4.1 Fruit thinning
The experimentation results presented in the above section (Fig. 1) have revealed that the foliar PBZ application at the beginning of the pit hardening stage has significantly induced fruit thinning in Early Grande peach by way of increasing the initial fruit drop. The effect was found higher with the higher dose of PBZ. This influence of PBZ application on fruit drop may be supported by the findings of Monge et al. (1993) who reported that it is the interaction between the developing fruit and the PBZ which affects the absorption of different mineral elements and results in fruit thinning in peaches. These findings are also in close conformity with those of Webster and Andrew (1986) who reported that PBZ application was the most effective treatment for inducing fruit thinning in Victoria plum when it is applied at the pit hardening stage of fruit development. Further, they added that 1000 to 2000 mg/litre of PBZ are quite effective in most of the plum cultivars for inducing fruit thinning; a similar effect has been observed under the current studies for Early Grande peach wherein the 2500 ppm PBZ application resulted in effective fruit thinning.
The higher initial fruit drop observed under the LFR treatments than control (Fig. 1) may be attributed to the physiological shock that the fruits in the vicinity of leaves might have received upon removal of some of these leaves for maintenance of specific LFR. The findings of Dejong and Walton (1989), Lai et al. (1989), Corelli-Grappadelli and Coston (1991) are also supportive in this regard; they reported that the developing fruits require leaves in the vicinity for meeting their requirement of carbohydrates for respiration and other fruit developmental processes.
The additive effect of PBZ and LFR treatments in inducing 1st or 2nd periodic fruit drop may be attributed both to the inhibitory effect of PBZ (Blanco, 1988; Monge 1993) and the sudden reduction in carbohydrates supply which was induced by the LFR treatments (Dejong and Walton, 1989; Lai et al. 1989; Corelli-Grappadelli and Coston, 1991) which might have resulted in higher fruit drop under the treatment combination of 2500 ppm PBZ and 40:1 LFR. Gernard et al. (1998) attributed such type of influence to the preferential orientation of available assimilates toward the shoot growth rather than fruit growth in late-maturing peach cultivars. However, in medium or early-maturing peach cultivars fruit and vegetative growth occur concurrently, so the balance between assimilating fruits and leaves is reported to be important in peaches. Further, Mimoun et al. (1996) while estimating the effect of LFR on peach leaf light-saturated photosynthesis concluded that there exists a significant effect of the LFR on leaf light-saturated photosynthesis, it happens to be highest at a specific LFR and gets reduced at higher LFR or lower LFR.
4.2 Days from full bloom to fruit harvest
In the present findings, the PBZ application @ 2500 ppm has been found to advance fruit maturity by 2.5 days (Table 1). It is now well-established that PBZ is widely used to advance the harvest maturity of many fruits crop (Ashraf and Ashraf, 2020). It restricts the vegetative growth and guides the assimilates toward the developing fruit resulting in relatively early maturity of the fruits (Arzani et al., 2009). The effect of PBZ on fruit developmental processes is dose-dependent, Upreti et al. (2013) reported that its application not only influences the gibberellins but also increases the contents of ABA and cytokinin viz. zeatin, zeatin riboside and dehydrogenation riboside concomitant with C: N ratio and leaf water potential which ultimately enhance the rate of fruit development.
4.3 Fruit Retention, Weight and Yield
The effect of PBZ on higher fruit retention (Table 1) may be attributed to the higher initial fruit drop induced by PBZ application (Fig. 1); the focussed supply of the assimilates to the fruits which remained there on the plants after the initial fruit drop might have helped in better fruit retention at harvest. Similar results were reported in McIntosh apples by Stan et al. (1989) who reported that the foliar application of paclobutrazol reduced pre-harvest drop when applied within 5 weeks after full bloom. The highest final fruit retention specifically at 40:1 LFR may be supported by the findings of Hansen (1971), Dann et al. (1984) and Dejong et al. (1987) which can be used for explaining the balanced assimilates partitioning between developing fruit and vegetative parts.
The positive effect of PBZ on fruit weight may be attributed to the optimum number of fruits which remained after high initial thinning (Fig. 1) induced by the PBZ treatment. PBZ application not only increased the assimilate translocation toward the developing fruits but also reduced their translocation towards the vegetative sinks which are evident from the data presented in Table 4. Similar findings have been reported by Blanco (1985), Costa (2000), Wu et al. (2005) and Deshmukh et al. (2012).
Further, the appropriate number of leaves maintained through LFR treatment 40:1 not only restricted the excessive vegetative growth but also provided the target assimilate supply to the developing fruits. Thus, the combination of PBZ @ 2500 ppm LFR @ 40:1 resulted in the observance of the highest fruit weight. These findings are in close conformity with those of Gucci et al. (1994), Urban and Lechudel (2005) and Li et al. (2007) who stated that reduction in leaf area to a certain extent leads to a rise in photosynthetic rates of remaining leaves which compensates the sink demands.
As yield is a function of fruit number and fruit weight; the higher values for these parameters recorded under the PBZ and LFR treatments thus resulted in higher per-plant yield (Table 1). Higher yields in peaches due to PBZ application and at a specific LFR have also been reported by Yoshikawa et al. (1988), Zhang et al. (1990), Palmer (1992) and Patel et al. (2014). As both these treatment factors exerted a positive influence on fruit number and fruit weight, their interactions thus produced an additive effect on fruit yield.
4.4 Fruit Colour
The significant improvement in fruit surface and pulp colour together with a higher proportion of surface redness due to PBZ application may be attributed to the suppression of gibberellins activity. These findings are in consonation with the findings of Saure (1990) who reported that anthocyanin formation can be stimulated directly by decreasing the level of gibberellins or by increasing the level of inhibitors in developing fruits. Better red colour development in apples and cherries with PBZ application has also been reported by Elfving et al. (1990) and Wani (2004), respectively. Further, the better colour development with the lower dose of PBZ is supported by the findings of Looney and Mckeller (1987) who reported that in Lambert sweet cherry 1000 ppm produced less colour than 500 ppm treatment of paclobutrazol may be due to the attainment of anthocyanin synthesis maxima even with the lower dose of PBZ. Similar observations were recorded under the present experimentation also. 1500ppm resulted in better colour development though it was at par with the 2500 ppm PBZ application rate.
Similarly, the reduction in leaf number under LFR treatments resulted in better exposure of the fruits to sunlight and this might have improved the fruit surface redness and colour. The plantation under high density are known to induce shading through their dense canopies, therefore, improving light levels at the individual plant through Leaf-to-fruit ratio modifications might have accelerated the photo-sensory pathways of the fruit colour development. These findings are in harmony with those of Jone (2018) who reported that fruit colour and commercial value can be improved by improving light at the canopy level. Better colour development in peaches with a specific proportion of leaves to fruit has also been reported by Corelli and Coston (1991), Okie and Blackburn, 2011 and Zhang et al. (2018). As PBZ and LFR treatments have both been found positive for fruit colour development, this might be the reason that the additive effect of these treatment combinations was obtained.
4.5 Fruit quality
Paclobutrazol application @ 2500 ppm produced a significant gain in almost all fruit quality parameters, like TSS, total sugars, and reducing and non-reducing sugars. On the other hand, the titratable acidity, firmness and shelf life of the fruits were found at the lowest at the higher dose of PBZ application. Such changes in fruit quality parameters might have happened due to modification in the source and sink relationship induced by the PBZ application (Yadav et al. 2005). The increase of TSS and sugars thus may be attributed to the reduced allocation of the assimilates to the vegetative growth (Table 4) and this might have resulted in more nutrient partitioning to the fruits.
The decrease in fruit acid content with PBZ application in a dose-dependent manner may be explained under the light of the fact that paclobutrazol while modifying the source-sink relation, may tend to reduce photo-assimilate demand of the growing shoot in favour of the growing sinks (fruits) and increased fruit soluble solids with a corresponding decrease in acidity. But, more appropriately it is speculated that the hastening of fruit developmental processes by PBZ application might be essentially coupled with the process of conversion of acid to sugars with the early onset of the fruit ripening process and this might have resulted in a lower level of fruit acidity at the higher dose of PBZ application. Allan et al., (1992) also reported that paclobutrazol significantly reduced the competing spring shoot growth and resulted in earlier maturity of a greater crop of larger, better-quality fruits. Further, a reduced malic acid concentration at high application rates of PBZ was also reported in pear by Williams (1983).
The reduction in fruit firmness and shelf life of peaches under the present experimentation is also justified in the light of the fact that PBZ application has hastened the fruit maturity and reduced the number of days from full bloom to maturity (Table 1) and this might have resulted into early onset of ripening processes leading to the loosening of cohesive strength of the fruit cells. Deshmukh et al. (2012) also reported similar findings of reduction in fruit firmness and subsequent decrease in the shelf life of peaches due to PBZ application.
The positive effect of LFR treatments was not noticed as significant as far as sugars, firmness or shelf life was concerned but the effect on total soluble solids (TSS), acidity, and ratio was significant. The LFR treatment 40:1 resulted in higher TSS and lower fruit acidity, almost similar to L3 i.e. 50:1 LFR treatment. Similar findings were reported by Gucci et al., (1994), Urban and Lechudel (2005), Li et al. (2007), and Caruso et al. (1991) who reported that maintaining a specific proportion of leaves increased total soluble solids and total sugars while decreasing the level of acidity due to enhancement photosynthetic efficiency of the remaining leaves after removal of extra leaves. The positive and additive effect of PBZ and LFR treatments on fruit quality parameters can be explained in the light of present findings that both these treatment factors restricted vegetative growth (Table 4) and this might have favoured assimilate production at greater photosynthetic efficiency thereby supporting the fruit sink in respiration, growth and storage in a preferential manner (Wareing and Patrick, 1975).
4.6 Vegetative growth
The PBZ application has exerted a significant effect on tree size control (trunk diameter, shoot extension growth and shoot dry weight) under current experimentation and this may be attributed to the inhibitory effect of PBZ on vegetative growth. The PBZ is well known to inhibit gibberellins activity by modifying the isoprenoid pathway as a result of a block in the transformation of the gibberellin biosynthesis pathway from ent-kaurene to ent-kaurenoic acid (Dalziel and Lawrence, 1984). The present findings are in close conformity with those of Edgerton (1986) who reported foliar spray of PBZ @ 500 to 1000 ppm provides 40 to 60% control over the vegetative growth of sweet cherry, sour cherry and peaches.
The significant influence of LFR treatments on reducing shoot length increase might be due to the utilization of the assimilates more by the developing fruit rather than the vegetative part of the plant under the condition of decreased leaf number. Mimoun et al. 1998 also demonstrated a reduction in vegetative growth at reduced level leaf area in peaches. It implies that modification in the number of leaves per unit of fruit also restricted vegetative growth and it can be attributed to the ability of the developing fruit to act as a stronger sink and attract more assimilates than the vegetative parts (Wareing and Patrick, 1975). Lowering the leaf area, the photosynthesis is compensated by the remaining leaves but translocation of assimilates is forwarded first to the developing fruit load than the vegetative parts (Gucci et al. 1994, Urban and Lechudel, 2005, Li et al. 2007).
The positive interaction effects of PBZ and LFR treatments on the reduction of increment in diameter trunk and length of shoot growth can also be supported by the above-discussed research stating strong control of these factors on vegetative growth which might have led to the additive effect on the control of trunk diameter and shoot growth, also.