**3. Papaya responses to environment stresses**

Environmental factors including soil, temperature, and water availability are external factors that significantly impact plant growth and development. The increased demand for papaya fruit as a source of food and plant-derived products and the marginalization of land available for cultivation due to increased housing needs has pushed papaya cultivation to less productive farms or in more developed countries, expensive protective housing cultivation. Further, climate change involving prolonged periods of adverse temperatures and extremes in weather patterns are contributing to the environmental stresses on papaya cultivation. These changes are further limiting not just the productivity, but also the zones of cultivation. Traditional cultivation areas are realizing significant reductions in yield or are forced to grow alternative crops. Papaya plants have optimal cultivation temperatures of between 25°C and 30°C. Temperature, moisture, light, and wind are also major environmental factors impacting papaya production [37, 38]. Temperatures lower than 16°C and higher than 36°C for extended periods negatively impact plant growth [39]. Under these climate extremes, different plant tissue types and organs, including roots, leaves, flowers, and fruit, exhibit variations in responses [40]. The use of traditional genotypes with desired plant fitness and plant developmental stages, fruit types, and yields are being negatively impacted pushing papaya breeding programs to develop varieties that are more adaptable and stable to seasonal changes [41].

In one study, the performance of a number of papaya cultivars including Solo, Formosa, and local commercial hybrids over two harvest seasons was compared. The results indicated that the summer harvest season with average temperatures of 24.9°C and maximums of 34.2°C were more productive than winter harvests where the average temperature was 21.9°C and the minimum 13.4°C [42]. Under low temperatures (<11°C), papaya plants produced fewer new leaves with no fruit set [43]. The optimal temperature for germination of papaya pollen was between 20 and 25°C at 72–80% humidity [44]. Further, extreme temperatures below 15°C and above 30°C negatively impacted germination with rates dropping to between 0 and 56%. Upon applying heat stress to papaya plants, it was shown that plants recovered from mild (37–41°C) and moderate (46°C), but not severe (49°C) heat stress. Photosynthesis was delayed while stress volatile production was induced [45].

Water stress has also been demonstrated to negatively impact papaya growth and development. For short periods of water stress, papaya leaves become droopy. If the water shortage continues, papaya plants will drop flower buds, and delay new fruit, flower, and leaf production. Water stress also results in leaf water potential and a reduction of stomatal opening. This, in turn, reduces carbon dioxide availability and consequently photosynthesis [46]. As a result of limited water availability, seed germination in many papaya cultivars is also delayed [47]. The genotype "Golden" papaya with less chlorophyll content outperformed high chlorophyll "Alianca" papaya under limited water availability [48]. Drought tolerant papaya was shown to have greater photosynthetic II (PSII) efficiency than susceptible ones. Susceptible plants displayed greater leaf abscission, less turgid shoots, and lower plant growth than those of more tolerant ones [30].
