In a previous project’s news (November 2023) we emphasized that a great deal of plant-plant interactions in intercropping take place in the hidden root zone, and presented a rather simple phenotyping pipeline to measure root traits based on rhizoboxes. These boxes have one transparent wall enabling us to observe the root growth in time and can be unscrewed at the end of the assessment period to observe and recover the undisturbed whole root system.
We then used the described rhizobox pipeline to test two white lupin genotypes of known contrasting morphology (one being taller and more vigorous than the other in field conditions) grown either as sole crops in the box or associated with a bread wheat genotype. The experiment was carried out for three weeks in a growth chamber mimicking early-spring conditions in terms of temperature and daylight duration (Fig. 1). While still waiting for the completion of image analyses of scans and photos taken on each rhizobox to measure traits such as visible and total root length, we can anticipate some features of the lupin genotypes in the early growth phases when intercropped with wheat or grown by themselves.
Figure 1
In parallel, we also set up a field experiment (Fig. 2) including five lupin genotypes (the two evaluated in rhizoboxes and three more) and the same wheat genotype, grown either in binary intercropping (lupin-wheat) in alternate rows (Fig. 3) or as sole crops. In this trial, a recent, simple root phenotyping method called shovelomics (Trachsel et al., 2011; Burridge et al., 2016) was applied, which involves the excavation and washing of blocks of soil within which the root system to be analyzed resides (Fig. 4). The method then allows a standardized analysis by a description and quantification on a dedicated board of key root characters, such as the gravitropic angle, the density of lateral roots and, in the case of legumes, the nodulation and the taproot diameter (Fig. 5). To our knowledge, this was the first application of shovelomics to intercropping.
Figure 2
In the rhizobox trial, the more vigorous lupin genotype (coded as 5/23) seemed to take advantage of the association with wheat in the early development phases, showing 10% higher dry weight per plant, and 6% greater number of fully expanded leaves in intercropping than in the sole crop. The other genotype showed instead a 2% decrease for both traits passing from the pure stand to the intercropping. Both genotypes had a larger area explored by roots when intercropped with wheat than when in pure stand, with a 21% area increase for the more vigorous genotype, and 14% for the other. Interestingly, the roots of both genotypes overlapped extensively with the widespread root system of the wheat companion when intercropped in the rhizobox (over 86% of lupin root area overlapping with that of wheat), whereas the proportion of root overlapping with the neighbor was much lower (40-50%) in the lupin pure stands. Any possible facilitation effect exerted by wheat on lupin remains to be proven.
Figure 3
The field trial indicated that intercropping reduced the lupin dry weight per plant (–41%) at an adult stage (late flowering time), thus partly disagreeing with the observation made in the rhizobox trial for the more vigorous genotype at an early stage (although the two trials were not directly comparable in terms of plant development and conditions). However, the vigorous genotype 5/23 was the least affected by the plant weight reduction in intercropping (–27%), confirming its possible advantage when associated with wheat. The gravitropic angle measuring the root growth in the inter-row showed that lupin roots tended to grow more towards wheat roots in intercropping than towards lupin roots in pure stand (8% wider angle in intercropping). The two genotypes with narrower angles when grown in pure stands were those with greater increase of the angle when intercropped with wheat (+46% e +20%). The taproot diameter was affected by the association with wheat (–22% along the taproot length compared with the sole crop), but the vigorous genotype 5/23 had the least reduction (–6%).
Figure 4
In conclusion: (i) the two phenotyping methods proved to be manageable and useful to record several root traits; (ii) white lupin did not shy away from wheat at the root level but, rather, it seemed to look for the cereal company; and (iii) there seemed to be enough genetic variation in lupin as to enable finding types more promising than others for intercropping.
Figure 5
This news item was written by Luciano Pecetti, Filippo Rossi, Tommaso Notario, Daniele Cavalli, Roberta Rossi (CREA)
Bibliography
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Burridge J., Jochua C.N., Bucksch A., Jonathan P. Lynch J.P. Legume shovelomics: High-Throughput phenotyping of common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata subsp, unguiculata) root architecture in the field. Field Crops Research (2016) 192: 21–32.