Traditional varieties of tomato (Lycopersicon esculentum Mill.) possess greater sources than sink because they are leafy. Greater source capacity leads to poor crop performance as fertilization and other cultural practices result in greater foliage and poor productivity [1]. It means instead of large physical dimensions of the sources, optimum and more stable functional efficiency at moderate source size are more advantageous to realize the potential sink size under field conditions. Even increased LAI is not associated with increased fruit production but reaches a plateau [2]. In some situations, physical leaf area is adequate and even more than required, but the functional efficiency is far lower due to utilizing resources as a respiratory burden of those parasitic leaves (leaves in the lower canopy) on the other [3]. Defoliation up to certain limit may, therefore, be useful to overcome this problem of excessive vegetative growth. Greater light penetration in the canopy through defoliation may reduce the abortion of flowers and increase fruit yield [2, 4, 5, 6]. Andriolo et al. [7] suggested that for commercial purposes, should practice leaf pruning simultaneously to maximize light interception and fruit yield.

The effect of manipulation of source (leaf) size in field crops has been studied and reported both advantageous and disadvantageous effect of defoliation in many crops [8, 9, 10, 11, 12]. For example, one-third leaf removal from basal portion of the canopy in tomato increased fruit yield over control and severe defoliation decreased fruit yield [2, 6]. Similarly, mild defoliations (16.6-33%) during reproductive phase do not adversely affect the seed yield in mungbean [11] and in soybean [13]. On the other hand, reverse results due to defoliation was also reported in mungbean [14] and in soybean [15]. No detail information is available about source-sink relationships under discriminated levels in tomato during early reproductive growth stage. These aspects need investigation in tomato genotypes to develop the high yielding variety and to assist in the development of practices under Sub-tropical condition.

In tropical and sub-tropical countries, loss of foliage in tomato by leaf-eating insects and diseases is common. The tomato plant can sustain such source (leaf) damages up to a certain extent without significant yield loss [4]. Tomato crop is vulnerable to be infected by bacterial, viral, nematode and fungal diseases. Among the fungal diseases, early leaf blight of tomato caused by Alternaria solani is the worst damaging one that cause reduction in quantity and quality of the tomato crop [16]. The pathogen causes infection first on old lower leaves, then spread on upper leaves, stem, petiole, twig and fruits as well as leads to the defoliation, drying of twigs and premature fruit drop which ultimately reduce the yield [17]. Primary methods of controlling Alternaria leaf blight is application of mancozeb. Unplanned and wide use of fungicides often leads to serious environmental problems besides affecting the health of users and consumers. So, it is necessary to minimize the use of chemicals for controlling disease.

In our earlier study, we observed that defoliation of lower leaves at flowering stage did not affect the rest leaves by A. solani. Anyway, removal of full-grown leaves from below is common practice in tomato cultivation. The main reasons for leaf removal are prevention of diseases, obtaining faster fruit ripening and easier harvest as trusses are no longer hidden by leaves. Old leaves are also believed not to contribute to the crop photosynthesis anymore [2]. This favours dry matter partitioning towards the fruits [6].

The purpose of this study was to investigate the extent to which and what portion of leaf removal during the beginning of reproductive phase affects physiological and biochemical parameters thereby fruit yield under field condition and to identify the yield components responsible for yield reduction in tomato.

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Results indicated that at harvest, tomato plant compensated its leaf loss. Losses of 3 and 6 leaves plant-1 at flowering stage, which was equivalent to 18 and 36% leaf loss of the total, compensated the leaf loss fully even some times greater that control, whereas leaves loss of 9 and 12 plant-1 compensated up to 90 and 82%, respectively, due to regrowth of leaves. This result indicates that tomato plant has high compensatory capacity of leaf loss during flowering. The result is consistent with the findings of Fukuchi et al. [23], who reported that leaf number did not decreased at harvest due to partial defoliation in tomato. Andnolo et al. [5] reported that tomato plant had high compensatory capacity of leaf loss at early growth stages that supported the present results. Further, removal of 36% or less of tomato leaves at flowering start stage had no significant negative effect on morphological characters, physiological and biochemical parameters, reproductive characters and fruit yield. Thus, up to 36% of the total leaf area in healthy tomato plants was apparently not required to supported normal fruit yield. However, fruit yield plant-1 increased under 3 and 6 leaves defoliated plants was due to greater number of fruits plant-1 and larger fruit size compared to control. This result is consistent with the findings of Heuvelink and Buiskool [1] and Fukuchi et al. [23] in tomato. They observed that fruit yields were not affect under mild or partial defoliation in tomato. Xiao et al. [6] found that removing one in every three young leaves did not result in any significant loss in yield of tomato. Again, lower fruit yield per plant under heavy defoliated condition was due to fewer numbers of fruit and smaller size fruits. Reduction in the number of fruits plant-1 under high defoliated condition might be due to lesser leaf area plant-1 which consequence production of lower amount of assimilate that is not sufficient for bearing maximum fruits. Similar result was also reported by many workers in tomato [1, 10, 24]. They observed that fruits plant-1 decreased under heavy defoliated condition in tomato.

Again, the fruit size was lower in higher defoliated plants. It might be due to lower amount of assimilate translocation from leaf to fruits which consequence smaller size fruits. The potential size of the fruit is defined by the number of cells in the ovary fixed in pre-anthesis, while its actual size is a result of cell elongation during the period of rapid growth [25]. At the time of the different defoliation treatments, the potential size of the fruit was already defined. Cell elongation depends on the supply of assimilates to the fruits and climatic conditions [26]. Under heavy defoliated condition, less number of leaves unavailable to supply sufficient assimilates to the fruits, thereby produced small size fruits.

Pn rate and NR activity increased in defoliated plant. It is possible because of leaf photosynthesis or whole canopy gas exchange per unit leaf area was positively related to crop load. In the experiment, fruit number did not decrease proportional to the leaf loss. In heavy defoliated plant, fruit bearing lost only 14% against 71% leaf reduction. It means to fulfill the assimilate demand by the sink, the remaining leaves increase Pn rate and NR activity. These results suggest that under normal condition, assimilate accumulation is operating below its maximum potential. When source-sink ratios of whole plants were lowered experimentally, net photosynthetic and net assimilation rates of the remaining leaves increased 20-40% in soybean [27] and 38% in okra [9]. Other authors have found that partial defoliation of plants stimulated the photosynthetic rates of the remaining leaves [1, 28]. Further, moderate defoliation may improve light penetration and distribution within the canopy, thereby improving whole plant CO2 assimilation. This indicates involvement of an effective compensatory mechanism, which helps in production of more assimilate in the remaining leaves. This could be the reason that fruit yield did not reduce proportionally to the degree of defoliation.

In general, heavy pruning decreased the number of flowers but RE was not affect by heavy defoliation, even increased over control. This could be explained in a way that less competition for assimilates by being their fewer flowers and this has certainly facilitated to produce maximum number of pods to flowers and vice versa [29]. Positive and significant correlation of yield with flowers, but number of flowers was negatively and significantly correlated with RE suggests that it might be difficult to get higher flower production with increased RE simultaneously. Similar results were also reported by Fakir et al. [30] in mungbean and Saitoh et al. [31] in soybean that it would be difficult to incorporate high flower production capacity and low flower abortion (FA) into one strain because of a positive correlation between FA and flower number, which also support the present result.

Conclusion

Leaf loss through insect attack, disease and other environmental hazards at bud initiation stage up to 36% may not affect fruit yield in tomato as was investigated in the experiment. This suggests that removal of lower leaves up to 36% check early leaf blight which other ways save environmental hazards by avoiding fungicide spray. Therefore, it may not advisable to spray pesticide for controlling pests in tomato variety at one-third loss of leaf surface to save environment from pollution.

This work was supported by the Research and Development Project of BINA, Ministry of Agriculture, Bangladesh. Authors have no conflict of interest.

The authors have no conflicts of interest relevant to this study to disclose.

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