Physico-chemical properties of guava fruits as influenced by solar radiation penetration in plants canopies

The study was conducted to observe the solar radiation distribution in different parts of 7year’s old guava plants and its subsequent effect on fruit quality. The interception of solar radiation decreased markedly with the depth of plant canopy from top to bottom as well as with increase in plant density. More than 3/4th of incoming radiations were found to be intercepted by upper one meter periphery of guava plants irrespective of plant spacing. The fruit quality in terms of size, weight, TSS, vitamin C and overall palatability reduced significantly with the depth of plant canopy and decrease in plant spacing. The upper canopy fruits particularly of widely spaced plants were better than others. Winter season fruits were double in weight and more palatable as compared to rainy season fruits.

Guava (Psidium guajava L.) is an important fruit crop grown in India. Although, the area and production of guava increased significantly in last decade, but the productivity could not increase as expected. Therefore, to increase the productivity to its maximum potential, certain important strategies have to be made. High density plantation (HDP) may be one of them. However, in high density planting system, interception of solar radiations and other microclimatic conditions such as canopy temperature and relative humidity are important aspects which directly or indirectly affect the vegetative growth, yield and quality of the fruits. Guava has a higher proportion of 'shade' to 'sun' leaves and their leaves are found photosynthetically inactive under deeper shade and act as unproductive sink (Singh et al, 2005).Therefore, vegetative growth, fruit yield and quality are functions of light interception and translocation of light energy into chemical energy. Production of good quality fruit is function of absorption of light and light is directly proportional to the yield and quality of fruit trees (Jackson, 1980;Palmer, 1989). Brar et al (2009) investigated that light interception was more in guava trees planted at wider spacing and decrease significantly with the depth of the canopies irrespective of the planting densities. Similarly Singh and Dhaliwal (2007) reported that fruit yield and quality of guava fruits decreased with poor light interception at higher planting densities. Therefore, the present investigations were made to study the radiation penetration in guava plants at different spacing and its effect on physico -chemical propories of fruits.

MATERIALS AND METHODS
The present investigation on seven year old guava plants cv. 'Allahabad Safeda' at different spacing viz. 6x2m, 6x3m, 6x4m and 6x5m were carried out at the New Orchard, Department of Horticulture, PAU, Ludhiana in the year 2007 to 2009 for both rainy (March-August) and winter (September-February) crop seasons. The solar radiation measurements were recorded in clear days thrice a day viz. 8.00-10 am, 12.00-2.00 pm and 4.00-6.00 pm by recording the sensor output from Pyranometer using a Digital Multi-Volt Meter. The Pyranometer measures the total direct and diffuse solar radiation. Incoming solar radiation measurements (Cal cm -2 min -1 ) were recorded at one foot above the canopy and at the centre of the upper, middle and lower parts of the tree by facing Pyranometer upward. The Pyranometer was inverted at a height of one foot above the canopy to see the tree canopy below and thus the amount of reflected short wave radiations [Albedo (A)] was recorded. The radiation/light interception was calculated as the difference between incoming radiations received in each of the three different parts of the tree canopy and was expressed as intercepted radiation at a particular [Vol. 13,No. 1      The observations on physical characters of fruits of all canopy parts of plants at different spacing were noted in terms of fruit size, fruit weight and seed number per fruit. The data on quality characters of fruits were determined in terms of palatability rating, total soluble solids, acidity and vitamin C Physio-Chemical properties of guava June 2011] 52 content according to the method of AOAC (2000).

RESULTS AND DISCUSSION
The total radiation intercepted by Allahabad Safeda guava plants was maximum i.e. 72.7 % and 67.9% of 6x4m followed by 70.9% and 66.5% in 6x5m spaced plants during rainy and winter season, respectively (Table 1). Decrease in radiation interception with the increasing plant density and depth of canopy was observed during both seasons. Minimum total radiations i.e. 61.13% and 57.64% were intercepted in 6x2m spaced plants during rainy and winter crop seasons, respectively.
The upper part of plants at 6x4m spacing intercepted highest solar radiations during rainy (56.1%) as well as winter (51.2%) season and least in 6x2m spaced plants with 48.7% (rainy) and 45.2% (winter) radiation interception. Similar trend of radiation interception with plant density and depth of plant canopy was recorded during both seasons. During winter crop season (September-February), radiation interception in the upper part of plants in all spacing levels as well as total radiation intercepted was recorded lesser as compared to the rainy crop season i.e. March -August due to the difference in inclination of solar radiations during winter and summer seasons. Moreover the old leaves turning yellow as well as the new leaves have relatively high transmissibility (Mavi,1986), thus less radiations were intercepted during this period in the upper part of the plant canopies and slightly more of radiations were reached in the middle and lower parts of the canopies.
Somewhat vertical orientation of auxiliary shoot and leaves causing less absorption and more reflection of incoming solar radiations results reduction in radiation interception in plants at closer spacing of 6x2m and 6x3m. The plants at 6x4m spacing were found to intercept highest radiations owing to higher foliage and more horizontal orientation of shoots and leaves.

Quality characteristics of guava fruits
Fruit size: Reduction in fruit size was recorded with the increase in plant density as well as with the depth of plant canopy ( Table 2). Size of fruits obtained from the upper parts of plants at 6x4m spacing during rainy season was found maximum (length 5.1cm and breadth 5.2cm). During winter season fruit length was recorded maximum i.e. 6.70cm in 6x4m and fruit breadth 6.6cm in 6x5m spaced plants. Smallest fruits were obtained from the lower parts of plant canopy i.e length (4.4 and 5.7cm) and breadth (4.6 and 5.8cm) during rainy and winter season, respectively. Higher fruit size during winter season might be attributed to quite production of less number of fruits during winter season and also due to more accumulation of more carbohydrates at low temperature in winter season. Higher availability of photosynthates and less competition for nutrients and microclimatic parameters in plants at wider spacing and upper parts of canopy contributed increment in fruit size.

Fruit weight:
The mean fruit weight was found to increase Table 3: Quality obtained from upper (U), middle (M) and lower (L) parts of plants at different spacing [Vol. 13,No. 1 with increase in plant spacings particularly during winter season. Maximum mean fruit weight of 94 and 95g per fruit during rainy season (Table 2) and 162 and 160 g per fruit during winter season was recorded in upper parts of plants at 6x5m and 6x4 m spacing, respectively. Size of fruits obtained from the lower parts of plants reduced significantly in all spacing levels during both seasons. The winter season fruits were much heavier than rainy season fruits Seed number: Seed numbers in fruit of both season exhibited positive relationship with increase in spacing. However, middle canopy fruits contained higher seed content as compared to lower and upper canopy fruits during rainy and winter (Table 3) seasons. Maximum mean number of seeds were counted in fruits obtained from middle parts of the plants during rainy (239) and winter (326) season. Fruits taken of lower canopy of plants at closest spacing of 6x2m exhibited least seed numbers during rainy (185) as well as winter (267) season.

Palatability rating (PLR):
The palatability rating of fruits was increased with increase in plant spacing as well as height of plant canopy in both rainy as well as winter (Table 3) season crops. The upper canopy fruits of plants at 6x5m spacing were maximum palatable with rating of 8.38 during winter and 6.70 during rainy season. The fruit PLR of both seasons decreased significantly with the depth of plant canopy and increase in plant density. The least PLR during rainy (6.10) and winter (8.0) was recorded in lower canopy of plants at highest density plants. Decrease in palatability rating with decrease in plant spacing and depth of plant canopy may be due to reduced interception of solar radiations and uncongenial microclimatic conditions leading to reduced TSS, reducing sugars, higher acidity, poor fruit size and lesser colour development as compared to fruits obtained from plants at wider spacing and upper parts of canopy.

Total soluble solids (TSS):
The upper canopy fruits of plants exhibited maximum mean TSS i.e. 9.7 and 11.2% and minimum of 9.4 and 10.3% in lower canopy fruits during rainy and winter (Table 3) season crop, respectively. Highest TSS content in rainy (9.9%) and winter (11.8%) season was recorded in upper canopy fruits of 6x5m spaced plant and least 9.1% (rainy) and 10.2% (winter) in lower canopy fruits of 6x2m spaced plants.
Acidity: Decreasing trend of acidity in guava fruits with increasing plant spacing and height of plant canopy was observed during both seasons. Lower canopy fruits of both seasons contained average maximum acidity of 0.213% (Table 3) followed by middle and upper canopy fruits. Least ascorbic acid content was analyzed in fruits of upper canopy of plant at wider spacing of 6x5m during rainy (0.185%) seasons.

Vitamin C:
In rainy season, average vitamin C content of upper canopy fruits was significantly higher i.e. 152.3 mg /100g fruit in rainy and 178.4 mg/100 g fruit in winter season. Highest vitamin C content was recorded in upper canopy fruits of plants in rainy (158.1 and 160.5 mg) and winter (190.0 and 185.5 mg) season at 6x5 m and 6x4m spaced plants, respectively. Increment in vitamin C content of fruits of widely spaced and upper canopy fruits may be owing to higher radiation interception and assimilation of better nutrition and photosynthates as compared to the plants at closer spacing.