Similarly, in the outer model, changes in MVs reflect a change in their LV, and thus connect the outer model with MVs to the inner model of LVs. For instance, the MV PC3 has a negative correlation with the LV leaf shape , so that as the value of PC3 decreases, it reflects as a corresponding increase in LV leaf shape . This change is represented as an increase in the roundness of the leaf. This then corresponds to a positive change in yield , which is in turn a reflection of fruit biomass . The model indicates that photosynthesis has a strong positive influence on both fruit BRIX and vegetative biomass but has a negative impact on fruit yield. As photosynthetic rates increase , fruit BRIX increases, but at the sacrifice of yield, an inverse relationship which has long been known . Leaf shape has a negative relationship with vegetative biomass, which corresponds to the decreased leaf complexity with the Potato Leaf Morph . However, leaf shape has a strong positive influence on both fruit BRIX and yield , suggesting that leaf shape influences fruit quality as seen previously by Chitwood et al. . The effect of leaf shape on fruit quality does not work through leaf sugar, as this correlation was not significant. Our leaf sugar measurements were completed in the glasshouse, owing to the complexity of the chemical analyses required, growing raspberries in container and as such the model was tested without leaf sugar. No significant causative relationship changes occurred in the model upon omitting the leaf sugar values.
While our work does not implicitly study mechanisms, the negative relationship between leaf sugar and fruit BRIX is of interest, and may provide some avenues for future research into the mechanisms underlying impact of leaf shapes on fruit quality in tomato. Fig. 6 displays the effect of each trait on the overall output of the plants . Leaf shape has no strong contribution to vegetative biomass. Although shape shows a negative relationship with biomass, this influence is minimal when compared with photosynthesis . However, leaf shape shows the largest influence on both yield and fruit BRIX, with photosynthesis second, and is the only positive contributor to yield . This positive correlation is from rounder, Potato Leaf Morph-like leaves, while narrower leaves have the opposite effect based on the PC contributions to leaf shape. The negative effect of photosynthesis on tomato fruit yield and the strong contribution of leaf shape to yield and BRIX are novel findings that run counter to the interpretation of fruit quality improvement, as increased photo assimilate should result in more available sucrose to stronger sinks such as fruit . To test the model performance we used PLSPREDICT on the entire heirloom dataset used to build the structural model. Table S8 shows the mean absolute percentage error and Q 2 value for the complete model. We also used part of the dataset that included ABC Potato Leaf and Aunt Ginny’s Purple in a similar analysis . The complete model has c. 20–30% error for each LV, which is expected given the diversity of genotypes in the dataset, with fruit weight giving the highest MAPE, at 93.2% . The Q 2 value for most variables is positive and shows that they have relevance in the predictive performance, with the exception of leaf sugar, which is slightly negative .
In the case of ABC Potato Leaf and Aunt Ginny’s Purple, two lines selected randomly to test the model on individual cultivars, a significant increase in Q 2 and decrease in MAPE is seen for all LVs except leaf sugar . This indicates that the model is substantially stronger in predictive performance for individual cultivars, but also predicts well with the complete model. To evaluate the predictive performance of our model on additional datasets, we used data from two other cultivars grown in the same field, M82 and ‘Lukullus’, that were not used to construct the model. PLSPREDICT was used in SMARTPLS 3.0, along with the structural model constructed using the heirloom cultivars, to test the model performance by use of training sets and hold out samples, both taken from the M82/’Lukullus’ dataset. By using the leaf shape PC values, we were able to compare the predicted mean values for the remaining MVs, or the predicted measured values, against the actual measured values and evaluate the relative performance of the model. Tables 2 and 3 show the results for M82 and ‘Lukullus’, respectively. PC values for leaf shape are not included as they are input variables and used for predicting the other values. For M82 the predicted median values compared with the actual median values showed under 1% difference for all except leaf complexity, which had a percentage difference of 8.42% . This indicates that the model was under predicting the leaf complexity of M82 by c. 8%. ‘Lukullus’-predicted values were also under 1% different, except for leaf complexity and stomatal conductance which varied by 2.56% and 1.31%, respectively .
In addition to the predicted values PLSPREDICT also tests the model performance and reports the root mean square error, mean absolute error, and MAPE for each of the MVs tested . The MAPE shows the accuracy of the predictions, with lower percentages representing better performance. Leaf complexity for both cultivars showed the largest MAPE values, 201.2% and 26.5% in M82 and ‘Lukullus’, respectively . The M82 MAPE indicates that the model does not predict leaf complexity well for mid-level complexities such as 18 but does improve at high-end leaf complexities near 40 . Most heirloom cultivars had low leaf complexities , potentially explaining the poor performance in predicting leaf complexity for M82. Contrary to previous findings , we found that leaf complexity does not impact yield or BRIX, and only impacts vegetative biomass, so this inaccuracy would only impact vegetative output predictions by the model. ‘Lukullus’ has indeterminate growth like the heirlooms analyzed here, but M82 is determinate; however, the predictive accuracy of the model was still good, indicating its usefulness in assessing field performance of other tomato cultivars.The primary focus of crop improvement has been on fruit traits and photosynthesis , with some studies focusing on how sugars are moved from source to sink. Despite heirloom varieties with the Potato Leaf Morph being prized for fruit quality by the gardening community, vegetative traits such as leaf shape have been relatively ignored in breeding efforts. In this study we investigated the role of leaf shape on fruit quality by measuring both input traits and output traits for 18 heirloom cultivars. All these cultivars were classified as Potato Leaf, but varied greatly in their leaf shapes, development, and fruit quality . We found that these lines do not vary significantly in overall photosynthetic capacity, or their usage of light when available , suggesting that the variation in BY among these cultivars was not a result of improved/decreased photosynthetic capacity. While our measurements for photosynthesis do not show significant difference when PAR is available, the PARi differed between cultivars based on their growth patterns . All cultivars exceeded 1200 lmols m2 s 1 of PARi but varied in the later weeks between 1200 and 2000 lmols m2 s 1 . Combining multiple complex physiological and morphological measurements into informative relationships has proven difficult and has limited our understanding of how these different traits impact each other . Focusing on any one part, such as photosynthesis or fruit sink strength, raspberry container size while providing improvements , occurs at the expense of a comprehensive understanding of the overall relationships between these traits. Analyzing the individual PCs revealed significant differences in leaf shape among the heirloom cultivars, with several having stronger Potato Leaf Morphs and higher BY values , with some correlation between these traits. Potential epidermal shape changes that could arise from leaf shape changes and that could influence yield would relate to stomatal number. A previous study in 2002 analyzed several tomato cultivars developmentally and histologically and found no real differences between these cultivars. This and another study in 2010 suggest that there are no gross anatomical differences between these tomato cultivars. We used PLS-PM to combine all these measured traits, using the modeled final harvest data as input to find causative relationships . Strong relationships among gas exchange, light, and photosynthesis were expected, along with a strong positive effect of photosynthesis on vegetative biomass .
Photosynthesis has a strong positive effect on fruit BRIX, both directly and indirectly . Increased photosynthesis results in lowered leaf sugar content, and a concomitant increase in fruit BRIX. It is possible that increased sugar production from photosynthesis results in higher rates of transport of sugars out of the leaves and into sinks. The mechanisms that regulate source–sink relations and sugar distribution are still not fully understood on a whole-plant physiological level ; however, based on our model, increased photosynthesis negatively impacts total yield . While photosynthesis does lead to increased sugar production and is shown in our model to drive higher sugar content within existing fruit, it does not provide a means to increase yield. Leaf shape, specifically rounder, less lobed leaves, has a positive effect on both fruit BRIX and yield . Of all the factors measured here, only leaf shape positively influenced yield, with other paths having negative influences . Rounder leaves still drive slightly increased photosynthesis indicated by the thin arrow , which results in increased fruit BRIX. This path should also result in decreased yield. However, leaf shape has a strong positive and direct correlation with yield that overcomes the negative impact of photosynthesis and leads to increased yield as well as BRIX . Conversely, with narrow leaflets there is a small negative impact on photosynthesis which should result in increased yield, but narrow leaves have a direct negative impact on yield which is stronger than the photosynthetic pathway . The strong causative relationship among leaf shape, fruit BRIX, and yield suggests that leaf shape impacts both high fruit BRIX and increased number of fruits, probably by modulating sugar distribution, therefore bypassing the direct impacts of photosynthesis itself . How leaf shape affects this distribution is unclear, as it does not act directly through leaf sugar content, or through strong regulation of photosynthesis to improve yield . A recent study looked at the diversity of leaf shape in sweet potato . Any correlations between leaf shape and yield traits in this species would be of interest and help to establish general principles. The whole-genome phylogenetic analysis of 23 tomato cultivars showed many of the Potato Leaf Morph cultivars were closely related to each other, with the exception of Brandywine, though it did not show the origin of the C-locus mutation . To address this and identify if this morphology was selected for in breeding, we performed PHYLONETWORKS analysis . This analysis showed several hybridizations between Potato Leaf Morph and nonPotato Leaf Morph cultivars, and probably a unique incidence of the C-locus mutation in Prudens Purple . PHYLONETWORKS analysis of chromosome 1, 6, and 12 specific common SNPs each showed unique hybridization events, distinct from those seen in the WGS analysis . The PHYLONETWORKS analysis suggests multiple hybridization events with Potato Leaf Morph-containing cultivars. Potato leaf cultivars have been suggested to increase disease resistance compared with regular leaf varieties and may have been selected for this reason or for other asyet-unknown benefits present. We have shown that leaf shape strongly impacts the overall fruit quality in tomato, with rounder, less lobed leaves giving rise to higher yield and higher fruit BRIX. Photosynthesis, surprisingly, has a negative impact on yield while still positively contributing to fruit BRIX. Using data from cultivars not included in making our path model, we also showed that the model has a strong predictive performance for linking leaf shape to BY and could be used to potentially predict the outputs of a cultivar using leaf shape data . Our work shows the importance of leaf shape to yield and BRIX across a wide array of genetic backgrounds, implicating leaf morphology in playing a significant and previously unidentified role in tomato fruit quality.Eighteen heirloom tomato varieties identified as having a range of fruit types, including cherry and beefsteak tomatoes, and several intermediate types, were analyzed. These tomato varieties also differed in fruit production timing from early to late, and the type of leaf morphology.