate, is often the major nutrient limiting plant development and yield. In natural soils, Pi availability is generally low, depending on various factors such as fixation in the soil, leaching and conversion into organic forms that are not readily absorbed by plant roots. These same factors contribute to the inefficient use of applied fertilizer-Pi by agricultural crops, which can lead to overuse of fertilizer-Pi and the eutrophication of terrestrial and aquatic ecosystems. More efficient fertilization strategies and technologies are required to minimize environmental concerns.
Subirrigation systems were developed to reduce nutrient and water usage in greenhouse floriculture operations, but their utility could be further enhanced by the optimization of nutrient delivery. Recent research with chrysanthemum combined a moderate rate of nitrogen (N) or sulphur (S) supply during vegetative growth with the removal of the entire suite of nutrients during reproductive growth. The delivery of N and S, respectively, was reduced by approximately 75 and 87.5 per cent over the crop cycle, compared to industry standards, and without adverse effects on plant and flower quality.
Recently, we tested two methods for reducing Pi delivery to subirrigated, potted, disbudded chrysanthemums: (1) Pi removal during the reproductive growth stage; and (2) reduction of Pi rate during the vegetative growth stage. All the Pi regimens tested produced market quality plants, even when Pi delivered over the crop cycle was substantially less than typical industry recommendations. Together, these two methods offer a novel delivery strategy for efficiently reducing the Pi nutrition of chrysanthemum.
Disbudded, potted chrysanthemums were grown in a peat-perlite mix under greenhouse conditions using two cultivars, ‘Olympia’ and ‘Covington,’ and four different Pi treatment rates, each replicated in four different areas of the greenhouse. One treatment was similar to the industry standard, where 80 ppm Pi was supplied as a complete nutrient solution over both vegetative and reproductive growth stages (labelled as 80VR). The three remaining experimental treatments, 80, 60 and 40 ppm Pi, were supplied in an otherwise complete nutrient solution until bud crack (i.e., the vegetative stage only), and then replaced with deionized water thereafter (labelled as 80V, 60V and 40V, respectively). Some plants were harvested at bud crack (i.e., early flowering) and again when the flowers were fully expanded (i.e., late flowering, 8 weeks after short days were imposed to induce flowering), and assessed for visual quality, plant characteristics, and tissue-P in various plant parts.
FLOWER QUALITY UNCHANGED
Being distinct cultivars, the morphological characteristics (i.e., shoot length, bloom diameter and dry mass yields) differed between ‘Olympia’ and ‘Covington’. However, neither cultivar was affected by removing Pi during the reproductive phase and/or reducing Pi during the vegetative stage. Market quality flowers were produced without any decrease in bloom diameter or visible symptoms of P deficiency on either leaves or flowers (Fig. 1), even when given the lowest Pi treatment of 40 ppm Pi during the vegetative stage, and zero Pi during the reproductive stage (40V). Notably, ‘Olympia’ had much lower levels of leaf-P than ‘Covington’ across all Pi treatments, but the acquisition of P was similar in 40V ‘Olympia’ and 60V ‘Covington.’ Based on literature values for tissue-P in a diagnostic leaf during early flowering, these findings indicate that none of the Pi regimens limited plant development and growth, and that ‘Olympia’ utilized the available Pi more efficiently than ‘Covington’ over the crop cycle.
PLANT CHANGES PHOSPHORUS UPTAKE AND UTILIZATION
Phosphorus can be stored, primarily in the leaves and stems, during the vegetative stage, then drawn from during the reproductive stage when root uptake is relatively poor to satisfy some of the P demands for flower and seed development. P deficiency typically promotes the movement of Pi into and through the roots, as well as the release of P stored either as Pi or organic forms, for metabolism and/or transport to other areas of the plant as needed.
To better understand how chrysanthemums were able to compensate for the different Pi treatments, we estimated the actual Pi inputs delivered with each treatment in the nutrient solution over the crop cycle, and tracked three measures of P use in the plant:
- Input-based P-use efficiency – A measure of how efficiently the plant used up the applied P (PUEi = shoot dry weight divided by weight of Pi input);
- Input-based P-uptake efficiency – A measure of how efficiently the plant took up the applied P through its roots (PUpEi = shoot P content divided by weight of Pi input);
- Content-based P-use efficiency: A measure of how efficiently the plant used its stored P in the shoots (PUEc = shoot dry weight divided by shoot P content).
All measures increased in both cultivars with decreasing Pi delivery, indicating that the plant compensated for lower P applied using all three methods above. The primary strategy for improving P use efficiency in the chrysanthemum plant was increased uptake efficiency (Fig. 2). This could be explained by stimulation of multiple transport proteins that mediate the movement of Pi from the soil into and through the root, driven by a reduction in Pi availability. These findings confirm that ‘Olympia’ has greater internal utilization efficiency than ‘Covington’ over the crop cycle, especially when one considers that 40V ‘Olympia’ and 60V ‘Covington’ acquired the same amount of Pi.
FROM RESEARCH TO APPLICATION
From the differences in biomass yields, tissue-P concentrations, and content-based P-use efficiency with constant Pi acquisition, it is clear that ‘Olympia’ has greater P-utilization efficiency than ‘Covington’ when Pi availability is non-limiting. Other research has established that stored organic-P, and to a lesser extent Pi, are more important sources of P for flower development in ‘Covington’ than in ‘Olympia’, likely due to the higher tissue-P levels at flower emergence. Despite their differences, both cultivars produced market quality plants and flowers, even when Pi delivery over the crop cycle was reduced to approximately 25 per cent of typical industry recommendations.
The success of our modified subirrigation practice (i.e., using moderate levels of a specific nutrient during vegetative growth combined with removal of all nutrients during reproductive growth) was previously demonstrated with N and S. We are continuing the study of the remaining macronutrients (i.e., potassium, calcium and magnesium), as well as the micronutrients (i.e., zinc, copper, iron, manganese, boron and molybdenum), in order to develop a complete, optimized nutrient regimen, which will then be validated in a commercial setting. Since there does not appear to be cultivar-specific responses in chrysanthemum to the modified management practice, it should be readily adoptable by growers. In the longer term, the modified practice could be applied to other agriculturally important, indoor-grown crops.
Acknowledgements: This research was funded by the Canadian Ornamental Horticulture Alliance and the Ontario Ministry of Agriculture, Food and Rural Affairs through the OMAFRA-University of Guelph partnership. Thanks to Berger for providing the growing medium.
For more details, see the article,“Strategic timing and rate of phosphorus fertilization improves phosphorus use efficiency in two contrasting cultivars of subirrigated greenhouse-grown chrysanthemum”, which is published in the Canadian Journal of Plant Science (DOI: 10.1139/CJPS-2019-0173).
For more background, see the articles, “Moving towards low-input floricultural operations”, and “Managing sulphur nutrition of sub-irrigated greenhouse-grown chrysanthemum”, which are, respectively, published in the December 2018 and July 2019 issues of Greenhouse Canada magazine.
Barry Shelp, William Sutton and Edward Flaherty are Professor Emeritus, graduate student and research technician, respectively, in the Department of Plant Agriculture at the University of Guelph. For more information, please contact Dr. Shelp at 518-824-4120 ext. 53089, or firstname.lastname@example.org.