Daily light integral is a measure of the total photosynthetic photon flux (PPF) density delivered over the course of 1 day. This measurement has proven to be an extremely useful tool for assessing the irradiance delivered to various horticultural crops, and thus has become widely adopted by professional horticulturists. A review of the recent academic research describing plant responses to DLI is presented in the Discussion section.
Published articles have used various terminologies to describe DLI, such as photosynthetic DLI, average daily PPF, daily irradiance, and daily PAR integral. These terms are synonymous; therefore, we propose that the term DLI, by itself, adequately describes the daily accumulated photosynthetically available number of photons delivered to a given area over the course of 1 day. Predicating DLI with the terms mean or average may be appropriate for studies using solar radiation as the light source because the DLI value may vary widely during the experimental period. The concept of critical DLI was introduced in the 1980s to describe the minimum DLI required for flower initiation, development, or both. Armitage et al. (1981) reported that seed geraniums required 1.94 mol·m−2·d−1 for flower initiation and 3.25 mol·m−2·d−1 for flower development. More recently, Christiaens et al. (2014) used the term minimum DLI to describe the DLI required to maintain a steady state between photosynthesis and respiration, i.e., the light compensation point was expressed as a function of DLI rather than as the usual PPF. This value was estimated to be 1.7–2.1 mol·m−2·d−1 for two azalea (Rhododendron simsii) cultivars, whereas the critical DLI for good quality post-production flowering quality was 3.3 mol·m−2·d−1. Meanwhile, Garland et al. (2010) used the term minimum DLI to describe the lowest light level needed to produce high-quality plants, which they determined to be 10.0 mol·m−2·d−1 for coleus (Solenostemon scutellarioides).
New terms have been introduced with the potential to enhance the value and meaning of DLI measurements. Gent (2014) presented hydroponic lettuce data using a normalized daily irradiance measurement, i.e., DLI expressed on leaf area rather than on ground area. This technique accounted for the effects of plant size on tissue concentrations of various plant metabolites. At any given DLI delivered based on ground area, smaller plants with a lower leaf area index will have a greater light intensity delivered per unit leaf area and concomitantly higher sugar contents. Therefore, higher DLIs (based on ground area) are required for larger plants to deliver an equivalent normalized DLI to smaller plants, i.e., the normalized DLI measurement compensates for changes in leaf area over time. Kjaer et al. (2012) introduced the term daily photosynthesis integral which also expresses DLI on a leaf area basis and thus is synonymous with normalized DLI. Kjaer et al. (2012) also introduced the term cumulative DLI which is equivalent to how the term degree-day represents the accumulated thermal time. This cumulative DLI concept has merit when a mean DLI provided over a long duration of time may not accurately represent the light environment due to seasonal fluctuations.
Irradiance response group is a term introduced to describe flowering responses to DLI (Erwin and Warner, 2002; Warner and Erwin, 2003). Two irradiance response groups were introduced: facultative irradiance and irradiance indifference. Facultative irradiance describes responses in which flower initiation occurs at an earlier developmental stage, i.e., lower node position, in response to a higher DLI. Plant species in this category demonstrate a reduced juvenile period under higher DLI. The irradiance indifference response occurs when a species exhibits no reduction in node number below the flower or inflorescence in response to DLI.
Daily light integral maps of the 48 contiguous states were first published in 2002 (Korczynski et al., 2002). At this time, DLI measurements were not widely used among professional horticulturists. However, the availability of affordable sensors, extension outreach (Torres and Lopez, 2010) and educational materials (Faust, 2012), and educational programs for professional horticulturists has resulted in rapid acceptance of the DLI concept and integration of this measurement into the horticultural jargon. The original DLI maps served as a valuable resource to educators in communicating the importance of DLI measurements in horticulture production systems. Today, growers frequently reference DLI measurements when conversing about environmental conditions and plant responses to those environments. This growth in knowledge has contributed to improved management of the light environment in the commercial production of a wide range of horticultural crops (Faust, 2012).
The DLI maps created in 2002 used spatial interpolation of solar radiation for 237 observation sites unequally distributed throughout the lower 48 states, or ≈34,000 km2 per site. Solar radiation was directly measured with radiation sensors and was likely very accurate because global horizontal incidence measurement accuracy is estimated at 5% when directly measured; however, many stations had missing data and several different models were used to fill in the gaps (Marion and Wilcox, 1994). Also, spatial interpolation using a limited number of sites added a higher degree of inaccuracy to these maps. Data for Hawaii and Alaska were not available at that time. Because the original maps were published, updated, high-resolution data have been released from the National Renewable Energy Laboratory (Renewable Resource Data Center, 2018). Data for the contiguous 48 states and Hawaii are now available in 100 km2 resolution from 1998 to 2009, whereas Alaska data are available in 1600 km2 sections. These improvements in the available data justified recreating DLI maps for the United States. The objectives of this project were 1) to provide updated monthly DLI maps and 2) to review the research literature concerning plant growth and flowering responses of horticultural crops published since the original DLI maps were created.
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