Effect of temperature variations on phenology and horticultural traits of guava under North-west Indian conditions

Guava (Psidium guajava L.) is an evergreen fruit tree, which is grown throughout India. In Punjab, it is the second most important fruit crop after Kinnow and occupies an area of 9,580 hectares area (Anon., 2019). In Punjab, guava produces two distinct crops in a year i.e. rainy and winter season crops. Generally, the flowering for rainy season crop occurs in April-May and for winter season crop, it occurs in July-August (Bal, 2015). The fruits of rainy and winter season crops mature during July-August and November-December, respectively. Rainy season crop has high yields while the winter season fruits are of superior quality by virtue of low incidences of fruit fly and fruit-borer (Boora et al., 2016). Although, guava is a tropical fruit crop, but in Punjab (North-Western India), it is being cultivated under subtropical climate. Under this climate, the physiological growth of the guava starts with the rise of temperature in spring. However, the start of spring is determined by the length of winters in north-west India, which is variable. The information on impact of temperature variability on phenological clock of guava is lacking. Further, for onset of a particular phenological phase, the specific temperature requirement must be met (Islam et al., 2019), which can be reliably measured in terms of heat units (Padilla-Ramirez et al., 2012). The heat unit information for different phenological phases may prove useful in formulating area specific crop management practices to avert sudden climatic risks (Islam et al., 2019). In Punjab (North-Western India), the year 2019 was a relatively cooler than 2018. The study aimed to assess the impact of these inter-annual temperature variations on phenology and horticultural traits of guava; and also to estimate the accumulated heat units for onset of different phenological stages in guava.

Guava (Psidium guajava L.) is an evergreen fruit tree, which is grown throughout India. In Punjab, it is the second most important fruit crop after Kinnow and occupies an area of 9,580 hectares area (Anon., 2019). In Punjab, guava produces two distinct crops in a year i.e. rainy and winter season crops. Generally, the flowering for rainy season crop occurs in April-May and for winter season crop, it occurs in July-August (Bal, 2015). The fruits of rainy and winter season crops mature during July-August and November-December, respectively. Rainy season crop has high yields while the winter season fruits are of superior quality by virtue of low incidences of fruit fly and fruit-borer (Boora et al., 2016). Although, guava is a tropical fruit crop, but in Punjab (North-Western India), it is being cultivated under subtropical climate. Under this climate, the physiological growth of the guava starts with the rise of temperature in spring. However, the start of spring is determined by the length of winters in north-west India, which is variable. The information on impact of temperature variability on phenological clock of guava is lacking. Further, for onset of a particular phenological phase, the specific temperature requirement must be met (Islam et al., 2019), which can be reliably measured in terms of heat units (Padilla-Ramirez et al., 2012). The heat unit information for different phenological phases may prove useful in formulating area specific crop management practices to avert sudden climatic risks (Islam et al., 2019). In Punjab (North-Western India), the year 2019 was a relatively cooler than 2018. The study aimed to assess the impact of these inter-annual temperature variations on phenology and horticultural traits of guava; and also to estimate the accumulated heat units for onset of different phenological stages in guava.
The study was carried out on 7 years old trees of guava cv. Shweta growing at Fruit Research Farm, Punjab Agricultural University, Ludhiana, India. The observations were recorded from a total of nine trees with three trees constituting a replication. The experimental trees were managed under uniform cultural practices. The weather data of monthly temperature (maximum, minimum and mean) of the experimental period was collected from PAU Agro-meteorology Observatory, Ludhiana (Table 1). The experimental location received a total rainfall of 919 mm in 2018 and 1147 mm in 2019. To find the impact of yearly temperature variations on phenological cycle, the data was recorded on the onset of phenological stages (bud sprouting, flower initiation, fruit maturity) and days required to reach flower initiation and fruit maturity for rainy and winter season crops. The temperature required for transition of sprouting phase to flowering (SF) and flowering to fruit maturity (FM) was determined in terms of heat units. The heat units were calculated by subtracting the base temperature from the daily average temperature (T max +T min /2). The accumulative heat units were worked out by summing up daily heat units for duration of SF and FM. The temperature of 15 o C was taken as base temperature, as shoot growth was arrested below this temperature under Ludhiana conditions. Among the horticultural traits, the data on fruit set, fruit drop and final fruit retention was recorded.

Temperature variability and its impact on guava phenology
The perusal of temperature data of two years revealed that the year 2019 was relatively cooler than 2018 (Table 1) Table 2). The duration of bud sprouting to flower initiation for rainy season crop was 49 days in 2019 as against 42 days in 2018. Unlike flower initiation, the transition period of flower initiation to fruit maturity for rainy season crop was almost similar in the two years (Table 2).
Due to delay of cycle of rainy season crop in 2019, the phenology of winter season crop was also altered. The sprouting of vegetative bud and initiation of flowering was delayed by 13 and 15 days, respectively. The most apparent effect was noticed for fruit maturity, which took 33 more days in 2019 compared to 2018 (Table 2). Leaf sprouting and flower development in spring season starts early with an increase of temperature during late winters and early spring (Chmielewski and Rotzer, 2001). In contrast, the extreme cold and frosty conditions for longer period may delay the flower initiation. Similarly, the duration of flowering is also influenced by temperature (Lomas andBurd, 1983: Singh andBhatia, 2011).

Heat units required for onset of different phenophases
The accumulated heat units for flower initiation from sprouting (SF) was highly different for rainy season crop (351.5 and 439.2 day 0 C), but almost similar for winter season crop (217.2 versus 238.9 day 0 C) in both the years i.e. 2018 and 2019 ( Table 2). The lower   temperature retards shoot growth in guava as the reserved carbohydrates are diverted to protect against for low temperature stress (Hao et al., 2009). In geophytes, the exposure of plants to a temperature higher than the optimal (25-30 o C) during the period of flower initiation also delays the onset of flowering (Khodorova and Boitel-Conti, 2013). Hence, the delay of flowering of rainy season crop in 2019 could be a consequence of bi-faceted effect of prevailing temperature. The lower temperature first retarded the vegetative growth and prolonged the duration to attain optimum growth stage for flower induction. The later concurrence of flower induction period with higher temperature in SMW 17 further extended it.
For transition of flowering to fruit maturity (FM), the accumulated heat units for rainy season crop were almost similar (1545 day 0 C in 2018 versus 1574 day 0 C in 2019) and highly variable for winter season crop in both the years ( Table 2). The actual relationship of fruit maturity with temperature is not known under fluctuating temperature conditions in guava, though, in tomato, when plants were grown under different temperature regimes of 14, 18, 22 and 26 o C, the fruit maturity was advanced at higher temperature compared to lower temperatures (Adams et al., 2001). There was a significant difference of 33 days in fruit maturity (FM) duration of winter season crop among both the years (Table 2). Thus, the early onset of flowering (by two weeks) and initiation of fruit development under the optimum temperature regimes might have helped in inducing timely fruit maturity of winter season crop in 2018 compared to 2019.

Fruit set and fruit drop pattern
For rainy season crop, the number of fruits set was higher (46.1%) in 2019 than 2018 (Table 3). However, for winter season crop, the number of fruits set in both the years was almost similar. The higher fruit set for rainy season crop of 2019 may be due to longer exposure to lower temperature regimes. The higher accumulation of low temperature hours preceding bloom is known to increase intensity of flowering in citrus (Valiente and Albrigo, 2004). The fruit drop in winter season crop of 2019 was substantially higher when compared to 2018. Comparatively low temperature and associated weather conditions during fruiting in second season may be the cause of more fruit drops when compared with first season's fewer fruit drops in guava. It might have coincidence of fruit maturity with the low temperature and foggy weather/low radiation availability. Similarly, the off season winter fruits of longan had developmental problems which led to heavy fruit drop under field conditions (Yang et al., 2010).
In conclusion, the prevalence of low temperature during pre-sprouting period (February and March) substantially delayed the onset of phenological clock both of rainy as well as winter season crops in 2019. This delay caused the fruit maturity of winter season guava to coincide with peak winters of 2019-20, which led to excessive fruit drop. The future thrust should be to manage such fruit drop by advancing fruit maturity with some chemicals interventions. The heat unit concept based information will serve as a valuable guide for such innovative interventions.

Conflict of Interest Statement :
The author(s) declare (s) that there is no conflict of interest.

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